Route generated chunks over transient events

.github/workflows/pipeline.yml

@@ -36,7 +36,6 @@ jobs:
         with:
           name: exo
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
-          pushFilter: '-docker-image.tar.gz$'
 
       - name: Build Metal packages (macOS only)
         if: runner.os == 'macOS'

python/parts.nix

@@ -183,24 +183,7 @@ let
       inherit venv;
       editablePythonSet = pythonSet.overrideScope editableOverlay;
       mkPythonScript = path: mkApp ''python ${path} "$@"'';
-      mkOutputs = name:
-        let package = mkApp ''exo "$@"'' name;
-        in {
-          ${name} = package;
-          "${name}-docker-image" = pkgs.dockerTools.buildLayeredImage {
-            name = "${name}-docker-image";
-            config = {
-              Entrypoint = [ (lib.getExe package) ];
-              Env = [
-                "EXO_HOME=/var/lib/${name}"
-              ];
-            };
-
-            extraCommands = ''
-              mkdir -p  var/lib/${name}
-            '';
-          };
-        };
+      mkExo = mkApp ''exo "$@"'';
     };
 in
 {
@@ -208,7 +191,7 @@ in
     { self', pkgs, unfreePkgs, lib, ... }:
     let
       inherit (pkgs.stdenv.hostPlatform) isLinux;
-      inherit (mkPythonSet { inherit self' pkgs lib; members.exo = [ "cpu" ]; }) editablePythonSet mkOutputs;
+      inherit (mkPythonSet { inherit self' pkgs lib; members = { exo = [ "cpu" ]; }; }) editablePythonSet mkExo;
 
       # Virtual environment with dev dependencies for testing
       testVenv = (mkPythonSet {
@@ -230,13 +213,10 @@ in
         text = ''exec python ${path} "$@"'';
       };
 
-      defaultOutputs = mkOutputs "exo";
-      cuda12Outputs = (mkPythonSet { inherit self' lib; inherit (unfreePkgs.pkgsCuda.cudaPackages_12) pkgs; members.exo=["cuda12"]; }).mkOutputs "exo-cuda-12";
-      cuda13Outputs = (mkPythonSet { inherit self' lib; inherit (unfreePkgs.pkgsCuda.cudaPackages_13) pkgs; members.exo=["cuda13"]; }).mkOutputs "exo-cuda-13";
     in
     {
       packages = {
-        # for devShell
+        exo = mkExo "exo";
         editableVenv = editablePythonSet.mkVirtualEnv "exo-dev-env" { exo = [ "dev" ]; };
         # for running tests in ci
         exo-test-env = testVenv;
@@ -245,9 +225,10 @@ in
         exo-eval-tool-calls = mkBenchScript "exo-eval-tool-calls" (inputs.self + /bench/eval_tool_calls.py);
         # used by ./tests/run_exo_on.sh
         exo-get-all-models-on-cluster = mkSimplePythonScript "exo-get-all-models-on-cluster" (inputs.self + /tests/get_all_models_on_cluster.py);
-      } // defaultOutputs
-      // lib.optionalAttrs isLinux cuda12Outputs
-      // lib.optionalAttrs isLinux cuda13Outputs;
+      } // lib.optionalAttrs isLinux {
+        exo-cuda-12 = (mkPythonSet { inherit self' lib; inherit (unfreePkgs.pkgsCuda.cudaPackages_12) pkgs; members = { exo = [ "cuda12" ]; }; }).mkExo "exo-cuda-12";
+        exo-cuda-13 = (mkPythonSet { inherit self' lib; inherit (unfreePkgs.pkgsCuda.cudaPackages_13) pkgs; members = { exo = [ "cuda13" ]; }; }).mkExo "exo-cuda-13";
+      };
 
       checks = {
         lint = pkgs.runCommand "ruff-lint" { } ''

src/exo/api/main.py

@@ -121,6 +121,8 @@ from exo.api.types.openai_responses import (
 )
 from exo.master.image_store import ImageStore
 from exo.master.placement import place_instance as get_instance_placements
+from exo.routing.event_router import EventRouter
+from exo.routing.snapshot_receiver import SnapshotReceiver
 from exo.shared.apply import apply
 from exo.shared.constants import (
     DASHBOARD_DIR,
@@ -133,10 +135,12 @@ from exo.shared.constants import (
 )
 from exo.shared.election import ElectionMessage
 from exo.shared.logging import InterceptLogger
-from exo.shared.models import model_cards
 from exo.shared.models.model_cards import (
     ModelCard,
     ModelId,
+    add_to_card_cache,
+    get_card,
+    get_model_cards,
 )
 from exo.shared.tracing import TraceEvent, compute_stats, export_trace, load_trace_file
 from exo.shared.types.chunks import (
@@ -162,6 +166,7 @@ from exo.shared.types.commands import (
     ImageEdits,
     ImageGeneration,
     PlaceInstance,
+    RequestSnapshot,
     SendInputChunk,
     SetInstanceLink,
     StartDownload,
@@ -169,16 +174,17 @@ from exo.shared.types.commands import (
     TaskFinished,
     TextGeneration,
 )
-from exo.shared.types.common import CommandId, Id, NodeId, SystemId
+from exo.shared.types.common import CommandId, Id, NodeId, SessionId, SystemId
 from exo.shared.types.events import (
     ChunkGenerated,
     Event,
     IndexedEvent,
-    InstanceDeleted,
     TracesMerged,
+    TransientEvent,
 )
 from exo.shared.types.instance_link import InstanceLink, InstanceLinkId
 from exo.shared.types.memory import Memory
+from exo.shared.types.snapshots import SnapshotChunk
 from exo.shared.types.state import State
 from exo.shared.types.tasks import (
     ImageEdits as ImageEditsTask,
@@ -205,6 +211,8 @@ from exo.utils.task_group import TaskGroup
 _API_EVENT_LOG_DIR = EXO_EVENT_LOG_DIR / "api"
 ONBOARDING_COMPLETE_FILE = EXO_CACHE_HOME / "onboarding_complete"
 
+_SNAPSHOT_FETCH_TIMEOUT_SECONDS = 30
+
 
 def _format_to_content_type(image_format: Literal["png", "jpeg", "webp"] | None) -> str:
     return f"image/{image_format or 'png'}"
@@ -234,9 +242,13 @@ class API:
     def __init__(
         self,
         node_id: NodeId,
+        session_id: SessionId,
         *,
         port: int,
+        event_router: EventRouter,
         event_receiver: Receiver[IndexedEvent],
+        transient_event_receiver: Receiver[TransientEvent],
+        snapshot_chunk_receiver: Receiver[SnapshotChunk],
         command_sender: Sender[ForwarderCommand],
         download_command_sender: Sender[ForwarderDownloadCommand],
         # This lets us pause the API if an election is running
@@ -245,14 +257,17 @@ class API:
         self.state = State()
         self._event_log = DiskEventLog(_API_EVENT_LOG_DIR)
         self._system_id = SystemId()
+        self.session_id = session_id
+        self.event_router = event_router
         self.command_sender = command_sender
         self.download_command_sender = download_command_sender
         self.event_receiver = event_receiver
+        self.transient_event_receiver = transient_event_receiver
+        self.snapshot_chunk_receiver = snapshot_chunk_receiver
         self.election_receiver = election_receiver
         self.node_id: NodeId = node_id
         self.last_completed_election: int = 0
         self.port = port
-        self._sent_image_hashes: set[str] = set()
 
         self.paused: bool = False
         self.paused_ev: anyio.Event = anyio.Event()
@@ -287,22 +302,38 @@ class API:
         self._image_generation_queues: dict[
             CommandId, Sender[ImageChunk | ErrorChunk]
         ] = {}
+        self._observed_generation_commands: set[CommandId] = set()
         self._image_store = ImageStore(EXO_IMAGE_CACHE_DIR)
         self._tg: TaskGroup = TaskGroup()
 
-    def reset(self, result_clock: int, event_receiver: Receiver[IndexedEvent]):
+    def reset(
+        self,
+        result_clock: int,
+        session_id: SessionId,
+        event_router: EventRouter,
+        event_receiver: Receiver[IndexedEvent],
+        transient_event_receiver: Receiver[TransientEvent],
+        snapshot_chunk_receiver: Receiver[SnapshotChunk],
+    ):
         logger.info("Resetting API State")
         self._event_log.close()
         self._event_log = DiskEventLog(_API_EVENT_LOG_DIR)
         self.state = State()
         self._system_id = SystemId()
+        self.session_id = session_id
+        self.event_router = event_router
         self._text_generation_queues = {}
         self._image_generation_queues = {}
+        self._observed_generation_commands = set()
         self.unpause(result_clock)
         self.event_receiver.close()
         self.event_receiver = event_receiver
-        self._tg.start_soon(self._apply_state)
-        self._sent_image_hashes = set()
+        self.transient_event_receiver.close()
+        self.transient_event_receiver = transient_event_receiver
+        self.snapshot_chunk_receiver.close()
+        self.snapshot_chunk_receiver = snapshot_chunk_receiver
+        self._tg.start_soon(self._bootstrap_then_apply_state)
+        self._tg.start_soon(self._apply_transient)
 
     def unpause(self, result_clock: int):
         logger.info("Unpausing API")
@@ -479,7 +510,6 @@ class API:
                 topology=self.state.topology,
                 current_instances=self.state.instances,
                 download_status=self.state.downloads,
-                node_rdma_ctl=self.state.node_rdma_ctl,
             )
         except ValueError as exc:
             raise HTTPException(status_code=400, detail=str(exc)) from exc
@@ -543,7 +573,6 @@ class API:
                     current_instances=self.state.instances,
                     required_nodes=required_nodes,
                     download_status=self.state.downloads,
-                    node_rdma_ctl=self.state.node_rdma_ctl,
                 )
             except ValueError as exc:
                 if (model_card.model_id, sharding, instance_meta, 0) not in seen:
@@ -826,18 +855,8 @@ class API:
         )
         command = TextGeneration(task_params=task_params)
 
-        new_images: list[tuple[int, str]] = []
-        for idx, (img, h) in enumerate(zip(images, hashes, strict=True)):
-            if h not in self._sent_image_hashes:
-                self._sent_image_hashes.add(h)
-                new_images.append((idx, img))
-
-        if not new_images:
-            await self._send(command)
-            return command
-
         all_chunks: list[tuple[int, str]] = []
-        for img_idx, img_data in new_images:
+        for img_idx, img_data in enumerate(images):
             for i in range(0, len(img_data), EXO_MAX_CHUNK_SIZE):
                 all_chunks.append((img_idx, img_data[i : i + EXO_MAX_CHUNK_SIZE]))
 
@@ -1633,16 +1652,17 @@ class API:
     async def ollama_tags(self) -> OllamaTagsResponse:
         """Returns list of models in Ollama tags format. We return the downloaded ones only."""
 
-        downloaded_model_ids: set[ModelId] = set()
+        def none_if_empty(value: str) -> str | None:
+            return value or None
+
+        downloaded_model_ids: set[str] = set()
         for node_downloads in self.state.downloads.values():
             for dl in node_downloads:
                 if isinstance(dl, DownloadCompleted):
                     downloaded_model_ids.add(dl.shard_metadata.model_card.model_id)
 
         cards = [
-            c
-            for c in await model_cards.card_cache.list_all()
-            if c.model_id in downloaded_model_ids
+            c for c in await get_model_cards() if c.model_id in downloaded_model_ids
         ]
 
         now = time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())
@@ -1655,8 +1675,8 @@ class API:
                     size=card.storage_size.in_bytes,
                     digest="sha256:000000000000",
                     details=OllamaModelDetails(
-                        family=card.family or None,
-                        quantization_level=card.quantization or None,
+                        family=none_if_empty(card.family),
+                        quantization_level=none_if_empty(card.quantization),
                     ),
                 )
                 for card in cards
@@ -1719,7 +1739,7 @@ class API:
 
     async def get_models(self, status: str | None = Query(default=None)) -> ModelList:
         """Returns list of available models, optionally filtered by being downloaded."""
-        cards = await model_cards.card_cache.list_all()
+        cards = await get_model_cards()
 
         if status == "downloaded":
             downloaded_model_ids: set[str] = set()
@@ -1770,7 +1790,7 @@ class API:
 
         # Immediately update the local cache so the subsequent GET /models
         # returns the new model without waiting for the event round-trip.
-        model_cards.card_cache.cc[card.model_id] = card
+        add_to_card_cache(card)
 
         return ModelListModel(
             id=card.model_id,
@@ -1786,7 +1806,7 @@ class API:
 
     async def delete_custom_model(self, model_id: ModelId) -> JSONResponse:
         """Delete a user-added custom model card and sync deletion across the cluster."""
-        card = model_cards.card_cache.get(model_id)
+        card = get_card(model_id)
         if card is None or not card.is_custom:
             raise HTTPException(status_code=404, detail="Custom model card not found")
 
@@ -1847,7 +1867,9 @@ class API:
         try:
             async with self._tg as tg:
                 logger.info("Starting API")
-                tg.start_soon(self._apply_state)
+                tg.start_soon(self._bootstrap_then_apply_state)
+                tg.start_soon(self._apply_transient)
+                tg.start_soon(self._reconcile_streams)
                 tg.start_soon(self._pause_on_new_election)
                 tg.start_soon(self._cleanup_expired_images)
                 print_startup_banner(self.port)
@@ -1861,6 +1883,8 @@ class API:
             self._event_log.close()
             self.command_sender.close()
             self.event_receiver.close()
+            self.transient_event_receiver.close()
+            self.snapshot_chunk_receiver.close()
 
     async def run_api(self, ev: anyio.Event):
         cfg = Config()
@@ -1876,47 +1900,108 @@ class API:
                 shutdown_trigger=ev.wait,
             )
 
+    async def _bootstrap_then_apply_state(self):
+        await self._fetch_snapshot()
+        await self._apply_state()
+
+    async def _fetch_snapshot(self) -> None:
+        receiver = SnapshotReceiver(self.node_id, self.session_id)
+        await self.command_sender.send(
+            ForwarderCommand(
+                origin=self._system_id,
+                command=RequestSnapshot(requester_node_id=self.node_id),
+            )
+        )
+
+        with anyio.move_on_after(_SNAPSHOT_FETCH_TIMEOUT_SECONDS):
+            with self.snapshot_chunk_receiver as chunks:
+                async for chunk in chunks:
+                    received = receiver.ingest(chunk)
+                    if received is None:
+                        continue
+                    self.state = received.state
+                    self.event_router.set_buffer_start(
+                        received.last_event_applied_idx + 1
+                    )
+                    logger.info(
+                        f"API bootstrapped from snapshot at idx "
+                        f"{received.last_event_applied_idx}"
+                    )
+                    return
+        logger.info(
+            "API: no snapshot received before timeout; falling back to full event-log replay"
+        )
+
     async def _apply_state(self):
         with self.event_receiver as events:
             async for i_event in events:
+                if i_event.idx <= self.state.last_event_applied_idx:
+                    continue
                 self._event_log.append(i_event.event)
                 self.state = apply(self.state, i_event)
                 event = i_event.event
 
-                if isinstance(event, ChunkGenerated):
-                    if queue := self._image_generation_queues.get(
-                        event.command_id, None
-                    ):
-                        assert isinstance(event.chunk, ImageChunk)
-                        try:
-                            await queue.send(event.chunk)
-                        except (BrokenResourceError, ClosedResourceError):
-                            self._image_generation_queues.pop(event.command_id, None)
-                    if queue := self._text_generation_queues.get(
-                        event.command_id, None
-                    ):
-                        assert not isinstance(event.chunk, ImageChunk)
-                        try:
-                            await queue.send(event.chunk)
-                        except (BrokenResourceError, ClosedResourceError):
-                            self._text_generation_queues.pop(event.command_id, None)
-                if isinstance(event, InstanceDeleted):
-                    self._close_streams_for_instance(event.instance_id)
                 if isinstance(event, TracesMerged):
                     self._save_merged_trace(event)
 
-    def _close_streams_for_instance(self, instance_id: InstanceId) -> None:
-        """Close any active generation streams for commands running on the given instance."""
-        for task in self.state.tasks.values():
-            if task.instance_id != instance_id:
-                continue
-            if not isinstance(
+    async def _apply_transient(self) -> None:
+        with self.transient_event_receiver as events:
+            async for event in events:
+                if isinstance(event, ChunkGenerated):
+                    await self._dispatch_chunk(event)
+
+    async def _dispatch_chunk(self, event: ChunkGenerated) -> None:
+        if queue := self._image_generation_queues.get(event.command_id, None):
+            assert isinstance(event.chunk, ImageChunk)
+            try:
+                await queue.send(event.chunk)
+            except (BrokenResourceError, ClosedResourceError):
+                self._image_generation_queues.pop(event.command_id, None)
+        if queue := self._text_generation_queues.get(event.command_id, None):
+            assert not isinstance(event.chunk, ImageChunk)
+            try:
+                await queue.send(event.chunk)
+            except (BrokenResourceError, ClosedResourceError):
+                self._text_generation_queues.pop(event.command_id, None)
+
+    async def _reconcile_streams(self) -> None:
+        while True:
+            await anyio.sleep(1)
+            self._reconcile_streams_once()
+
+    def _reconcile_streams_once(self) -> None:
+        generation_tasks = [
+            task
+            for task in self.state.tasks.values()
+            if isinstance(
                 task, (TextGenerationTask, ImageGenerationTask, ImageEditsTask)
-            ):
-                continue
-            if sender := self._text_generation_queues.pop(task.command_id, None):
+            )
+        ]
+        state_command_ids = {task.command_id for task in generation_tasks}
+        self._observed_generation_commands.update(state_command_ids)
+
+        live_command_ids = {
+            task.command_id
+            for task in generation_tasks
+            if task.instance_id in self.state.instances
+        }
+        queued_command_ids = set(self._text_generation_queues) | set(
+            self._image_generation_queues
+        )
+        stale_command_ids = (
+            self._observed_generation_commands - live_command_ids
+        ) & queued_command_ids
+        self._close_streams_for_commands(stale_command_ids)
+
+        self._observed_generation_commands = (
+            self._observed_generation_commands & queued_command_ids
+        ) | state_command_ids
+
+    def _close_streams_for_commands(self, command_ids: set[CommandId]) -> None:
+        for command_id in command_ids:
+            if sender := self._text_generation_queues.pop(command_id, None):
                 sender.close()
-            if sender := self._image_generation_queues.pop(task.command_id, None):
+            if sender := self._image_generation_queues.pop(command_id, None):
                 sender.close()
 
     def _save_merged_trace(self, event: TracesMerged) -> None:

src/exo/api/tests/test_api_snapshot_bootstrap.py

@@ -0,0 +1,191 @@
+# pyright: reportPrivateUsage=false
+
+import hashlib
+
+import anyio
+import pytest
+import zstandard
+
+from exo.api.main import API
+from exo.routing.event_router import EventRouter
+from exo.shared.models.model_cards import ModelId
+from exo.shared.types.chunks import (
+    ErrorChunk,
+    PrefillProgressChunk,
+    TokenChunk,
+    ToolCallChunk,
+)
+from exo.shared.types.commands import ForwarderCommand, RequestSnapshot
+from exo.shared.types.common import CommandId, NodeId, SessionId, SystemId
+from exo.shared.types.events import (
+    ChunkGenerated,
+    Event,
+    GlobalForwarderEvent,
+    IndexedEvent,
+    LocalForwarderEvent,
+    TestEvent,
+    TransientEvent,
+)
+from exo.shared.types.snapshots import SnapshotChunk, SnapshotTransferId
+from exo.shared.types.state import State
+from exo.utils.channels import Receiver, Sender, channel
+
+
+class _FakeEventLog:
+    def __init__(self) -> None:
+        self.appended: list[Event] = []
+
+    def append(self, event: Event) -> None:
+        self.appended.append(event)
+
+
+def _snapshot_chunk(
+    state: State, *, requester_node_id: NodeId, session_id: SessionId
+) -> SnapshotChunk:
+    body = zstandard.ZstdCompressor().compress(state.model_dump_json().encode("utf-8"))
+    return SnapshotChunk.from_data(
+        data=body,
+        transfer_id=SnapshotTransferId("transfer-1"),
+        requester_node_id=requester_node_id,
+        session_id=session_id,
+        schema_version=state.schema_version,
+        last_event_applied_idx=state.last_event_applied_idx,
+        chunk_index=0,
+        total_chunks=1,
+        sha256_hex=hashlib.sha256(body).hexdigest(),
+    )
+
+
+def _api(
+    node_id: NodeId, session_id: SessionId
+) -> tuple[
+    API,
+    EventRouter,
+    Receiver[ForwarderCommand],
+    Sender[SnapshotChunk],
+    Sender[TransientEvent],
+    Sender[IndexedEvent],
+    _FakeEventLog,
+]:
+    router_command_sender, _router_command_receiver = channel[ForwarderCommand]()
+    _global_event_sender, global_event_receiver = channel[GlobalForwarderEvent]()
+    local_event_sender, _local_event_receiver = channel[LocalForwarderEvent]()
+    event_router = EventRouter(
+        session_id=session_id,
+        command_sender=router_command_sender,
+        external_inbound=global_event_receiver,
+        external_outbound=local_event_sender,
+    )
+
+    event_sender, event_receiver = channel[IndexedEvent]()
+    transient_sender, transient_receiver = channel[TransientEvent]()
+    command_sender, command_receiver = channel[ForwarderCommand]()
+    snapshot_sender, snapshot_receiver = channel[SnapshotChunk]()
+
+    api = object.__new__(API)
+    api.node_id = node_id
+    api.session_id = session_id
+    api.event_router = event_router
+    api.event_receiver = event_receiver
+    api.transient_event_receiver = transient_receiver
+    api.snapshot_chunk_receiver = snapshot_receiver
+    api.command_sender = command_sender
+    api._system_id = SystemId("api-system")
+    api.state = State()
+    event_log = _FakeEventLog()
+    api._event_log = event_log  # pyright: ignore[reportAttributeAccessIssue]
+    api._image_generation_queues = {}
+    api._text_generation_queues = {}
+    return (
+        api,
+        event_router,
+        command_receiver,
+        snapshot_sender,
+        transient_sender,
+        event_sender,
+        event_log,
+    )
+
+
+@pytest.mark.asyncio
+async def test_api_fetch_snapshot_applies_state_and_fast_forwards_router() -> None:
+    node_id = NodeId("api")
+    session_id = SessionId(master_node_id=NodeId("master"), election_clock=1)
+    (
+        api,
+        event_router,
+        command_receiver,
+        snapshot_sender,
+        _transient_sender,
+        _event_sender,
+        _event_log,
+    ) = _api(node_id, session_id)
+    state = State(last_event_applied_idx=7)
+
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(api._fetch_snapshot)
+        command = await command_receiver.receive()
+        assert isinstance(command.command, RequestSnapshot)
+        assert command.command.requester_node_id == node_id
+
+        await snapshot_sender.send(
+            _snapshot_chunk(state, requester_node_id=node_id, session_id=session_id)
+        )
+
+    assert api.state.last_event_applied_idx == 7
+    assert event_router.event_buffer.next_idx_to_release == 8
+
+
+@pytest.mark.asyncio
+async def test_api_apply_state_ignores_events_covered_by_snapshot() -> None:
+    node_id = NodeId("api")
+    session_id = SessionId(master_node_id=NodeId("master"), election_clock=1)
+    (
+        api,
+        _event_router,
+        _command_receiver,
+        _snapshot_sender,
+        _transient_sender,
+        event_sender,
+        event_log,
+    ) = _api(node_id, session_id)
+    api.state = State(last_event_applied_idx=7)
+
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(api._apply_state)
+        await event_sender.send(IndexedEvent(idx=7, event=TestEvent()))
+        await event_sender.send(IndexedEvent(idx=8, event=TestEvent()))
+
+        while api.state.last_event_applied_idx != 8:
+            await anyio.sleep(0.001)
+        tg.cancel_scope.cancel()
+
+    assert len(event_log.appended) == 1
+
+
+@pytest.mark.asyncio
+async def test_api_apply_transient_dispatches_generated_chunks() -> None:
+    node_id = NodeId("api")
+    session_id = SessionId(master_node_id=NodeId("master"), election_clock=1)
+    (
+        api,
+        _event_router,
+        _command_receiver,
+        _snapshot_sender,
+        transient_sender,
+        _event_sender,
+        _event_log,
+    ) = _api(node_id, session_id)
+    command_id = CommandId("cmd-a")
+    chunk_sender, chunk_receiver = channel[
+        TokenChunk | ErrorChunk | ToolCallChunk | PrefillProgressChunk
+    ]()
+    api._text_generation_queues[command_id] = chunk_sender
+
+    chunk = ErrorChunk(model=ModelId("test-model"), error_message="test chunk")
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(api._apply_transient)
+        await transient_sender.send(ChunkGenerated(command_id=command_id, chunk=chunk))
+
+        assert await chunk_receiver.receive() == chunk
+        tg.cancel_scope.cancel()

src/exo/api/tests/test_instance_deleted_stream_cleanup.py

@@ -1,11 +1,11 @@
 # pyright: reportUnusedFunction=false, reportAny=false
-"""Tests that InstanceDeleted events close active generation streams."""
+"""Tests that streaming queues reconcile against durable State."""
 
 from unittest.mock import MagicMock
 
 from exo.api.main import API
 from exo.api.types import ImageGenerationTaskParams
-from exo.shared.types.common import CommandId, ModelId
+from exo.shared.types.common import CommandId, ModelId, NodeId
 from exo.shared.types.state import State
 from exo.shared.types.tasks import ImageGeneration, TextGeneration
 from exo.shared.types.text_generation import (
@@ -13,15 +13,16 @@ from exo.shared.types.text_generation import (
     InputMessageContent,
     TextGenerationTaskParams,
 )
-from exo.shared.types.worker.instances import InstanceId
+from exo.shared.types.worker.instances import InstanceId, MlxRingInstance
+from exo.shared.types.worker.runners import ShardAssignments
 
 
 def _make_api_with_state(state: State) -> API:
-    """Create a minimal API instance with pre-set state."""
     api = object.__new__(API)
     api.state = state
     api._text_generation_queues = {}  # pyright: ignore[reportPrivateUsage]
     api._image_generation_queues = {}  # pyright: ignore[reportPrivateUsage]
+    api._observed_generation_commands = set()  # pyright: ignore[reportPrivateUsage]
     return api
 
 
@@ -38,45 +39,90 @@ def _make_text_gen_task(
     )
 
 
-def test_close_streams_for_deleted_instance() -> None:
-    """Deleting an instance closes the text generation sender for commands on that instance."""
+def _make_instance(instance_id: InstanceId) -> MlxRingInstance:
+    return MlxRingInstance(
+        instance_id=instance_id,
+        shard_assignments=ShardAssignments(
+            model_id=ModelId("test-model"),
+            node_to_runner={},
+            runner_to_shard={},
+        ),
+        hosts_by_node={NodeId("node-1"): []},
+        ephemeral_port=1,
+    )
+
+
+def test_reconcile_closes_stream_when_task_instance_is_missing() -> None:
     instance_id = InstanceId("inst-1")
     command_id = CommandId("cmd-1")
     task = _make_text_gen_task(instance_id, command_id)
-
-    state = State(tasks={task.task_id: task})
-    api = _make_api_with_state(state)
+    api = _make_api_with_state(State(tasks={task.task_id: task}, instances={}))
 
     sender = MagicMock()
     api._text_generation_queues[command_id] = sender  # pyright: ignore[reportPrivateUsage]
 
-    api._close_streams_for_instance(instance_id)  # pyright: ignore[reportPrivateUsage]
+    api._reconcile_streams_once()  # pyright: ignore[reportPrivateUsage]
 
     sender.close.assert_called_once()
     assert command_id not in api._text_generation_queues  # pyright: ignore[reportPrivateUsage]
 
 
-def test_close_streams_ignores_unrelated_instances() -> None:
-    """Deleting an instance does NOT close streams for commands on other instances."""
-    target_id = InstanceId("inst-delete")
-    other_id = InstanceId("inst-keep")
-    other_cmd = CommandId("cmd-keep")
-    other_task = _make_text_gen_task(other_id, other_cmd)
-
-    state = State(tasks={other_task.task_id: other_task})
-    api = _make_api_with_state(state)
+def test_reconcile_keeps_stream_for_live_task_instance() -> None:
+    instance_id = InstanceId("inst-live")
+    command_id = CommandId("cmd-live")
+    task = _make_text_gen_task(instance_id, command_id)
+    api = _make_api_with_state(
+        State(
+            tasks={task.task_id: task},
+            instances={instance_id: _make_instance(instance_id)},
+        )
+    )
 
     sender = MagicMock()
-    api._text_generation_queues[other_cmd] = sender  # pyright: ignore[reportPrivateUsage]
+    api._text_generation_queues[command_id] = sender  # pyright: ignore[reportPrivateUsage]
 
-    api._close_streams_for_instance(target_id)  # pyright: ignore[reportPrivateUsage]
+    api._reconcile_streams_once()  # pyright: ignore[reportPrivateUsage]
 
     sender.close.assert_not_called()
-    assert other_cmd in api._text_generation_queues  # pyright: ignore[reportPrivateUsage]
+    assert command_id in api._text_generation_queues  # pyright: ignore[reportPrivateUsage]
 
 
-def test_close_streams_for_deleted_instance_image_generation() -> None:
-    """Deleting an instance closes the image generation sender for commands on that instance."""
+def test_reconcile_does_not_close_command_before_state_observes_it() -> None:
+    command_id = CommandId("cmd-not-created-yet")
+    api = _make_api_with_state(State())
+
+    sender = MagicMock()
+    api._text_generation_queues[command_id] = sender  # pyright: ignore[reportPrivateUsage]
+
+    api._reconcile_streams_once()  # pyright: ignore[reportPrivateUsage]
+
+    sender.close.assert_not_called()
+    assert command_id in api._text_generation_queues  # pyright: ignore[reportPrivateUsage]
+
+
+def test_reconcile_closes_stream_after_observed_task_leaves_state() -> None:
+    instance_id = InstanceId("inst-live")
+    command_id = CommandId("cmd-deleted")
+    task = _make_text_gen_task(instance_id, command_id)
+    api = _make_api_with_state(
+        State(
+            tasks={task.task_id: task},
+            instances={instance_id: _make_instance(instance_id)},
+        )
+    )
+
+    sender = MagicMock()
+    api._text_generation_queues[command_id] = sender  # pyright: ignore[reportPrivateUsage]
+    api._reconcile_streams_once()  # pyright: ignore[reportPrivateUsage]
+
+    api.state = State(instances={instance_id: _make_instance(instance_id)})
+    api._reconcile_streams_once()  # pyright: ignore[reportPrivateUsage]
+
+    sender.close.assert_called_once()
+    assert command_id not in api._text_generation_queues  # pyright: ignore[reportPrivateUsage]
+
+
+def test_reconcile_closes_image_stream_when_task_instance_is_missing() -> None:
     instance_id = InstanceId("inst-img")
     command_id = CommandId("cmd-img")
     task = ImageGeneration(
@@ -84,14 +130,12 @@ def test_close_streams_for_deleted_instance_image_generation() -> None:
         command_id=command_id,
         task_params=ImageGenerationTaskParams(prompt="a cat", model="test-model"),
     )
-
-    state = State(tasks={task.task_id: task})
-    api = _make_api_with_state(state)
+    api = _make_api_with_state(State(tasks={task.task_id: task}, instances={}))
 
     sender = MagicMock()
     api._image_generation_queues[command_id] = sender  # pyright: ignore[reportPrivateUsage]
 
-    api._close_streams_for_instance(instance_id)  # pyright: ignore[reportPrivateUsage]
+    api._reconcile_streams_once()  # pyright: ignore[reportPrivateUsage]
 
     sender.close.assert_called_once()
     assert command_id not in api._image_generation_queues  # pyright: ignore[reportPrivateUsage]

src/exo/download/coordinator.py

@@ -16,8 +16,7 @@ from exo.download.download_utils import (
 )
 from exo.download.shard_downloader import ShardDownloader
 from exo.shared.constants import EXO_DEFAULT_MODELS_DIR, EXO_MODELS_READ_ONLY_DIRS
-from exo.shared.models import model_cards
-from exo.shared.models.model_cards import ModelId
+from exo.shared.models.model_cards import ModelId, get_model_cards
 from exo.shared.types.commands import (
     CancelDownload,
     DeleteDownload,
@@ -423,7 +422,7 @@ class DownloadCoordinator:
                     )
                 # Scan read-only directories for pre-downloaded models
                 if EXO_MODELS_READ_ONLY_DIRS:
-                    for card in await model_cards.card_cache.list_all():
+                    for card in await get_model_cards():
                         mid = card.model_id
                         if mid in self.active_downloads:
                             continue

src/exo/download/impl_shard_downloader.py

@@ -11,11 +11,11 @@ from exo.download.download_utils import (
     download_shard,
 )
 from exo.download.shard_downloader import ShardDownloader
-from exo.shared.models import model_cards
 from exo.shared.models.model_cards import (
     ModelCard,
     ModelId,
     ModelTask,
+    get_model_cards,
 )
 from exo.shared.types.memory import Memory
 from exo.shared.types.worker.shards import (
@@ -258,7 +258,7 @@ class ResumableShardDownloader(ShardDownloader):
 
         tasks = [
             create_task(download_with_semaphore(model_card))
-            for model_card in await model_cards.card_cache.list_all()
+            for model_card in await get_model_cards()
         ]
 
         for task in asyncio.as_completed(tasks):

src/exo/main.py

@@ -17,6 +17,7 @@ from exo.download.impl_shard_downloader import exo_shard_downloader
 from exo.master.main import Master
 from exo.routing.event_router import EventRouter
 from exo.routing.router import Router, get_node_id_keypair
+from exo.routing.transient_router import TransientRouter
 from exo.shared.constants import EXO_LOG
 from exo.shared.election import Election, ElectionResult
 from exo.shared.logging import logger_cleanup, logger_setup
@@ -31,6 +32,7 @@ from exo.worker.main import Worker
 class Node:
     router: Router
     event_router: EventRouter
+    transient_router: TransientRouter
     download_coordinator: DownloadCoordinator | None
     worker: Worker | None
     election: Election  # Every node participates in election, as we do want a node to become master even if it isn't a master candidate if no master candidates are present.
@@ -55,16 +57,24 @@ class Node:
         )
         await router.register_topic(topics.GLOBAL_EVENTS)
         await router.register_topic(topics.LOCAL_EVENTS)
+        await router.register_topic(topics.TRANSIENT_EVENTS)
         await router.register_topic(topics.COMMANDS)
         await router.register_topic(topics.ELECTION_MESSAGES)
         await router.register_topic(topics.CONNECTION_MESSAGES)
         await router.register_topic(topics.DOWNLOAD_COMMANDS)
+        await router.register_topic(topics.SNAPSHOT_RESPONSES)
         event_router = EventRouter(
             session_id,
             command_sender=router.sender(topics.COMMANDS),
             external_outbound=router.sender(topics.LOCAL_EVENTS),
             external_inbound=router.receiver(topics.GLOBAL_EVENTS),
         )
+        transient_router = TransientRouter(
+            node_id=node_id,
+            session_id=session_id,
+            external_outbound=router.sender(topics.TRANSIENT_EVENTS),
+            external_inbound=router.receiver(topics.TRANSIENT_EVENTS),
+        )
 
         logger.info(f"Starting node {node_id}")
 
@@ -83,8 +93,12 @@ class Node:
         if args.spawn_api:
             api = API(
                 node_id,
+                session_id,
                 port=args.api_port,
+                event_router=event_router,
                 event_receiver=event_router.receiver(),
+                transient_event_receiver=transient_router.receiver(),
+                snapshot_chunk_receiver=router.receiver(topics.SNAPSHOT_RESPONSES),
                 command_sender=router.sender(topics.COMMANDS),
                 download_command_sender=router.sender(topics.DOWNLOAD_COMMANDS),
                 election_receiver=router.receiver(topics.ELECTION_MESSAGES),
@@ -95,8 +109,13 @@ class Node:
         if not args.no_worker:
             worker = Worker(
                 node_id,
+                session_id,
+                event_router=event_router,
                 event_receiver=event_router.receiver(),
                 event_sender=event_router.sender(),
+                transient_event_receiver=transient_router.receiver(),
+                transient_event_sender=transient_router.sender(),
+                snapshot_chunk_receiver=router.receiver(topics.SNAPSHOT_RESPONSES),
                 command_sender=router.sender(topics.COMMANDS),
                 download_command_sender=router.sender(topics.DOWNLOAD_COMMANDS),
                 api_port=args.api_port,
@@ -109,9 +128,12 @@ class Node:
             node_id,
             session_id,
             event_sender=event_router.sender(),
+            transient_event_receiver=transient_router.receiver(),
+            transient_event_sender=transient_router.sender(),
             global_event_sender=router.sender(topics.GLOBAL_EVENTS),
             local_event_receiver=router.receiver(topics.LOCAL_EVENTS),
             command_receiver=router.receiver(topics.COMMANDS),
+            snapshot_chunk_sender=router.sender(topics.SNAPSHOT_RESPONSES),
             download_command_sender=router.sender(topics.DOWNLOAD_COMMANDS),
         )
 
@@ -132,6 +154,7 @@ class Node:
         return cls(
             router,
             event_router,
+            transient_router,
             download_coordinator,
             worker,
             election,
@@ -149,6 +172,7 @@ class Node:
             signal.signal(signal.SIGTERM, lambda _, __: self.shutdown())
             tg.start_soon(self.router.run)
             tg.start_soon(self.event_router.run)
+            tg.start_soon(self.transient_router.run)
             tg.start_soon(self.election.run)
             if self.download_coordinator:
                 tg.start_soon(self.download_coordinator.run)
@@ -192,6 +216,13 @@ class Node:
                         self.router.receiver(topics.GLOBAL_EVENTS),
                         self.router.sender(topics.LOCAL_EVENTS),
                     )
+                    self.transient_router.shutdown()
+                    self.transient_router = TransientRouter(
+                        node_id=self.node_id,
+                        session_id=result.session_id,
+                        external_outbound=self.router.sender(topics.TRANSIENT_EVENTS),
+                        external_inbound=self.router.receiver(topics.TRANSIENT_EVENTS),
+                    )
 
                 if (
                     result.session_id.master_node_id == self.node_id
@@ -207,9 +238,14 @@ class Node:
                         self.node_id,
                         result.session_id,
                         event_sender=self.event_router.sender(),
+                        transient_event_receiver=self.transient_router.receiver(),
+                        transient_event_sender=self.transient_router.sender(),
                         global_event_sender=self.router.sender(topics.GLOBAL_EVENTS),
                         local_event_receiver=self.router.receiver(topics.LOCAL_EVENTS),
                         command_receiver=self.router.receiver(topics.COMMANDS),
+                        snapshot_chunk_sender=self.router.sender(
+                            topics.SNAPSHOT_RESPONSES
+                        ),
                         download_command_sender=self.router.sender(
                             topics.DOWNLOAD_COMMANDS
                         ),
@@ -246,8 +282,15 @@ class Node:
                         # TODO: add profiling etc to resource monitor
                         self.worker = Worker(
                             self.node_id,
+                            result.session_id,
+                            event_router=self.event_router,
                             event_receiver=self.event_router.receiver(),
                             event_sender=self.event_router.sender(),
+                            transient_event_receiver=self.transient_router.receiver(),
+                            transient_event_sender=self.transient_router.sender(),
+                            snapshot_chunk_receiver=self.router.receiver(
+                                topics.SNAPSHOT_RESPONSES
+                            ),
                             command_sender=self.router.sender(topics.COMMANDS),
                             download_command_sender=self.router.sender(
                                 topics.DOWNLOAD_COMMANDS
@@ -256,8 +299,16 @@ class Node:
                         )
                         self._tg.start_soon(self.worker.run)
                     if self.api:
-                        self.api.reset(result.won_clock, self.event_router.receiver())
+                        self.api.reset(
+                            result.won_clock,
+                            result.session_id,
+                            self.event_router,
+                            self.event_router.receiver(),
+                            self.transient_router.receiver(),
+                            self.router.receiver(topics.SNAPSHOT_RESPONSES),
+                        )
                     self._tg.start_soon(self.event_router.run)
+                    self._tg.start_soon(self.transient_router.run)
                 else:
                     if self.api:
                         self.api.unpause(result.won_clock)

src/exo/master/main.py

@@ -1,6 +1,8 @@
+import hashlib
 from datetime import datetime, timedelta, timezone
 
 import anyio
+from anyio import to_thread
 from loguru import logger
 
 from exo.master.placement import (
@@ -25,6 +27,7 @@ from exo.shared.types.commands import (
     ImageGeneration,
     PlaceInstance,
     RequestEventLog,
+    RequestSnapshot,
     SendInputChunk,
     SetInstanceLink,
     TaskCancelled,
@@ -52,8 +55,10 @@ from exo.shared.types.events import (
     TraceEventData,
     TracesCollected,
     TracesMerged,
+    TransientEvent,
 )
 from exo.shared.types.instance_link import InstanceLink
+from exo.shared.types.snapshots import SnapshotChunk, SnapshotTransferId
 from exo.shared.types.state import State
 from exo.shared.types.tasks import (
     ImageEdits as ImageEditsTask,
@@ -74,6 +79,15 @@ from exo.utils.disk_event_log import DiskEventLog
 from exo.utils.event_buffer import MultiSourceBuffer
 from exo.utils.task_group import TaskGroup
 
+_SNAPSHOT_CHUNK_BYTES = 512 * 1024
+_MAX_EVENT_LOG_REPLAY_BATCH = 1000
+
+
+def _encode_state_for_transfer(state: State) -> bytes:
+    import zstandard
+
+    return zstandard.ZstdCompressor().compress(state.model_dump_json().encode("utf-8"))
+
 
 def _prefill_endpoint_for(state: State, decode_instance_id: InstanceId) -> str | None:
     decode = state.instances.get(decode_instance_id)
@@ -123,8 +137,11 @@ class Master:
         *,
         command_receiver: Receiver[ForwarderCommand],
         event_sender: Sender[Event],
+        transient_event_receiver: Receiver[TransientEvent],
+        transient_event_sender: Sender[TransientEvent],
         local_event_receiver: Receiver[LocalForwarderEvent],
         global_event_sender: Sender[GlobalForwarderEvent],
+        snapshot_chunk_sender: Sender[SnapshotChunk],
         download_command_sender: Sender[ForwarderDownloadCommand],
     ):
         self.node_id = node_id
@@ -134,7 +151,10 @@ class Master:
         self.command_task_mapping: dict[CommandId, TaskId] = {}
         self.command_receiver = command_receiver
         self.local_event_receiver = local_event_receiver
+        self.transient_event_receiver = transient_event_receiver
+        self.transient_event_sender = transient_event_sender
         self.global_event_sender = global_event_sender
+        self.snapshot_chunk_sender = snapshot_chunk_sender
         self.download_command_sender = download_command_sender
         self.event_sender = event_sender
         self._system_id = SystemId()
@@ -155,6 +175,9 @@ class Master:
             self._event_log.close()
             self.global_event_sender.close()
             self.local_event_receiver.close()
+            self.transient_event_receiver.close()
+            self.transient_event_sender.close()
+            self.snapshot_chunk_sender.close()
             self.command_receiver.close()
 
     async def shutdown(self):
@@ -365,7 +388,6 @@ class Master:
                                 self.state.node_memory,
                                 self.state.node_network,
                                 download_status=self.state.downloads,
-                                node_rdma_ctl=self.state.node_rdma_ctl,
                             )
                             transition_events = get_transition_events(
                                 self.state.instances, placement, self.state.tasks
@@ -442,12 +464,19 @@ class Master:
                         case RequestEventLog():
                             # We should just be able to send everything, since other buffers will ignore old messages
                             # rate limit to 1000 at a time
-                            end = min(command.since_idx + 1000, len(self._event_log))
+                            end = min(
+                                command.since_idx + _MAX_EVENT_LOG_REPLAY_BATCH,
+                                len(self._event_log),
+                            )
                             for i, event in enumerate(
                                 self._event_log.read_range(command.since_idx, end),
                                 start=command.since_idx,
                             ):
                                 await self._send_event(IndexedEvent(idx=i, event=event))
+                        case RequestSnapshot():
+                            self._tg.start_soon(
+                                self._serve_snapshot, command.requester_node_id
+                            )
                     for event in generated_events:
                         await self.event_sender.send(event)
                 except ValueError as e:
@@ -507,6 +536,42 @@ class Master:
                     self._event_log.append(event)
                     await self._send_event(indexed)
 
+    async def _serve_snapshot(self, requester_node_id: NodeId) -> None:
+        state = self.state
+        if state.last_event_applied_idx < 0:
+            logger.info(
+                f"RequestSnapshot from {requester_node_id} but master has no events yet"
+            )
+            return
+
+        body = await to_thread.run_sync(_encode_state_for_transfer, state)
+        sha256 = hashlib.sha256(body).hexdigest()
+        chunks = [
+            body[i : i + _SNAPSHOT_CHUNK_BYTES]
+            for i in range(0, len(body), _SNAPSHOT_CHUNK_BYTES)
+        ] or [b""]
+        transfer_id = SnapshotTransferId()
+
+        logger.info(
+            f"Serving snapshot to {requester_node_id}: "
+            f"idx={state.last_event_applied_idx}, "
+            f"{len(chunks)} chunk(s), {len(body)} bytes total"
+        )
+        for index, chunk in enumerate(chunks):
+            await self.snapshot_chunk_sender.send(
+                SnapshotChunk.from_data(
+                    data=chunk,
+                    transfer_id=transfer_id,
+                    requester_node_id=requester_node_id,
+                    session_id=self.session_id,
+                    schema_version=state.schema_version,
+                    last_event_applied_idx=state.last_event_applied_idx,
+                    chunk_index=index,
+                    total_chunks=len(chunks),
+                    sha256_hex=sha256,
+                )
+            )
+
     # This function is re-entrant, take care!
     async def _send_event(self, event: IndexedEvent):
         # Convenience method since this line is ugly

src/exo/master/placement.py

@@ -28,7 +28,7 @@ from exo.shared.types.events import (
     TaskStatusUpdated,
 )
 from exo.shared.types.memory import Memory
-from exo.shared.types.profiling import MemoryUsage, NodeNetworkInfo, NodeRdmaCtlStatus
+from exo.shared.types.profiling import MemoryUsage, NodeNetworkInfo
 from exo.shared.types.tasks import Task, TaskId, TaskStatus
 from exo.shared.types.worker.downloads import (
     DownloadCompleted,
@@ -105,7 +105,6 @@ def place_instance(
     node_network: Mapping[NodeId, NodeNetworkInfo],
     required_nodes: set[NodeId] | None = None,
     download_status: Mapping[NodeId, Sequence[DownloadProgress]] | None = None,
-    node_rdma_ctl: Mapping[NodeId, NodeRdmaCtlStatus] | None = None,
 ) -> dict[InstanceId, Instance]:
     cycles = topology.get_cycles()
     candidate_cycles = list(filter(lambda it: len(it) >= command.min_nodes, cycles))
@@ -167,18 +166,8 @@ def place_instance(
 
     smallest_cycles = get_smallest_cycles(cycles_with_sufficient_memory)
 
-    rdma_ctl_status = node_rdma_ctl or {}
-
-    def _all_rdma_ctl_enabled(cycle: Cycle) -> bool:
-        return all(
-            ((status := rdma_ctl_status.get(node_id)) is not None and status.enabled)
-            for node_id in cycle
-        )
-
     smallest_rdma_cycles = [
-        cycle
-        for cycle in smallest_cycles
-        if topology.is_rdma_cycle(cycle) and _all_rdma_ctl_enabled(cycle)
+        cycle for cycle in smallest_cycles if topology.is_rdma_cycle(cycle)
     ]
 
     if command.instance_meta == InstanceMeta.MlxJaccl:

src/exo/master/tests/test_master.py

@@ -7,12 +7,14 @@ from loguru import logger
 
 from exo.master.main import Master
 from exo.routing.router import get_node_id_keypair
+from exo.routing.snapshot_receiver import SnapshotReceiver
 from exo.shared.models.model_cards import ModelCard, ModelTask
 from exo.shared.types.commands import (
     CommandId,
     ForwarderCommand,
     ForwarderDownloadCommand,
     PlaceInstance,
+    RequestSnapshot,
     TextGeneration,
 )
 from exo.shared.types.common import ModelId, NodeId, SessionId, SystemId
@@ -24,11 +26,13 @@ from exo.shared.types.events import (
     LocalForwarderEvent,
     NodeGatheredInfo,
     TaskCreated,
+    TransientEvent,
 )
 from exo.shared.types.memory import Memory
 from exo.shared.types.profiling import (
     MemoryUsage,
 )
+from exo.shared.types.snapshots import SnapshotChunk
 from exo.shared.types.tasks import TaskStatus
 from exo.shared.types.tasks import TextGeneration as TextGenerationTask
 from exo.shared.types.text_generation import (
@@ -56,6 +60,8 @@ async def test_master():
     local_event_sender, le_receiver = channel[LocalForwarderEvent]()
     fcds, _fcdr = channel[ForwarderDownloadCommand]()
     ev_send, ev_recv = channel[Event]()
+    transient_send, transient_recv = channel[TransientEvent]()
+    snapshot_chunk_send, _snapshot_chunk_recv = channel[SnapshotChunk]()
 
     async def mock_event_router():
         idx = 0
@@ -89,9 +95,12 @@ async def test_master():
         node_id,
         session_id,
         event_sender=ev_send,
+        transient_event_receiver=transient_recv,
+        transient_event_sender=transient_send,
         global_event_sender=ge_sender,
         local_event_receiver=le_receiver,
         command_receiver=co_receiver,
+        snapshot_chunk_sender=snapshot_chunk_send,
         download_command_sender=fcds,
     )
     logger.info("run the master")
@@ -229,3 +238,55 @@ async def test_master():
 
         ev_send.close()
         await master.shutdown()
+
+
+@pytest.mark.asyncio
+async def test_master_serves_snapshot_for_current_state():
+    node_id = NodeId("master")
+    requester_node_id = NodeId("worker")
+    session_id = SessionId(master_node_id=node_id, election_clock=0)
+
+    ge_sender, _global_event_receiver = channel[GlobalForwarderEvent]()
+    command_sender, command_receiver = channel[ForwarderCommand]()
+    _local_event_sender, local_event_receiver = channel[LocalForwarderEvent]()
+    download_command_sender, _download_command_receiver = channel[
+        ForwarderDownloadCommand
+    ]()
+    event_sender, _event_receiver = channel[Event]()
+    transient_event_sender, transient_event_receiver = channel[TransientEvent]()
+    snapshot_chunk_sender, snapshot_chunk_receiver = channel[SnapshotChunk]()
+
+    master = Master(
+        node_id,
+        session_id,
+        event_sender=event_sender,
+        transient_event_receiver=transient_event_receiver,
+        transient_event_sender=transient_event_sender,
+        global_event_sender=ge_sender,
+        local_event_receiver=local_event_receiver,
+        command_receiver=command_receiver,
+        snapshot_chunk_sender=snapshot_chunk_sender,
+        download_command_sender=download_command_sender,
+    )
+    master.state = master.state.model_copy(update={"last_event_applied_idx": 12})
+
+    receiver = SnapshotReceiver(requester_node_id, session_id)
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(master.run)
+        await command_sender.send(
+            ForwarderCommand(
+                origin=SystemId("api"),
+                command=RequestSnapshot(requester_node_id=requester_node_id),
+            )
+        )
+
+        received = None
+        while received is None:
+            chunk = await snapshot_chunk_receiver.receive()
+            received = receiver.ingest(chunk)
+
+        assert received.last_event_applied_idx == 12
+        assert received.state.last_event_applied_idx == 12
+
+        await master.shutdown()
+        tg.cancel_scope.cancel()

src/exo/master/tests/test_placement.py

@@ -21,11 +21,7 @@ from exo.shared.types.events import (
 )
 from exo.shared.types.memory import Memory
 from exo.shared.types.multiaddr import Multiaddr
-from exo.shared.types.profiling import (
-    NetworkInterfaceInfo,
-    NodeNetworkInfo,
-    NodeRdmaCtlStatus,
-)
+from exo.shared.types.profiling import NetworkInterfaceInfo, NodeNetworkInfo
 from exo.shared.types.tasks import TaskId, TaskStatus, TextGeneration
 from exo.shared.types.text_generation import (
     InputMessage,
@@ -443,21 +439,8 @@ def test_tensor_rdma_backend_connectivity_matrix(
         min_nodes=1,
     )
 
-    node_rdma_ctl = {
-        node_a: NodeRdmaCtlStatus(enabled=True),
-        node_b: NodeRdmaCtlStatus(enabled=True),
-        node_c: NodeRdmaCtlStatus(enabled=True),
-    }
-
     # act
-    placements = place_instance(
-        cic,
-        topology,
-        {},
-        node_memory,
-        node_network,
-        node_rdma_ctl=node_rdma_ctl,
-    )
+    placements = place_instance(cic, topology, {}, node_memory, node_network)
 
     # assert
     assert len(placements) == 1
@@ -499,131 +482,6 @@ def test_tensor_rdma_backend_connectivity_matrix(
             assert len(ip_part.split(".")) == 4
 
 
-def _build_three_node_rdma_topology() -> tuple[
-    Topology, NodeId, NodeId, NodeId, dict[NodeId, NodeNetworkInfo]
-]:
-    topology = Topology()
-    node_a = NodeId()
-    node_b = NodeId()
-    node_c = NodeId()
-
-    ethernet_interface = NetworkInterfaceInfo(name="en0", ip_address="10.0.0.1")
-    ethernet_conn = SocketConnection(
-        sink_multiaddr=Multiaddr(address="/ip4/10.0.0.1/tcp/8000")
-    )
-    node_network = {
-        node_a: NodeNetworkInfo(interfaces=[ethernet_interface]),
-        node_b: NodeNetworkInfo(interfaces=[ethernet_interface]),
-        node_c: NodeNetworkInfo(interfaces=[ethernet_interface]),
-    }
-
-    for n in (node_a, node_b, node_c):
-        topology.add_node(n)
-
-    rdma_pairs = [
-        (node_a, node_b, 3),
-        (node_b, node_a, 3),
-        (node_b, node_c, 4),
-        (node_c, node_b, 4),
-        (node_a, node_c, 5),
-        (node_c, node_a, 5),
-    ]
-    for src, sink, iface in rdma_pairs:
-        topology.add_connection(
-            Connection(source=src, sink=sink, edge=create_rdma_connection(iface))
-        )
-
-    socket_pairs = [
-        (node_a, node_b),
-        (node_b, node_c),
-        (node_c, node_a),
-        (node_a, node_c),
-        (node_b, node_a),
-        (node_c, node_b),
-    ]
-    for src, sink in socket_pairs:
-        topology.add_connection(Connection(source=src, sink=sink, edge=ethernet_conn))
-
-    return topology, node_a, node_b, node_c, node_network
-
-
-def test_place_mlx_jaccl_rejects_when_a_node_has_rdma_ctl_disabled(
-    model_card: ModelCard,
-):
-    # arrange
-    model_card = model_card.model_copy(
-        update={"n_layers": 12, "storage_size": Memory.from_bytes(1500)}
-    )
-    topology, node_a, node_b, node_c, node_network = _build_three_node_rdma_topology()
-    node_memory = {
-        node_a: create_node_memory(500),
-        node_b: create_node_memory(500),
-        node_c: create_node_memory(500),
-    }
-    node_rdma_ctl = {
-        node_a: NodeRdmaCtlStatus(enabled=True),
-        node_b: NodeRdmaCtlStatus(enabled=True),
-        node_c: NodeRdmaCtlStatus(enabled=False),
-    }
-    cic = PlaceInstance(
-        sharding=Sharding.Tensor,
-        instance_meta=InstanceMeta.MlxJaccl,
-        command_id=CommandId(),
-        model_card=model_card,
-        min_nodes=3,
-    )
-
-    # act / assert
-    with pytest.raises(
-        ValueError, match="Requested RDMA \\(MlxJaccl\\) but no RDMA-connected cycles"
-    ):
-        place_instance(
-            cic,
-            topology,
-            {},
-            node_memory,
-            node_network,
-            node_rdma_ctl=node_rdma_ctl,
-        )
-
-
-def test_place_mlx_jaccl_rejects_when_node_rdma_ctl_missing(model_card: ModelCard):
-    """A node with no observed rdma_ctl status must not participate in RDMA placement."""
-    # arrange
-    model_card = model_card.model_copy(
-        update={"n_layers": 12, "storage_size": Memory.from_bytes(1500)}
-    )
-    topology, node_a, node_b, node_c, node_network = _build_three_node_rdma_topology()
-    node_memory = {
-        node_a: create_node_memory(500),
-        node_b: create_node_memory(500),
-        node_c: create_node_memory(500),
-    }
-    # node_c has no rdma_ctl entry at all
-    node_rdma_ctl = {
-        node_a: NodeRdmaCtlStatus(enabled=True),
-        node_b: NodeRdmaCtlStatus(enabled=True),
-    }
-    cic = PlaceInstance(
-        sharding=Sharding.Tensor,
-        instance_meta=InstanceMeta.MlxJaccl,
-        command_id=CommandId(),
-        model_card=model_card,
-        min_nodes=3,
-    )
-
-    # act / assert
-    with pytest.raises(ValueError):
-        place_instance(
-            cic,
-            topology,
-            {},
-            node_memory,
-            node_network,
-            node_rdma_ctl=node_rdma_ctl,
-        )
-
-
 def _make_task(
     instance_id: InstanceId,
     status: TaskStatus = TaskStatus.Running,

src/exo/routing/event_router.py

@@ -80,6 +80,9 @@ class EventRouter:
     def shutdown(self) -> None:
         self._tg.cancel_tasks()
 
+    def set_buffer_start(self, idx: int) -> None:
+        self.event_buffer.fast_forward_to(idx)
+
     async def _ingest(self, system_id: SystemId, recv: Receiver[Event]):
         idx = 0
         with recv as events:
@@ -95,7 +98,6 @@ class EventRouter:
                 self.out_for_delivery[event.event_id] = (anyio.current_time(), f_ev)
 
     async def _run_ext_in(self):
-        buf = OrderedBuffer[Event]()
         with self.external_inbound as events:
             async for event in events:
                 if event.session != self.session_id:
@@ -103,12 +105,12 @@ class EventRouter:
                 if event.origin != self.session_id.master_node_id:
                     continue
 
-                buf.ingest(event.origin_idx, event.event)
+                self.event_buffer.ingest(event.origin_idx, event.event)
                 event_id = event.event.event_id
                 if event_id in self.out_for_delivery:
                     self.out_for_delivery.pop(event_id)
 
-                drained = buf.drain_indexed()
+                drained = self.event_buffer.drain_indexed()
                 if drained:
                     self._nack_attempts = 0
                     if self._nack_cancel_scope:
@@ -119,7 +121,9 @@ class EventRouter:
                     or self._nack_cancel_scope.cancel_called
                 ):
                     # Request the next index.
-                    self._tg.start_soon(self._nack_request, buf.next_idx_to_release)
+                    self._tg.start_soon(
+                        self._nack_request, self.event_buffer.next_idx_to_release
+                    )
                     continue
 
                 for idx, event in drained:

src/exo/routing/snapshot_receiver.py

@@ -0,0 +1,109 @@
+"""Reassembles a snapshot from a stream of `SnapshotChunk`s.
+
+A receiver belongs to one node; it ignores chunks addressed to other
+requesters and chunks from prior sessions. Once a transfer's chunks have
+all been collected and the SHA-256 checks out, the snapshot is decoded into
+a `State`. Concurrent transfers (for the same requester) are tolerated:
+each is keyed by `transfer_id`.
+"""
+
+from __future__ import annotations
+
+import hashlib
+from dataclasses import dataclass, field
+from typing import final
+
+import zstandard
+from loguru import logger
+
+from exo.shared.types.common import NodeId, SessionId
+from exo.shared.types.snapshots import SnapshotChunk, SnapshotTransferId
+from exo.shared.types.state import State
+
+
+@final
+@dataclass
+class _Assembly:
+    """Partial state for one in-flight snapshot transfer."""
+
+    total_chunks: int
+    sha256_hex: str
+    schema_version: int
+    last_event_applied_idx: int
+    chunks: dict[int, bytes] = field(default_factory=dict)
+
+    def is_complete(self) -> bool:
+        return len(self.chunks) == self.total_chunks
+
+    def assemble(self) -> bytes:
+        return b"".join(self.chunks[i] for i in range(self.total_chunks))
+
+
+@dataclass
+class ReceivedSnapshot:
+    last_event_applied_idx: int
+    state: State
+
+
+class SnapshotReceiver:
+    """Filters and reassembles inbound chunks into a `ReceivedSnapshot`.
+
+    Stateless w.r.t. delivery: callers feed `SnapshotChunk`s in via `ingest`
+    and check the return value for completion.
+    """
+
+    def __init__(self, my_node_id: NodeId, session_id: SessionId) -> None:
+        self._my_node_id = my_node_id
+        self._session_id = session_id
+        self._assemblies: dict[SnapshotTransferId, _Assembly] = {}
+
+    def ingest(self, chunk: SnapshotChunk) -> ReceivedSnapshot | None:
+        """Absorb a chunk; return the snapshot once a transfer completes.
+
+        Returns None for partial transfers, mismatched recipients, stale
+        sessions, version mismatches, or corrupt payloads.
+        """
+        if chunk.requester_node_id != self._my_node_id:
+            return None
+        if chunk.session_id != self._session_id:
+            return None
+
+        existing = self._assemblies.get(chunk.transfer_id)
+        if existing is None:
+            existing = _Assembly(
+                total_chunks=chunk.total_chunks,
+                sha256_hex=chunk.sha256_hex,
+                schema_version=chunk.schema_version,
+                last_event_applied_idx=chunk.last_event_applied_idx,
+            )
+            self._assemblies[chunk.transfer_id] = existing
+        existing.chunks[chunk.chunk_index] = chunk.data
+
+        if not existing.is_complete():
+            return None
+
+        # Transfer complete — finalise and remove from the in-flight map.
+        del self._assemblies[chunk.transfer_id]
+        body = existing.assemble()
+        if hashlib.sha256(body).hexdigest() != existing.sha256_hex:
+            logger.warning(f"Snapshot {chunk.transfer_id} failed checksum; discarding")
+            return None
+        try:
+            decompressed = zstandard.ZstdDecompressor().decompress(body)
+            state = State.model_validate_json(decompressed.decode("utf-8"))
+        except (zstandard.ZstdError, ValueError) as e:
+            logger.opt(exception=e).warning(
+                f"Snapshot {chunk.transfer_id} could not be decoded; discarding"
+            )
+            return None
+        if state.schema_version != existing.schema_version:
+            # Should not happen — the master writes schema_version into both
+            # the chunk meta and the State payload — but treat it as corrupt.
+            logger.warning(
+                f"Snapshot {chunk.transfer_id} schema version mismatch "
+                f"(chunk={existing.schema_version}, state={state.schema_version})"
+            )
+            return None
+        return ReceivedSnapshot(
+            last_event_applied_idx=existing.last_event_applied_idx, state=state
+        )

src/exo/routing/tests/test_event_buffer.py

@@ -141,3 +141,28 @@ async def test_drain_and_ingest_with_new_sequence(buffer: OrderedBuffer[Event]):
     assert [e[0] for e in drained] == [2]
     assert buffer.next_idx_to_release == 3
     assert 4 in buffer.store
+
+
+@pytest.mark.asyncio
+async def test_fast_forward_discards_buffered_stale_events(
+    buffer: OrderedBuffer[Event],
+):
+    buffer.ingest(*make_indexed_event(0))
+    buffer.ingest(*make_indexed_event(2))
+    buffer.ingest(*make_indexed_event(4))
+
+    buffer.fast_forward_to(3)
+
+    assert buffer.next_idx_to_release == 3
+    assert set(buffer.store) == {4}
+
+
+@pytest.mark.asyncio
+async def test_fast_forward_only_moves_forward(buffer: OrderedBuffer[Event]):
+    buffer.ingest(*make_indexed_event(0))
+    buffer.ingest(*make_indexed_event(1))
+    buffer.drain()
+
+    buffer.fast_forward_to(1)
+
+    assert buffer.next_idx_to_release == 2

src/exo/routing/tests/test_snapshot_receiver.py

@@ -0,0 +1,151 @@
+import hashlib
+
+import pytest
+import zstandard
+
+from exo.routing.snapshot_receiver import SnapshotReceiver
+from exo.shared.types.common import NodeId, SessionId
+from exo.shared.types.snapshots import SnapshotChunk, SnapshotTransferId
+from exo.shared.types.state import State
+
+
+@pytest.fixture
+def session_id() -> SessionId:
+    return SessionId(master_node_id=NodeId("master"), election_clock=0)
+
+
+@pytest.fixture
+def my_node() -> NodeId:
+    return NodeId("worker-1")
+
+
+def _encode(state: State) -> bytes:
+    return zstandard.ZstdCompressor().compress(state.model_dump_json().encode("utf-8"))
+
+
+def _make_chunks(
+    body: bytes,
+    *,
+    chunk_size: int,
+    requester_node_id: NodeId,
+    session_id: SessionId,
+    state: State,
+    transfer_id: SnapshotTransferId | None = None,
+) -> list[SnapshotChunk]:
+    sha256 = hashlib.sha256(body).hexdigest()
+    transfer_id = transfer_id or SnapshotTransferId()
+    pieces = [body[i : i + chunk_size] for i in range(0, len(body), chunk_size)] or [
+        b""
+    ]
+    return [
+        SnapshotChunk.from_data(
+            data=piece,
+            transfer_id=transfer_id,
+            requester_node_id=requester_node_id,
+            session_id=session_id,
+            schema_version=state.schema_version,
+            last_event_applied_idx=state.last_event_applied_idx,
+            chunk_index=i,
+            total_chunks=len(pieces),
+            sha256_hex=sha256,
+        )
+        for i, piece in enumerate(pieces)
+    ]
+
+
+def test_completes_on_full_transfer(my_node: NodeId, session_id: SessionId):
+    state = State(last_event_applied_idx=42)
+    chunks = _make_chunks(
+        _encode(state),
+        chunk_size=64,
+        requester_node_id=my_node,
+        session_id=session_id,
+        state=state,
+    )
+    receiver = SnapshotReceiver(my_node, session_id)
+
+    received = None
+    for chunk in chunks:
+        received = receiver.ingest(chunk)
+    assert received is not None
+    assert received.last_event_applied_idx == 42
+    assert received.state.last_event_applied_idx == 42
+
+
+def test_handles_out_of_order_chunks(my_node: NodeId, session_id: SessionId):
+    state = State(last_event_applied_idx=99)
+    chunks = _make_chunks(
+        _encode(state),
+        chunk_size=32,
+        requester_node_id=my_node,
+        session_id=session_id,
+        state=state,
+    )
+    receiver = SnapshotReceiver(my_node, session_id)
+
+    # Reverse them.
+    received = None
+    for chunk in reversed(chunks):
+        received = receiver.ingest(chunk)
+    assert received is not None
+    assert received.last_event_applied_idx == 99
+
+
+def test_ignores_chunks_for_other_recipients(my_node: NodeId, session_id: SessionId):
+    state = State(last_event_applied_idx=1)
+    other = NodeId("worker-2")
+    chunks = _make_chunks(
+        _encode(state),
+        chunk_size=64,
+        requester_node_id=other,
+        session_id=session_id,
+        state=state,
+    )
+    receiver = SnapshotReceiver(my_node, session_id)
+    for chunk in chunks:
+        assert receiver.ingest(chunk) is None
+
+
+def test_ignores_chunks_from_stale_session(my_node: NodeId, session_id: SessionId):
+    state = State(last_event_applied_idx=1)
+    other_session = SessionId(master_node_id=NodeId("other-master"), election_clock=99)
+    chunks = _make_chunks(
+        _encode(state),
+        chunk_size=64,
+        requester_node_id=my_node,
+        session_id=other_session,
+        state=state,
+    )
+    receiver = SnapshotReceiver(my_node, session_id)
+    for chunk in chunks:
+        assert receiver.ingest(chunk) is None
+
+
+def test_discards_on_checksum_mismatch(my_node: NodeId, session_id: SessionId):
+    state = State(last_event_applied_idx=1)
+    chunks = _make_chunks(
+        _encode(state),
+        chunk_size=64,
+        requester_node_id=my_node,
+        session_id=session_id,
+        state=state,
+    )
+    # Corrupt the last byte of the last chunk.
+    original = chunks[-1]
+    chunks[-1] = SnapshotChunk.from_data(
+        data=original.data + b"\x00garbage",
+        transfer_id=original.transfer_id,
+        requester_node_id=original.requester_node_id,
+        session_id=original.session_id,
+        schema_version=original.schema_version,
+        last_event_applied_idx=original.last_event_applied_idx,
+        chunk_index=original.chunk_index,
+        total_chunks=original.total_chunks,
+        sha256_hex=original.sha256_hex,
+    )
+
+    receiver = SnapshotReceiver(my_node, session_id)
+    received = None
+    for chunk in chunks:
+        received = receiver.ingest(chunk)
+    assert received is None

src/exo/routing/tests/test_snapshot_routing_types.py

@@ -0,0 +1,37 @@
+from exo.routing import topics
+from exo.shared.types.commands import ForwarderCommand, RequestSnapshot
+from exo.shared.types.common import NodeId, SessionId, SystemId
+from exo.shared.types.snapshots import SnapshotChunk, SnapshotTransferId
+
+
+def test_request_snapshot_round_trips_through_forwarder_command() -> None:
+    command = ForwarderCommand(
+        origin=SystemId("system-1"),
+        command=RequestSnapshot(requester_node_id=NodeId("worker-1")),
+    )
+
+    restored = ForwarderCommand.model_validate_json(command.model_dump_json())
+
+    assert isinstance(restored.command, RequestSnapshot)
+    assert restored.command.requester_node_id == NodeId("worker-1")
+
+
+def test_snapshot_response_topic_round_trips_chunk() -> None:
+    chunk = SnapshotChunk.from_data(
+        data=b"snapshot-bytes",
+        transfer_id=SnapshotTransferId("transfer-1"),
+        requester_node_id=NodeId("worker-1"),
+        session_id=SessionId(master_node_id=NodeId("master"), election_clock=1),
+        schema_version=1,
+        last_event_applied_idx=42,
+        chunk_index=0,
+        total_chunks=1,
+        sha256_hex="unused",
+    )
+
+    restored = topics.SNAPSHOT_RESPONSES.deserialize(
+        topics.SNAPSHOT_RESPONSES.serialize(chunk)
+    )
+
+    assert restored == chunk
+    assert restored.data == b"snapshot-bytes"

src/exo/routing/tests/test_transient_router.py

@@ -0,0 +1,69 @@
+import anyio
+import pytest
+
+from exo.routing import topics
+from exo.routing.transient_router import TransientRouter
+from exo.shared.types.common import NodeId, SessionId
+from exo.shared.types.events import (
+    GlobalForwarderTransientEvent,
+    TaskAcknowledged,
+)
+from exo.shared.types.tasks import TaskId
+from exo.utils.channels import channel
+
+
+def test_transient_topic_round_trips_wrapped_event() -> None:
+    wrapped = GlobalForwarderTransientEvent(
+        origin=NodeId("worker"),
+        session=SessionId(master_node_id=NodeId("master"), election_clock=1),
+        event=TaskAcknowledged(task_id=TaskId("task-1")),
+    )
+
+    restored = topics.TRANSIENT_EVENTS.deserialize(
+        topics.TRANSIENT_EVENTS.serialize(wrapped)
+    )
+
+    assert restored == wrapped
+
+
+@pytest.mark.asyncio
+async def test_transient_router_publishes_and_dispatches_session_events() -> None:
+    node_id = NodeId("worker")
+    session_id = SessionId(master_node_id=NodeId("master"), election_clock=1)
+    external_outbound_sender, external_outbound_receiver = channel[
+        GlobalForwarderTransientEvent
+    ]()
+    external_inbound_sender, external_inbound_receiver = channel[
+        GlobalForwarderTransientEvent
+    ]()
+    router = TransientRouter(
+        node_id=node_id,
+        session_id=session_id,
+        external_outbound=external_outbound_sender,
+        external_inbound=external_inbound_receiver,
+    )
+    local_sender = router.sender()
+    local_receiver = router.receiver()
+    event = TaskAcknowledged(task_id=TaskId("task-1"))
+
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(router.run)
+        await local_sender.send(event)
+
+        wrapped = await external_outbound_receiver.receive()
+        assert wrapped.origin == node_id
+        assert wrapped.session == session_id
+        assert wrapped.event == event
+
+        await external_inbound_sender.send(wrapped)
+        assert await local_receiver.receive() == event
+
+        stale_session = SessionId(master_node_id=NodeId("master"), election_clock=2)
+        await external_inbound_sender.send(
+            wrapped.model_copy(update={"session": stale_session})
+        )
+        await anyio.sleep(0)
+        assert local_receiver.collect() == []
+
+        router.shutdown()
+        tg.cancel_scope.cancel()

src/exo/routing/topics.py

@@ -6,8 +6,10 @@ from exo.shared.election import ElectionMessage
 from exo.shared.types.commands import ForwarderCommand, ForwarderDownloadCommand
 from exo.shared.types.events import (
     GlobalForwarderEvent,
+    GlobalForwarderTransientEvent,
     LocalForwarderEvent,
 )
+from exo.shared.types.snapshots import SnapshotChunk
 from exo.utils.pydantic_ext import FrozenModel
 
 
@@ -39,6 +41,9 @@ class TypedTopic[T: FrozenModel]:
 
 GLOBAL_EVENTS = TypedTopic("global_events", PublishPolicy.Always, GlobalForwarderEvent)
 LOCAL_EVENTS = TypedTopic("local_events", PublishPolicy.Always, LocalForwarderEvent)
+TRANSIENT_EVENTS = TypedTopic(
+    "transient_events", PublishPolicy.Always, GlobalForwarderTransientEvent
+)
 COMMANDS = TypedTopic("commands", PublishPolicy.Always, ForwarderCommand)
 ELECTION_MESSAGES = TypedTopic(
     "election_messages", PublishPolicy.Always, ElectionMessage
@@ -49,3 +54,6 @@ CONNECTION_MESSAGES = TypedTopic(
 DOWNLOAD_COMMANDS = TypedTopic(
     "download_commands", PublishPolicy.Always, ForwarderDownloadCommand
 )
+SNAPSHOT_RESPONSES = TypedTopic(
+    "snapshot_responses", PublishPolicy.Always, SnapshotChunk
+)

src/exo/routing/transient_router.py

@@ -0,0 +1,84 @@
+from dataclasses import dataclass, field
+
+from anyio import BrokenResourceError, ClosedResourceError
+from loguru import logger
+
+from exo.shared.types.common import NodeId, SessionId
+from exo.shared.types.events import (
+    GlobalForwarderTransientEvent,
+    TransientEvent,
+)
+from exo.utils.channels import Receiver, Sender, channel
+from exo.utils.task_group import TaskGroup
+
+
+@dataclass
+class TransientRouter:
+    """Routes unordered, non-durable events over the transient-events topic."""
+
+    node_id: NodeId
+    session_id: SessionId
+    external_outbound: Sender[GlobalForwarderTransientEvent]
+    external_inbound: Receiver[GlobalForwarderTransientEvent]
+    _outbound: list[Sender[TransientEvent]] = field(init=False, default_factory=list)
+    _inbound: list[Receiver[TransientEvent]] = field(init=False, default_factory=list)
+    _tg: TaskGroup = field(init=False, default_factory=TaskGroup)
+
+    def sender(self) -> Sender[TransientEvent]:
+        send, recv = channel[TransientEvent]()
+        if self._tg.is_running():
+            self._tg.start_soon(self._publish, recv)
+        else:
+            self._inbound.append(recv)
+        return send
+
+    def receiver(self) -> Receiver[TransientEvent]:
+        assert not self._tg.is_running(), (
+            "TransientRouter receivers must be registered before run()"
+        )
+        send, recv = channel[TransientEvent]()
+        self._outbound.append(send)
+        return recv
+
+    def shutdown(self) -> None:
+        self._tg.cancel_tasks()
+
+    async def run(self) -> None:
+        try:
+            async with self._tg as tg:
+                for recv in self._inbound:
+                    tg.start_soon(self._publish, recv)
+                tg.start_soon(self._dispatch_inbound)
+        finally:
+            self.external_outbound.close()
+            for send in self._outbound:
+                send.close()
+
+    async def _publish(self, recv: Receiver[TransientEvent]) -> None:
+        with recv as events:
+            async for event in events:
+                await self.external_outbound.send(
+                    GlobalForwarderTransientEvent(
+                        origin=self.node_id,
+                        session=self.session_id,
+                        event=event,
+                    )
+                )
+
+    async def _dispatch_inbound(self) -> None:
+        with self.external_inbound as wrapped_events:
+            async for wrapped in wrapped_events:
+                if wrapped.session != self.session_id:
+                    continue
+                stale: set[int] = set()
+                for index, send in enumerate(self._outbound):
+                    try:
+                        await send.send(wrapped.event)
+                    except (ClosedResourceError, BrokenResourceError):
+                        stale.add(index)
+                if stale:
+                    for index in sorted(stale, reverse=True):
+                        self._outbound.pop(index)
+                    logger.debug(
+                        f"TransientRouter dropped {len(stale)} closed receivers"
+                    )

src/exo/shared/apply.py

@@ -41,7 +41,14 @@ from exo.shared.types.profiling import (
     ThunderboltBridgeStatus,
 )
 from exo.shared.types.state import State
-from exo.shared.types.tasks import Task, TaskId, TaskStatus
+from exo.shared.types.tasks import (
+    ImageEdits,
+    ImageGeneration,
+    Task,
+    TaskId,
+    TaskStatus,
+    TextGeneration,
+)
 from exo.shared.types.topology import Connection, RDMAConnection
 from exo.shared.types.worker.downloads import DownloadProgress
 from exo.shared.types.worker.instances import Instance, InstanceId
@@ -66,18 +73,6 @@ from exo.utils.info_gatherer.info_gatherer import (
 )
 
 
-def _is_rdma_ctl_enabled(
-    node_id: NodeId, node_rdma_ctl: Mapping[NodeId, NodeRdmaCtlStatus]
-) -> bool:
-    """A node is RDMA-capable only if rdma_ctl status has been observed as enabled.
-
-    Missing entries default to ``False`` — if we have not yet observed (or the node
-    cannot run) ``rdma_ctl``, it must not participate in an RDMA-backed instance.
-    """
-    status = node_rdma_ctl.get(node_id)
-    return status is not None and status.enabled
-
-
 def event_apply(event: Event, state: State) -> State:
     """Apply an event to state."""
     match event:
@@ -85,7 +80,6 @@ def event_apply(event: Event, state: State) -> State:
             TestEvent()
             | ChunkGenerated()
             | TaskAcknowledged()
-            | InputChunkReceived()
             | TracesCollected()
             | TracesMerged()
         ):  # Pass-through events that don't modify state
@@ -108,6 +102,8 @@ def event_apply(event: Event, state: State) -> State:
             return apply_runner_status_updated(event, state)
         case TaskCreated():
             return apply_task_created(event, state)
+        case InputChunkReceived():
+            return apply_input_chunk_received(event, state)
         case TaskDeleted():
             return apply_task_deleted(event, state)
         case TaskFailed():
@@ -172,10 +168,32 @@ def apply_task_created(event: TaskCreated, state: State) -> State:
     return state.model_copy(update={"tasks": new_tasks})
 
 
+def apply_input_chunk_received(event: InputChunkReceived, state: State) -> State:
+    command_chunks = {
+        **state.input_chunks.get(event.command_id, {}),
+        event.chunk.chunk_index: event.chunk,
+    }
+    return state.model_copy(
+        update={
+            "input_chunks": {**state.input_chunks, event.command_id: command_chunks}
+        }
+    )
+
+
 def apply_task_deleted(event: TaskDeleted, state: State) -> State:
+    task = state.tasks.get(event.task_id)
     new_tasks: Mapping[TaskId, Task] = {
         tid: task for tid, task in state.tasks.items() if tid != event.task_id
     }
+    if isinstance(task, (TextGeneration, ImageGeneration, ImageEdits)):
+        new_input_chunks = {
+            command_id: chunks
+            for command_id, chunks in state.input_chunks.items()
+            if command_id != task.command_id
+        }
+        return state.model_copy(
+            update={"tasks": new_tasks, "input_chunks": new_input_chunks}
+        )
     return state.model_copy(update={"tasks": new_tasks})
 
 
@@ -412,9 +430,6 @@ def apply_node_gathered_info(event: NodeGatheredInfo, state: State) -> State:
                 for nid in state.node_thunderbolt
                 for tb_ident in state.node_thunderbolt[nid].interfaces
             }
-            source_is_rdma_enabled = _is_rdma_ctl_enabled(
-                event.node_id, state.node_rdma_ctl
-            )
             as_rdma_conns = [
                 Connection(
                     source=event.node_id,
@@ -427,10 +442,6 @@ def apply_node_gathered_info(event: NodeGatheredInfo, state: State) -> State:
                 for tb_conn in info.conns
                 if tb_conn.source_uuid in conn_map
                 if tb_conn.sink_uuid in conn_map
-                if source_is_rdma_enabled
-                and _is_rdma_ctl_enabled(
-                    conn_map[tb_conn.sink_uuid][0], state.node_rdma_ctl
-                )
             ]
             topology.replace_all_out_rdma_connections(event.node_id, as_rdma_conns)
         case ThunderboltBridgeInfo():
@@ -454,12 +465,6 @@ def apply_node_gathered_info(event: NodeGatheredInfo, state: State) -> State:
                 **state.node_rdma_ctl,
                 event.node_id: NodeRdmaCtlStatus(enabled=info.enabled),
             }
-            # If RDMA just got disabled on this node, drop any RDMA edges touching it
-            # so placement / topology consumers cannot pick a disabled node for an
-            # RDMA-backed instance. (Edges will repopulate on the next
-            # MacThunderboltConnections poll once both endpoints are enabled again.)
-            if not info.enabled:
-                topology.remove_all_rdma_connections_touching(event.node_id)
 
     return state.model_copy(update=update)
 

src/exo/shared/constants.py

@@ -4,13 +4,13 @@ from pathlib import Path
 
 from exo.utils.dashboard_path import find_dashboard, find_resources
 
-_EXO_HOME_ENV = os.environ.get("EXO_HOME", "")
+_EXO_HOME_ENV = os.environ.get("EXO_HOME", None)
 
 
 def _get_xdg_dir(env_var: str, fallback: str) -> Path:
     """Get XDG directory, prioritising EXO_HOME environment variable if its set. On non-Linux platforms, default to ~/.exo."""
 
-    if _EXO_HOME_ENV != "":
+    if _EXO_HOME_ENV is not None:
         return Path.home() / _EXO_HOME_ENV
 
     if sys.platform != "linux":

src/exo/shared/models/model_cards.py

@@ -39,57 +39,7 @@ _BUILTIN_CARD_DIRS = [
     Path(RESOURCES_DIR) / "image_model_cards",
 ]
 
-
-class _CardCache:
-    def __init__(self):
-        self.cc: dict[ModelId, "ModelCard"] = {}
-
-    def get(self, model_id: ModelId) -> "ModelCard | None":
-        return self.cc.get(model_id)
-
-    async def save(self, card: "ModelCard"):
-        self.cc[card.model_id] = card
-        try:
-            await card.save_to_custom_dir()
-        except OSError as e:
-            logger.warning(f"failed to save custom model card ({e.strerror})")
-
-    async def pop(self, model_id: ModelId) -> "ModelCard | None":
-        """Delete a user-added custom model card. Returns True if deleted."""
-        card_path = _custom_cards_dir / (ModelId(model_id).normalize() + ".toml")
-        try:
-            if await card_path.exists():
-                await card_path.unlink()
-                return self.cc.pop(model_id, None)
-        except OSError as e:
-            logger.warning(f"failed to delete custom model card ({e.strerror})")
-
-    async def list_all(self) -> list["ModelCard"]:
-        if len(self.cc) == 0:
-            await self.refresh()
-        if EXO_ENABLE_IMAGE_MODELS:
-            return list(self.cc.values())
-        return [c for c in self.cc.values() if not _is_image_card(c)]
-
-    async def _load_cards_from_dir(self, directory: Path, *, is_custom: bool) -> None:
-        """Load all TOML model cards from a directory into the cache."""
-        async for toml_file in directory.rglob("*.toml"):
-            try:
-                card = await ModelCard.load_from_path(toml_file)
-                if is_custom:
-                    card = card.model_copy(update={"is_custom": True})
-                if self.get(card.model_id) is None:
-                    self.cc[card.model_id] = card
-            except (ValidationError, TOMLKitError):
-                pass
-
-    async def refresh(self) -> None:
-        for path in _BUILTIN_CARD_DIRS:
-            await self._load_cards_from_dir(path, is_custom=False)
-        await self._load_cards_from_dir(_custom_cards_dir, is_custom=True)
-
-
-card_cache = _CardCache()
+_card_cache: dict[ModelId, "ModelCard"] = {}
 
 
 def detect_vision_from_config(model_id: ModelId) -> "VisionCardConfig | None":
@@ -109,10 +59,42 @@ def detect_vision_from_config(model_id: ModelId) -> "VisionCardConfig | None":
     return None
 
 
+async def _load_cards_from_dir(directory: Path, *, is_custom: bool) -> None:
+    """Load all TOML model cards from a directory into the cache."""
+    async for toml_file in directory.rglob("*.toml"):
+        try:
+            card = await ModelCard.load_from_path(toml_file)
+            if is_custom:
+                card = card.model_copy(update={"is_custom": True})
+            if card.model_id not in _card_cache:
+                _card_cache[card.model_id] = card
+        except (ValidationError, TOMLKitError):
+            pass
+
+
+async def _refresh_card_cache() -> None:
+    for path in _BUILTIN_CARD_DIRS:
+        await _load_cards_from_dir(path, is_custom=False)
+    await _load_cards_from_dir(_custom_cards_dir, is_custom=True)
+
+
 def _is_image_card(card: "ModelCard") -> bool:
     return any(t in (ModelTask.TextToImage, ModelTask.ImageToImage) for t in card.tasks)
 
 
+def get_card(model_id: ModelId) -> "ModelCard | None":
+    """Look up a single model card from the cache by ID."""
+    return _card_cache.get(model_id)
+
+
+async def get_model_cards() -> list["ModelCard"]:
+    if len(_card_cache) == 0:
+        await _refresh_card_cache()
+    if EXO_ENABLE_IMAGE_MODELS:
+        return list(_card_cache.values())
+    return [c for c in _card_cache.values() if not _is_image_card(c)]
+
+
 class ModelTask(str, Enum):
     TextGeneration = "TextGeneration"
     TextToImage = "TextToImage"
@@ -214,13 +196,14 @@ class ModelCard(FrozenModel):
     # Is it okay that model card.load defaults to network access if the card doesn't exist? do we want to be more explicit here?
     @staticmethod
     async def load(model_id: ModelId) -> "ModelCard":
-        if card_cache.get(model_id) is None:
-            await card_cache.refresh()
-        if (mc := card_cache.get(model_id)) is not None:
+        if model_id not in _card_cache:
+            await _refresh_card_cache()
+        if (mc := _card_cache.get(model_id)) is not None:
             return mc
 
         mc = await ModelCard.fetch_from_hf(model_id)
         await mc.save_to_custom_dir()
+        _card_cache[model_id] = mc
         return mc
 
     @staticmethod
@@ -250,6 +233,21 @@ class ModelCard(FrozenModel):
         )
 
 
+def add_to_card_cache(card: "ModelCard") -> None:
+    """Add or update a model card in the in-memory cache."""
+    _card_cache[card.model_id] = card
+
+
+async def delete_custom_card(model_id: ModelId) -> bool:
+    """Delete a user-added custom model card. Returns True if deleted."""
+    card_path = _custom_cards_dir / (ModelId(model_id).normalize() + ".toml")
+    if await card_path.exists():
+        await card_path.unlink()
+        _card_cache.pop(model_id, None)
+        return True
+    return False
+
+
 class ConfigData(BaseModel):
     model_config = {"extra": "ignore"}  # Allow unknown fields
 

src/exo/shared/tests/test_apply/test_apply_input_chunks.py

@@ -0,0 +1,85 @@
+from exo.shared.apply import apply
+from exo.shared.models.model_cards import ModelId
+from exo.shared.types.chunks import InputImageChunk
+from exo.shared.types.common import CommandId
+from exo.shared.types.events import (
+    IndexedEvent,
+    InputChunkReceived,
+    TaskCreated,
+    TaskDeleted,
+)
+from exo.shared.types.state import State
+from exo.shared.types.tasks import TaskId, TaskStatus, TextGeneration
+from exo.shared.types.text_generation import (
+    InputMessage,
+    InputMessageContent,
+    TextGenerationTaskParams,
+)
+from exo.shared.types.worker.instances import InstanceId
+
+
+def test_apply_input_chunk_received_stores_chunk_in_state() -> None:
+    command_id = CommandId("command-1")
+    chunk = InputImageChunk(
+        model=ModelId("mlx-community/test-model"),
+        command_id=command_id,
+        data="abc",
+        chunk_index=0,
+        total_chunks=1,
+        image_index=0,
+    )
+
+    state = apply(
+        State(),
+        IndexedEvent(
+            idx=0,
+            event=InputChunkReceived(command_id=command_id, chunk=chunk),
+        ),
+    )
+
+    assert state.input_chunks == {command_id: {0: chunk}}
+
+
+def test_apply_task_deleted_removes_chunks_for_generation_command() -> None:
+    command_id = CommandId("command-1")
+    task_id = TaskId("task-1")
+    chunk = InputImageChunk(
+        model=ModelId("mlx-community/test-model"),
+        command_id=command_id,
+        data="abc",
+        chunk_index=0,
+        total_chunks=1,
+        image_index=0,
+    )
+    task = TextGeneration(
+        task_id=task_id,
+        instance_id=InstanceId("instance-1"),
+        task_status=TaskStatus.Pending,
+        command_id=command_id,
+        task_params=TextGenerationTaskParams(
+            model=ModelId("mlx-community/test-model"),
+            input=[
+                InputMessage(role="user", content=InputMessageContent("hello")),
+            ],
+        ),
+    )
+
+    state = State()
+    state = apply(
+        state,
+        IndexedEvent(
+            idx=0,
+            event=InputChunkReceived(command_id=command_id, chunk=chunk),
+        ),
+    )
+    state = apply(
+        state,
+        IndexedEvent(idx=1, event=TaskCreated(task_id=task_id, task=task)),
+    )
+    state = apply(
+        state,
+        IndexedEvent(idx=2, event=TaskDeleted(task_id=task_id)),
+    )
+
+    assert state.tasks == {}
+    assert state.input_chunks == {}

src/exo/shared/tests/test_apply/test_apply_rdma_gating.py

@@ -1,231 +0,0 @@
-from datetime import datetime, timezone
-
-from exo.shared.apply import apply_node_gathered_info
-from exo.shared.topology import Topology
-from exo.shared.types.common import NodeId
-from exo.shared.types.events import NodeGatheredInfo
-from exo.shared.types.profiling import (
-    NodeRdmaCtlStatus,
-    NodeThunderboltInfo,
-)
-from exo.shared.types.state import State
-from exo.shared.types.thunderbolt import ThunderboltConnection, ThunderboltIdentifier
-from exo.shared.types.topology import RDMAConnection
-from exo.utils.info_gatherer.info_gatherer import (
-    MacThunderboltConnections,
-    RdmaCtlStatus,
-)
-
-
-def _now() -> str:
-    return datetime.now(timezone.utc).isoformat()
-
-
-def _make_state_with_thunderbolt_idents(
-    *node_ids_and_uuids: tuple[NodeId, str, str],
-    rdma_ctl: dict[NodeId, NodeRdmaCtlStatus] | None = None,
-) -> State:
-    """Build a State with Thunderbolt identifiers per node so the apply MacThunderboltConnections
-    case can resolve uuid -> (node, iface)."""
-    node_thunderbolt = {
-        nid: NodeThunderboltInfo(
-            interfaces=[ThunderboltIdentifier(rdma_interface=iface, domain_uuid=uuid)]
-        )
-        for nid, uuid, iface in node_ids_and_uuids
-    }
-    return State(
-        node_thunderbolt=node_thunderbolt,
-        node_rdma_ctl=rdma_ctl or {},
-    )
-
-
-def _has_rdma_edge(topology: Topology, source: NodeId, sink: NodeId) -> bool:
-    return any(
-        isinstance(edge, RDMAConnection)
-        for edge in topology.get_all_connections_between(source, sink)
-    )
-
-
-def test_mac_thunderbolt_connections_emits_rdma_when_both_endpoints_enabled():
-    node_a = NodeId()
-    node_b = NodeId()
-    state = _make_state_with_thunderbolt_idents(
-        (node_a, "uuid-a", "rdma_en1"),
-        (node_b, "uuid-b", "rdma_en1"),
-        rdma_ctl={
-            node_a: NodeRdmaCtlStatus(enabled=True),
-            node_b: NodeRdmaCtlStatus(enabled=True),
-        },
-    )
-
-    event = NodeGatheredInfo(
-        node_id=node_a,
-        when=_now(),
-        info=MacThunderboltConnections(
-            conns=[ThunderboltConnection(source_uuid="uuid-a", sink_uuid="uuid-b")]
-        ),
-    )
-
-    new_state = apply_node_gathered_info(event, state)
-
-    assert _has_rdma_edge(new_state.topology, node_a, node_b)
-
-
-def test_mac_thunderbolt_connections_skips_rdma_when_source_rdma_ctl_disabled():
-    node_a = NodeId()
-    node_b = NodeId()
-    state = _make_state_with_thunderbolt_idents(
-        (node_a, "uuid-a", "rdma_en1"),
-        (node_b, "uuid-b", "rdma_en1"),
-        rdma_ctl={
-            node_a: NodeRdmaCtlStatus(enabled=False),
-            node_b: NodeRdmaCtlStatus(enabled=True),
-        },
-    )
-
-    event = NodeGatheredInfo(
-        node_id=node_a,
-        when=_now(),
-        info=MacThunderboltConnections(
-            conns=[ThunderboltConnection(source_uuid="uuid-a", sink_uuid="uuid-b")]
-        ),
-    )
-
-    new_state = apply_node_gathered_info(event, state)
-
-    assert not _has_rdma_edge(new_state.topology, node_a, node_b)
-
-
-def test_mac_thunderbolt_connections_skips_rdma_when_sink_rdma_ctl_disabled():
-    node_a = NodeId()
-    node_b = NodeId()
-    state = _make_state_with_thunderbolt_idents(
-        (node_a, "uuid-a", "rdma_en1"),
-        (node_b, "uuid-b", "rdma_en1"),
-        rdma_ctl={
-            node_a: NodeRdmaCtlStatus(enabled=True),
-            node_b: NodeRdmaCtlStatus(enabled=False),
-        },
-    )
-
-    event = NodeGatheredInfo(
-        node_id=node_a,
-        when=_now(),
-        info=MacThunderboltConnections(
-            conns=[ThunderboltConnection(source_uuid="uuid-a", sink_uuid="uuid-b")]
-        ),
-    )
-
-    new_state = apply_node_gathered_info(event, state)
-
-    assert not _has_rdma_edge(new_state.topology, node_a, node_b)
-
-
-def test_mac_thunderbolt_connections_skips_rdma_when_rdma_ctl_status_missing():
-    """Missing rdma_ctl status defaults to not-enabled — node is RDMA-incapable."""
-    node_a = NodeId()
-    node_b = NodeId()
-    state = _make_state_with_thunderbolt_idents(
-        (node_a, "uuid-a", "rdma_en1"),
-        (node_b, "uuid-b", "rdma_en1"),
-        rdma_ctl={
-            node_a: NodeRdmaCtlStatus(enabled=True),
-            # node_b intentionally absent
-        },
-    )
-
-    event = NodeGatheredInfo(
-        node_id=node_a,
-        when=_now(),
-        info=MacThunderboltConnections(
-            conns=[ThunderboltConnection(source_uuid="uuid-a", sink_uuid="uuid-b")]
-        ),
-    )
-
-    new_state = apply_node_gathered_info(event, state)
-
-    assert not _has_rdma_edge(new_state.topology, node_a, node_b)
-
-
-def test_rdma_ctl_status_disabled_purges_existing_rdma_edges():
-    """When a node reports rdma_ctl disabled, all RDMA edges touching it must be removed."""
-    node_a = NodeId()
-    node_b = NodeId()
-
-    # Start with both nodes RDMA-enabled and existing RDMA edges in the topology.
-    state = _make_state_with_thunderbolt_idents(
-        (node_a, "uuid-a", "rdma_en1"),
-        (node_b, "uuid-b", "rdma_en1"),
-        rdma_ctl={
-            node_a: NodeRdmaCtlStatus(enabled=True),
-            node_b: NodeRdmaCtlStatus(enabled=True),
-        },
-    )
-    state = apply_node_gathered_info(
-        NodeGatheredInfo(
-            node_id=node_a,
-            when=_now(),
-            info=MacThunderboltConnections(
-                conns=[ThunderboltConnection(source_uuid="uuid-a", sink_uuid="uuid-b")]
-            ),
-        ),
-        state,
-    )
-    state = apply_node_gathered_info(
-        NodeGatheredInfo(
-            node_id=node_b,
-            when=_now(),
-            info=MacThunderboltConnections(
-                conns=[ThunderboltConnection(source_uuid="uuid-b", sink_uuid="uuid-a")]
-            ),
-        ),
-        state,
-    )
-    assert _has_rdma_edge(state.topology, node_a, node_b)
-    assert _has_rdma_edge(state.topology, node_b, node_a)
-
-    # Now node_a flips to rdma_ctl disabled — both directions of RDMA edge must drop.
-    state = apply_node_gathered_info(
-        NodeGatheredInfo(
-            node_id=node_a, when=_now(), info=RdmaCtlStatus(enabled=False)
-        ),
-        state,
-    )
-
-    assert not _has_rdma_edge(state.topology, node_a, node_b)
-    assert not _has_rdma_edge(state.topology, node_b, node_a)
-    assert state.node_rdma_ctl[node_a].enabled is False
-
-
-def test_topology_remove_all_rdma_connections_touching_keeps_socket_edges():
-    """Purging RDMA edges for a disabled node must not affect non-RDMA edges."""
-    from exo.shared.types.multiaddr import Multiaddr
-    from exo.shared.types.topology import Connection, SocketConnection
-
-    topology = Topology()
-    node_a = NodeId()
-    node_b = NodeId()
-    topology.add_node(node_a)
-    topology.add_node(node_b)
-    topology.add_connection(
-        Connection(
-            source=node_a,
-            sink=node_b,
-            edge=RDMAConnection(
-                source_rdma_iface="rdma_en1", sink_rdma_iface="rdma_en1"
-            ),
-        )
-    )
-    socket_edge = SocketConnection(
-        sink_multiaddr=Multiaddr(address="/ip4/10.0.0.1/tcp/8000")
-    )
-    topology.add_connection(Connection(source=node_a, sink=node_b, edge=socket_edge))
-
-    topology.remove_all_rdma_connections_touching(node_a)
-
-    assert not _has_rdma_edge(topology, node_a, node_b)
-    # Socket edge survives.
-    assert any(
-        isinstance(edge, SocketConnection)
-        for edge in topology.get_all_connections_between(node_a, node_b)
-    )

src/exo/shared/tests/test_state_serialization.py

@@ -25,6 +25,7 @@ def test_state_serialization_roundtrip() -> None:
     json_repr = state.model_dump_json()
     restored_state = State.model_validate_json(json_repr)
 
+    assert restored_state.schema_version == state.schema_version
     assert (
         state.topology.to_snapshot().nodes
         == restored_state.topology.to_snapshot().nodes

src/exo/shared/topology.py

@@ -169,22 +169,6 @@ class Topology:
         for conn in new_connections:
             self.add_connection(conn)
 
-    def remove_all_rdma_connections_touching(self, node_id: NodeId) -> None:
-        """Remove every RDMA edge incident to ``node_id`` (incoming or outgoing)."""
-        if node_id not in self._vertex_indices:
-            return
-        rx_idx = self._vertex_indices[node_id]
-        rdma_edge_idxs = [
-            edge_idx
-            for edge_idx in (
-                *self._graph.out_edge_indices(rx_idx),
-                *self._graph.in_edge_indices(rx_idx),
-            )
-            if isinstance(self._graph.get_edge_data_by_index(edge_idx), RDMAConnection)
-        ]
-        for edge_idx in rdma_edge_idxs:
-            self._graph.remove_edge_from_index(edge_idx)
-
     def remove_connection(self, conn: Connection) -> None:
         if (
             conn.source not in self._vertex_indices

src/exo/shared/types/commands.py

@@ -67,6 +67,12 @@ class RequestEventLog(BaseCommand):
     since_idx: int
 
 
+class RequestSnapshot(BaseCommand):
+    """Ask the current master to send a State snapshot to this node."""
+
+    requester_node_id: NodeId
+
+
 class StartDownload(BaseCommand):
     target_node_id: NodeId
     shard_metadata: ShardMetadata
@@ -106,6 +112,7 @@ DownloadCommand = StartDownload | DeleteDownload | CancelDownload
 Command = (
     TestCommand
     | RequestEventLog
+    | RequestSnapshot
     | TextGeneration
     | ImageGeneration
     | ImageEdits

src/exo/shared/types/events.py

@@ -172,6 +172,9 @@ Event = (
 )
 
 
+TransientEvent = TaskAcknowledged | ChunkGenerated | TracesCollected | TracesMerged
+
+
 class IndexedEvent(FrozenModel):
     """An event indexed by the master, with a globally unique index"""
 
@@ -195,3 +198,11 @@ class LocalForwarderEvent(FrozenModel):
     origin: SystemId
     session: SessionId
     event: Event
+
+
+class GlobalForwarderTransientEvent(FrozenModel):
+    """An unordered, non-durable event published to the cluster."""
+
+    origin: NodeId
+    session: SessionId
+    event: TransientEvent

src/exo/shared/types/snapshots.py

@@ -0,0 +1,54 @@
+"""Wire types for snapshot transfer between master and a joining node.
+
+Snapshots can be tens of MB; the gossipsub message ceiling is around 1 MB.
+We slice the compressed snapshot body into chunks and publish each chunk on
+the SNAPSHOT_RESPONSES topic. The receiver collects chunks for its own
+`requester_node_id`, validates the SHA-256 of the reassembled body, and
+materialises the State.
+"""
+
+import base64
+
+from exo.shared.types.common import Id, NodeId, SessionId
+from exo.utils.pydantic_ext import FrozenModel
+
+
+class SnapshotTransferId(Id):
+    """Identifies a single snapshot transfer (one master response to one
+    `RequestSnapshot`). Distinct transfers may interleave; the id lets
+    receivers keep them apart."""
+
+
+class SnapshotChunk(FrozenModel):
+    """One slice of a snapshot in flight.
+
+    `data_b64` carries a base64-encoded slice of the zstd-compressed JSON
+    dump of State. Concatenating the *decoded* bytes of all chunks for a
+    `transfer_id` in order of `chunk_index` yields the full compressed
+    body; `sha256_hex` is the SHA-256 of that decoded blob.
+
+    We use base64 explicitly because the topic layer JSON-encodes messages,
+    and JSON can't carry raw binary. Helpers `from_data` / `data` keep the
+    base64 detail at the boundaries.
+    """
+
+    transfer_id: SnapshotTransferId
+    requester_node_id: NodeId
+    session_id: SessionId
+    schema_version: int
+    last_event_applied_idx: int
+    chunk_index: int
+    total_chunks: int
+    sha256_hex: str
+    data_b64: str
+
+    @classmethod
+    def from_data(cls, *, data: bytes, **kwargs: object) -> "SnapshotChunk":
+        return cls(data_b64=base64.b64encode(data).decode("ascii"), **kwargs)  # pyright: ignore[reportArgumentType]
+
+    @property
+    def data(self) -> bytes:
+        return base64.b64decode(self.data_b64)
+
+
+__all__ = ["SnapshotChunk", "SnapshotTransferId"]

src/exo/shared/types/state.py

@@ -7,7 +7,8 @@ from pydantic.alias_generators import to_camel
 
 from exo.shared.models.model_cards import ModelCard
 from exo.shared.topology import Topology, TopologySnapshot
-from exo.shared.types.common import ModelId, NodeId
+from exo.shared.types.chunks import InputImageChunk
+from exo.shared.types.common import CommandId, ModelId, NodeId
 from exo.shared.types.instance_link import InstanceLink, InstanceLinkId
 from exo.shared.types.profiling import (
     DiskUsage,
@@ -42,10 +43,16 @@ class State(FrozenModel):
         strict=True,
         arbitrary_types_allowed=True,
     )
+    # Bump when a State change makes older snapshots unsafe to restore.
+    schema_version: int = Field(default=1, ge=1)
+
     instances: Mapping[InstanceId, Instance] = {}
     runners: Mapping[RunnerId, RunnerStatus] = {}
     downloads: Mapping[NodeId, Sequence[DownloadProgress]] = {}
     tasks: Mapping[TaskId, Task] = {}
+    # Durable request input chunks for active image requests. Workers rebuild
+    # local image caches from this state instead of reading events directly.
+    input_chunks: Mapping[CommandId, Mapping[int, InputImageChunk]] = {}
     last_seen: Mapping[NodeId, datetime] = {}
     topology: Topology = Field(default_factory=Topology)
     last_event_applied_idx: int = Field(default=-1, ge=-1)
@@ -66,7 +73,8 @@ class State(FrozenModel):
     instance_links: Mapping[InstanceLinkId, InstanceLink] = {}
     prefill_server_ports: Mapping[RunnerId, int] = {}
 
-    # User-added model cards. Workers can reconcile their on-disk custom card cache
+    # User-added model cards. Workers can reconcile their on-disk custom card
+    # cache from this state after snapshot bootstrap.
     custom_model_cards: Mapping[ModelId, ModelCard] = {}
 
     @field_serializer("topology", mode="plain")

src/exo/shared/types/text_generation.py

@@ -135,9 +135,9 @@ class TextGenerationTaskParams(BaseModel, frozen=True):
     prefill_endpoint: str | None = None
 
     def with_card_sampling_defaults(self) -> "TextGenerationTaskParams":
-        from exo.shared.models import model_cards
+        from exo.shared.models.model_cards import get_card
 
-        card = model_cards.card_cache.get(self.model)
+        card = get_card(self.model)
         if card is None:
             return self
 

src/exo/utils/event_buffer.py

@@ -47,6 +47,18 @@ class OrderedBuffer[T]:
         logger.trace(f"Releasing event {ret}")
         return ret
 
+    def fast_forward_to(self, idx: int) -> None:
+        """Skip every event before idx.
+
+        Snapshot restore uses this after applying state that already includes
+        events before idx. Any buffered or future event below idx is stale.
+        """
+        if idx <= self.next_idx_to_release:
+            return
+        self.next_idx_to_release = idx
+        for stale_idx in [i for i in self.store if i < idx]:
+            del self.store[stale_idx]
+
 
 class MultiSourceBuffer[SourceId, T]:
     """

src/exo/utils/keyed_backoff.py

@@ -29,6 +29,10 @@ class KeyedBackoff[K]:
         """Return the number of recorded attempts for a key."""
         return self._attempts.get(key, 0)
 
+    def tracked_keys(self) -> set[K]:
+        """Return keys that currently have recorded backoff state."""
+        return set(self._attempts) | set(self._last_time)
+
     def reset(self, key: K) -> None:
         """Reset backoff state for a key (e.g., on success)."""
         self._attempts.pop(key, None)

src/exo/utils/tests/test_keyed_backoff.py

@@ -0,0 +1,13 @@
+from exo.utils.keyed_backoff import KeyedBackoff
+
+
+def test_tracked_keys_reports_and_resets_backoff_state() -> None:
+    backoff = KeyedBackoff[str]()
+
+    backoff.record_attempt("instance-a")
+
+    assert backoff.tracked_keys() == {"instance-a"}
+
+    backoff.reset("instance-a")
+
+    assert backoff.tracked_keys() == set()

src/exo/worker/engines/image/builder.py

@@ -143,7 +143,6 @@ class ImageEngine(Engine):
         Generator[tuple[TaskId, Chunk | FinishedResponse | CancelledResponse]] | None
     ) = field(init=False, default=None)
     queue: deque[ImageTask] = field(init=False, default_factory=deque)
-    _cancelled_tasks: set[TaskId] = field(init=False, default_factory=set)
 
     def warmup(self) -> None:
         image = warmup_image_generator(model=self.image_model)
@@ -169,11 +168,7 @@ class ImageEngine(Engine):
             task = self.queue.popleft()
             self.current_gen = self._run_image_task(task.task_id, task.task_params)
             resp = next(self.current_gen, None)
-        return (
-            (resp,)
-            if resp is not None and _is_primary_output_node(self.shard_metadata)
-            else ()
-        )
+        return (resp,) if resp is not None else ()
 
     def close(self) -> None:
         with contextlib.suppress(NameError, AttributeError):

src/exo/worker/main.py

@@ -8,30 +8,38 @@ from loguru import logger
 
 from exo.api.types import ImageEditsTaskParams
 from exo.download.download_utils import is_read_only_model_dir, resolve_existing_model
+from exo.routing.event_router import EventRouter
+from exo.routing.snapshot_receiver import SnapshotReceiver
 from exo.shared.apply import apply
 from exo.shared.constants import EXO_MAX_INSTANCE_RETRIES
-from exo.shared.models.model_cards import ModelId, card_cache
+from exo.shared.models.model_cards import (
+    ModelCard,
+    ModelId,
+    add_to_card_cache,
+    delete_custom_card,
+)
 from exo.shared.types.chunks import InputImageChunk
 from exo.shared.types.commands import (
     DeleteInstance,
     ForwarderCommand,
     ForwarderDownloadCommand,
+    RequestSnapshot,
     StartDownload,
 )
-from exo.shared.types.common import CommandId, NodeId, SystemId
+from exo.shared.types.common import CommandId, NodeId, SessionId, SystemId
 from exo.shared.types.events import (
     Event,
     IndexedEvent,
-    InputChunkReceived,
-    InstanceDeleted,
     NodeDownloadProgress,
     NodeGatheredInfo,
     TaskCreated,
     TaskStatusUpdated,
     TopologyEdgeCreated,
     TopologyEdgeDeleted,
+    TransientEvent,
 )
 from exo.shared.types.multiaddr import Multiaddr
+from exo.shared.types.snapshots import SnapshotChunk
 from exo.shared.types.state import State
 from exo.shared.types.tasks import (
     CancelTask,
@@ -57,14 +65,21 @@ from exo.utils.task_group import TaskGroup
 from exo.worker.plan import plan
 from exo.worker.runner.supervisor import RunnerSupervisor
 
+_SNAPSHOT_FETCH_TIMEOUT_SECONDS = 30
+
 
 class Worker:
     def __init__(
         self,
         node_id: NodeId,
+        session_id: SessionId,
         *,
+        event_router: EventRouter,
         event_receiver: Receiver[IndexedEvent],
         event_sender: Sender[Event],
+        transient_event_receiver: Receiver[TransientEvent],
+        transient_event_sender: Sender[TransientEvent],
+        snapshot_chunk_receiver: Receiver[SnapshotChunk],
         # This is for requesting updates. It doesn't need to be a general command sender right now,
         # but I think it's the correct way to be thinking about commands
         command_sender: Sender[ForwarderCommand],
@@ -72,8 +87,13 @@ class Worker:
         api_port: int,
     ):
         self.node_id: NodeId = node_id
+        self.session_id: SessionId = session_id
+        self.event_router = event_router
         self.event_receiver = event_receiver
         self.event_sender = event_sender
+        self.transient_event_receiver = transient_event_receiver
+        self.transient_event_sender = transient_event_sender
+        self.snapshot_chunk_receiver = snapshot_chunk_receiver
         self.command_sender = command_sender
         self.download_command_sender = download_command_sender
         self.api_port = api_port
@@ -93,6 +113,7 @@ class Worker:
         self._instance_backoff: KeyedBackoff[InstanceId] = KeyedBackoff(
             base=0.5, cap=10.0
         )
+        self._synced_custom_cards: dict[ModelId, ModelCard] = {}
         self._stopped: anyio.Event = anyio.Event()
 
     async def run(self):
@@ -103,23 +124,61 @@ class Worker:
 
         try:
             async with self._tg as tg:
-                tg.start_soon(info_gatherer.run)
-                tg.start_soon(self._forward_info, info_recv)
-                tg.start_soon(self.plan_step)
-                tg.start_soon(self._event_applier)
-                tg.start_soon(self._poll_connection_updates)
-                tg.start_soon(self._reconcile_custom_cards)
-
+                tg.start_soon(self._bootstrap_then_run, info_gatherer, info_recv)
         finally:
             # Actual shutdown code - waits for all tasks to complete before executing.
             logger.info("Stopping Worker")
             self.event_sender.close()
+            self.transient_event_receiver.close()
+            self.transient_event_sender.close()
+            self.snapshot_chunk_receiver.close()
             self.command_sender.close()
             self.download_command_sender.close()
             for runner in self.runners.values():
                 runner.shutdown()
             self._stopped.set()
 
+    async def _bootstrap_then_run(
+        self, info_gatherer: InfoGatherer, info_recv: Receiver[GatheredInfo]
+    ) -> None:
+        await self._fetch_snapshot()
+        self._sync_input_views_from_state()
+        self._tg.start_soon(info_gatherer.run)
+        self._tg.start_soon(self._forward_info, info_recv)
+        self._tg.start_soon(self.plan_step)
+        self._tg.start_soon(self._event_applier)
+        self._tg.start_soon(self._reconcile_instance_backoff)
+        self._tg.start_soon(self._reconcile_custom_cards)
+        self._tg.start_soon(self._poll_connection_updates)
+
+    async def _fetch_snapshot(self) -> None:
+        receiver = SnapshotReceiver(self.node_id, self.session_id)
+        await self.command_sender.send(
+            ForwarderCommand(
+                origin=self._system_id,
+                command=RequestSnapshot(requester_node_id=self.node_id),
+            )
+        )
+
+        with anyio.move_on_after(_SNAPSHOT_FETCH_TIMEOUT_SECONDS):
+            with self.snapshot_chunk_receiver as chunks:
+                async for chunk in chunks:
+                    received = receiver.ingest(chunk)
+                    if received is None:
+                        continue
+                    self.state = received.state
+                    self.event_router.set_buffer_start(
+                        received.last_event_applied_idx + 1
+                    )
+                    logger.info(
+                        f"Worker bootstrapped from snapshot at idx "
+                        f"{received.last_event_applied_idx}"
+                    )
+                    return
+        logger.info(
+            "No snapshot received before timeout; falling back to full event-log replay"
+        )
+
     async def _forward_info(self, recv: Receiver[GatheredInfo]):
         with recv as info_stream:
             async for info in info_stream:
@@ -134,55 +193,69 @@ class Worker:
     async def _event_applier(self):
         with self.event_receiver as events:
             async for event in events:
+                if event.idx <= self.state.last_event_applied_idx:
+                    continue
                 # 2. for each event, apply it to the state
                 self.state = apply(self.state, event=event)
-                event = event.event
+                self._sync_input_views_from_state()
 
-                if isinstance(event, InstanceDeleted):
-                    self._instance_backoff.reset(event.instance_id)
+    async def _reconcile_instance_backoff(self) -> None:
+        while True:
+            await anyio.sleep(1)
+            self._reconcile_instance_backoff_once()
 
-                # Buffer input image chunks for image editing
-                if isinstance(event, InputChunkReceived):
-                    cmd_id = event.command_id
-                    if cmd_id not in self.input_chunk_buffer:
-                        self.input_chunk_buffer[cmd_id] = {}
-                        self.input_chunk_counts[cmd_id] = event.chunk.total_chunks
-
-                    self.input_chunk_buffer[cmd_id][event.chunk.chunk_index] = (
-                        event.chunk
-                    )
-                    if (
-                        len(self.input_chunk_buffer[cmd_id])
-                        == self.input_chunk_counts[cmd_id]
-                    ):
-                        per_image: defaultdict[int, list[InputImageChunk]] = (
-                            defaultdict(list)
-                        )
-                        for chunk in self.input_chunk_buffer[cmd_id].values():
-                            per_image[chunk.image_index].append(chunk)
-                        for chunks_for_image in per_image.values():
-                            sorted_chunks = sorted(
-                                chunks_for_image, key=lambda c: c.chunk_index
-                            )
-                            img = Base64Image("".join(c.data for c in sorted_chunks))
-                            self.image_cache[
-                                Base64ImageHash(
-                                    hashlib.sha256(img.encode("ascii")).hexdigest()
-                                )
-                            ] = img
+    def _reconcile_instance_backoff_once(self) -> None:
+        live_instances = set(self.state.instances)
+        for instance_id in self._instance_backoff.tracked_keys():
+            if instance_id not in live_instances:
+                self._instance_backoff.reset(instance_id)
 
     async def _reconcile_custom_cards(self) -> None:
         while True:
             await anyio.sleep(1)
-            target = dict(self.state.custom_model_cards)
-            for model_id, card in target.items():
-                if card_cache.get(model_id) == card:
-                    continue
-                await card_cache.save(card)
+            await self._sync_custom_cards_from_state()
 
-            for card in await card_cache.list_all():
-                if card.model_id not in target:
-                    await card_cache.pop(card.model_id)
+    async def _sync_custom_cards_from_state(self) -> None:
+        target = dict(self.state.custom_model_cards)
+        for model_id, card in target.items():
+            if self._synced_custom_cards.get(model_id) == card:
+                continue
+            await card.save_to_custom_dir()
+            add_to_card_cache(card)
+            self._synced_custom_cards[model_id] = card
+
+        for model_id in list(self._synced_custom_cards):
+            if model_id in target:
+                continue
+            await delete_custom_card(model_id)
+            self._synced_custom_cards.pop(model_id, None)
+
+    def _sync_input_views_from_state(self) -> None:
+        self.input_chunk_buffer = {
+            command_id: dict(chunks)
+            for command_id, chunks in self.state.input_chunks.items()
+        }
+        self.input_chunk_counts = {
+            command_id: next(iter(chunks.values())).total_chunks
+            for command_id, chunks in self.input_chunk_buffer.items()
+            if chunks
+        }
+
+        self.image_cache = {}
+        for command_id, chunks in self.input_chunk_buffer.items():
+            expected_chunks = self.input_chunk_counts.get(command_id)
+            if expected_chunks is None or len(chunks) != expected_chunks:
+                continue
+
+            per_image: defaultdict[int, list[InputImageChunk]] = defaultdict(list)
+            for chunk in chunks.values():
+                per_image[chunk.image_index].append(chunk)
+            for chunks_for_image in per_image.values():
+                sorted_chunks = sorted(chunks_for_image, key=lambda c: c.chunk_index)
+                image = Base64Image("".join(chunk.data for chunk in sorted_chunks))
+                self.image_cache[
+                    Base64ImageHash(hashlib.sha256(image.encode("ascii")).hexdigest())
+                ] = image
 
     async def plan_step(self):
         while True:
@@ -194,7 +267,7 @@ class Worker:
                 self.state.instances,
                 self.state.runners,
                 self.state.tasks,
-                self.input_chunk_buffer,
+                self.state.input_chunks,
                 self.image_cache,
                 self._instance_backoff,
                 self._download_backoff,
@@ -326,15 +399,9 @@ class Worker:
                             advanced_params=task.task_params.advanced_params,
                         ),
                     )
-                    # Cleanup buffers
-                    if cmd_id in self.input_chunk_buffer:
-                        del self.input_chunk_buffer[cmd_id]
-                    if cmd_id in self.input_chunk_counts:
-                        del self.input_chunk_counts[cmd_id]
                     await self._start_runner_task(modified_task)
 
                 case TextGeneration() if task.task_params.image_hashes:
-                    cmd_id = task.command_id
                     resolved_images = [
                         self.image_cache[h]
                         for _, h in sorted(task.task_params.image_hashes.items())
@@ -346,10 +413,6 @@ class Worker:
                             )
                         }
                     )
-                    if cmd_id in self.input_chunk_buffer:
-                        del self.input_chunk_buffer[cmd_id]
-                    if cmd_id in self.input_chunk_counts:
-                        del self.input_chunk_counts[cmd_id]
                     await self._start_runner_task(modified_task)
                 case LoadModel(instance_id=instance_id):
                     if (instance := self.state.instances.get(instance_id)) is not None:
@@ -375,6 +438,7 @@ class Worker:
         runner = RunnerSupervisor.create(
             bound_instance=task.bound_instance,
             event_sender=self.event_sender.clone(),
+            transient_event_sender=self.transient_event_sender.clone(),
         )
         self.runners[task.bound_instance.bound_runner_id] = runner
         self._tg.start_soon(runner.run)

src/exo/worker/runner/llm_inference/batch_generator.py

@@ -138,10 +138,8 @@ class SequentialGenerator(Engine):
     def agree_on_tasks(self) -> None:
         """Agree between all ranks about the task ordering (some may have received in different order or not at all)."""
         agreed, different = mx_all_gather_tasks(self._maybe_queue, self.group)
-        # Extend from `agreed` (sorted by task_id on all ranks) to guarantee every
-        # rank enqueues tasks in the same order, preventing TP collective deadlocks.
-        self._queue.extend(agreed)
-        self._maybe_queue = list(different)
+        self._queue.extend(task for task in self._maybe_queue if task in agreed)
+        self._maybe_queue = [task for task in self._maybe_queue if task in different]
 
     def agree_on_cancellations(self) -> None:
         """Agree between all ranks about which tasks to cancel."""
@@ -199,14 +197,9 @@ class SequentialGenerator(Engine):
             self._active = None
             raise
 
-        return filter(
-            lambda chunk: (
-                not isinstance(chunk[1], GenerationChunk) or self.device_rank == 0
-            ),
-            itertools.chain(
-                output,
-                map(lambda task: (task, CancelledResponse()), self._cancelled_tasks),
-            ),
+        return itertools.chain(
+            output,
+            map(lambda task: (task, CancelledResponse()), self._cancelled_tasks),
         )
 
     def _start_next(self) -> None:
@@ -375,10 +368,8 @@ class BatchGenerator(Engine):
     def agree_on_tasks(self) -> None:
         """Agree between all ranks about the task ordering (some may have received in different order or not at all)."""
         agreed, different = mx_all_gather_tasks(self._maybe_queue, self.group)
-        # Extend from `agreed` (sorted by task_id on all ranks) to guarantee every
-        # rank enqueues tasks in the same order, preventing TP collective deadlocks.
-        self._queue.extend(agreed)
-        self._maybe_queue = list(different)
+        self._queue.extend(task for task in self._maybe_queue if task in agreed)
+        self._maybe_queue = [task for task in self._maybe_queue if task in different]
 
     def agree_on_cancellations(self) -> None:
         """Agree between all ranks about which tasks to cancel."""
@@ -458,12 +449,7 @@ class BatchGenerator(Engine):
                 output.append((task.task_id, FinishedResponse()))
                 del self._active_tasks[uid]
 
-        return filter(
-            lambda chunk: (
-                not isinstance(chunk[1], GenerationChunk) or self.device_rank == 0
-            ),
-            itertools.chain(output, self._apply_cancellations()),
-        )
+        return itertools.chain(output, self._apply_cancellations())
 
     def _apply_cancellations(
         self,

src/exo/worker/runner/runner.py

@@ -390,5 +390,5 @@ class Runner:
         chunk: Chunk,
         command_id: CommandId,
     ):
-        assert isinstance(self.generator, Engine)
-        self.event_sender.send(ChunkGenerated(command_id=command_id, chunk=chunk))
+        if self.device_rank == 0:
+            self.event_sender.send(ChunkGenerated(command_id=command_id, chunk=chunk))

src/exo/worker/runner/supervisor.py

@@ -19,6 +19,7 @@ from exo.shared.types.events import (
     RunnerStatusUpdated,
     TaskAcknowledged,
     TaskStatusUpdated,
+    TransientEvent,
 )
 from exo.shared.types.tasks import (
     CANCEL_ALL_TASKS,
@@ -58,6 +59,7 @@ class RunnerSupervisor:
     _ev_recv: MpReceiver[Event]
     _task_sender: MpSender[Task]
     _event_sender: Sender[Event]
+    _transient_event_sender: Sender[TransientEvent]
     _cancel_sender: MpSender[TaskId]
     _tg: TaskGroup = field(default_factory=TaskGroup, init=False)
     status: RunnerStatus = field(default_factory=RunnerIdle, init=False)
@@ -75,6 +77,7 @@ class RunnerSupervisor:
         *,
         bound_instance: BoundInstance,
         event_sender: Sender[Event],
+        transient_event_sender: Sender[TransientEvent],
         initialize_timeout: float = 400,
     ) -> Self:
         ev_send, ev_recv = mp_channel[Event]()
@@ -104,6 +107,7 @@ class RunnerSupervisor:
             _task_sender=task_sender,
             _cancel_sender=cancel_sender,
             _event_sender=event_sender,
+            _transient_event_sender=transient_event_sender,
         )
 
         return self
@@ -124,6 +128,8 @@ class RunnerSupervisor:
                 self._task_sender.close()
             with contextlib.suppress(ClosedResourceError):
                 self._event_sender.close()
+            with contextlib.suppress(ClosedResourceError):
+                self._transient_event_sender.close()
             with contextlib.suppress(ClosedResourceError):
                 self._cancel_sender.send(CANCEL_ALL_TASKS)
             with contextlib.suppress(ClosedResourceError):
@@ -235,7 +241,10 @@ class RunnerSupervisor:
                         )
                         self.in_progress.pop(event.task_id, None)
                         self.completed.add(event.task_id)
-                    await self._event_sender.send(event)
+                    if isinstance(event, ChunkGenerated):
+                        await self._transient_event_sender.send(event)
+                    else:
+                        await self._event_sender.send(event)
         except (ClosedResourceError, BrokenResourceError) as e:
             await self._check_runner(e)
         finally:
@@ -275,7 +284,7 @@ class RunnerSupervisor:
         for task in self.in_progress.values():
             if isinstance(task, (TextGeneration, ImageGeneration, ImageEdits)):
                 with anyio.CancelScope(shield=True):
-                    await self._event_sender.send(
+                    await self._transient_event_sender.send(
                         ChunkGenerated(
                             command_id=task.command_id,
                             chunk=ErrorChunk(

src/exo/worker/tests/unittests/test_mlx/test_tokenizers.py

@@ -16,7 +16,7 @@ from exo.download.download_utils import (
     fetch_file_list_with_cache,
     resolve_model_dir,
 )
-from exo.shared.models.model_cards import ModelCard, ModelId, card_cache
+from exo.shared.models.model_cards import ModelCard, ModelId, get_model_cards
 from exo.worker.engines.mlx.utils_mlx import (
     get_eos_token_ids_for_model,
     load_tokenizer_for_model_id,
@@ -76,7 +76,7 @@ def get_test_models() -> list[ModelCard]:
     """Get a representative sample of models to test."""
     # Pick one model from each family to test
     families: dict[str, ModelCard] = {}
-    for card in asyncio.run(card_cache.list_all()):
+    for card in asyncio.run(get_model_cards()):
         # Extract family name (e.g., "llama-3.1" from "llama-3.1-8b")
         parts = card.model_id.short().split("-")
         family = "-".join(parts[:2]) if len(parts) >= 2 else parts[0]
@@ -298,7 +298,7 @@ async def test_tokenizer_special_tokens(model_card: ModelCard) -> None:
 async def test_kimi_tokenizer_specifically():
     """Test Kimi tokenizer with its specific patches and quirks."""
     kimi_models = [
-        card for card in await card_cache.list_all() if "kimi" in card.model_id.lower()
+        card for card in await get_model_cards() if "kimi" in card.model_id.lower()
     ]
 
     if not kimi_models:
@@ -350,7 +350,7 @@ async def test_glm_tokenizer_specifically():
 
     glm_model_cards = [
         card
-        for card in await card_cache.list_all()
+        for card in await get_model_cards()
         if contains(card, "glm")
         and not contains(card, "-5")
         and not contains(card, "4.7")

src/exo/worker/tests/unittests/test_runner/test_runner_supervisor.py

@@ -7,7 +7,12 @@ import pytest
 from exo.shared.models.model_cards import ModelId
 from exo.shared.types.chunks import ErrorChunk
 from exo.shared.types.common import CommandId, NodeId
-from exo.shared.types.events import ChunkGenerated, Event, RunnerStatusUpdated
+from exo.shared.types.events import (
+    ChunkGenerated,
+    Event,
+    RunnerStatusUpdated,
+    TransientEvent,
+)
 from exo.shared.types.tasks import Task, TaskId, TextGeneration
 from exo.shared.types.text_generation import (
     InputMessage,
@@ -43,6 +48,7 @@ class _DeadProcess:
 @pytest.mark.asyncio
 async def test_check_runner_emits_error_chunk_for_inflight_text_generation() -> None:
     event_sender, event_receiver = channel[Event]()
+    transient_sender, transient_receiver = channel[TransientEvent]()
     task_sender, _ = mp_channel[Task]()
     cancel_sender, _ = mp_channel[TaskId]()
     _, ev_recv = mp_channel[Event]()
@@ -62,6 +68,7 @@ async def test_check_runner_emits_error_chunk_for_inflight_text_generation() ->
         _ev_recv=ev_recv,
         _task_sender=task_sender,
         _event_sender=event_sender,
+        _transient_event_sender=transient_sender,
         _cancel_sender=cancel_sender,
     )
 
@@ -81,7 +88,7 @@ async def test_check_runner_emits_error_chunk_for_inflight_text_generation() ->
 
     await supervisor._check_runner(RuntimeError("boom"))  # pyright: ignore[reportPrivateUsage]
 
-    got_chunk = await event_receiver.receive()
+    got_chunk = await transient_receiver.receive()
     got_status = await event_receiver.receive()
 
     assert isinstance(got_chunk, ChunkGenerated)
@@ -93,5 +100,57 @@ async def test_check_runner_emits_error_chunk_for_inflight_text_generation() ->
     assert isinstance(got_status.runner_status, RunnerFailed)
 
     event_sender.close()
+    transient_sender.close()
     with anyio.move_on_after(0.1):
         await event_receiver.aclose()
+    with anyio.move_on_after(0.1):
+        await transient_receiver.aclose()
+
+
+@pytest.mark.asyncio
+async def test_forward_events_routes_generated_chunks_to_transient() -> None:
+    event_sender, event_receiver = channel[Event]()
+    transient_sender, transient_receiver = channel[TransientEvent]()
+    task_sender, _ = mp_channel[Task]()
+    cancel_sender, _ = mp_channel[TaskId]()
+    ev_send, ev_recv = mp_channel[Event]()
+
+    bound_instance: BoundInstance = get_bound_mlx_ring_instance(
+        instance_id=InstanceId("instance-a"),
+        model_id=ModelId("mlx-community/Llama-3.2-1B-Instruct-4bit"),
+        runner_id=RunnerId("runner-a"),
+        node_id=NodeId("node-a"),
+    )
+
+    supervisor = RunnerSupervisor(
+        shard_metadata=bound_instance.bound_shard,
+        bound_instance=bound_instance,
+        runner_process=cast("mp.Process", cast(object, _DeadProcess())),
+        initialize_timeout=400,
+        _ev_recv=ev_recv,
+        _task_sender=task_sender,
+        _event_sender=event_sender,
+        _transient_event_sender=transient_sender,
+        _cancel_sender=cancel_sender,
+    )
+
+    command_id = CommandId("cmd-a")
+    chunk = ChunkGenerated(
+        command_id=command_id,
+        chunk=ErrorChunk(
+            model=bound_instance.bound_shard.model_card.model_id,
+            error_message="test chunk",
+        ),
+    )
+
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(supervisor._forward_events)  # pyright: ignore[reportPrivateUsage]
+        ev_send.send(chunk)
+        assert await transient_receiver.receive() == chunk
+        tg.cancel_scope.cancel()
+
+    assert event_receiver.collect() == []
+
+    event_sender.close()
+    transient_sender.close()
+    ev_send.close()

src/exo/worker/tests/unittests/test_worker_custom_cards.py

@@ -0,0 +1,78 @@
+# pyright: reportPrivateUsage=false
+
+import pytest
+
+import exo.worker.main as worker_main
+from exo.shared.models.model_cards import ModelCard, ModelTask
+from exo.shared.types.common import ModelId
+from exo.shared.types.memory import Memory
+from exo.shared.types.state import State
+from exo.worker.main import Worker
+
+
+def _model_card(model_id: ModelId) -> ModelCard:
+    return ModelCard(
+        model_id=model_id,
+        n_layers=1,
+        storage_size=Memory.from_bytes(1),
+        hidden_size=1,
+        supports_tensor=True,
+        tasks=[ModelTask.TextGeneration],
+    )
+
+
+def _worker() -> Worker:
+    worker = object.__new__(Worker)
+    worker.state = State()
+    worker._synced_custom_cards = {}
+    return worker
+
+
+@pytest.mark.asyncio
+async def test_worker_syncs_custom_cards_from_state(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    saved: list[ModelId] = []
+    cached: list[ModelId] = []
+
+    async def save_to_custom_dir(card: ModelCard) -> None:
+        saved.append(card.model_id)
+
+    def add_to_card_cache(card: ModelCard) -> None:
+        cached.append(card.model_id)
+
+    monkeypatch.setattr(ModelCard, "save_to_custom_dir", save_to_custom_dir)
+    monkeypatch.setattr(worker_main, "add_to_card_cache", add_to_card_cache)
+
+    card = _model_card(ModelId("custom/model"))
+    worker = _worker()
+    worker.state = State(custom_model_cards={card.model_id: card})
+
+    await worker._sync_custom_cards_from_state()
+    await worker._sync_custom_cards_from_state()
+
+    assert saved == [card.model_id]
+    assert cached == [card.model_id]
+    assert worker._synced_custom_cards == {card.model_id: card}
+
+
+@pytest.mark.asyncio
+async def test_worker_deletes_custom_cards_missing_from_state(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    deleted: list[ModelId] = []
+
+    async def delete_custom_card(model_id: ModelId) -> bool:
+        deleted.append(model_id)
+        return True
+
+    monkeypatch.setattr(worker_main, "delete_custom_card", delete_custom_card)
+
+    card = _model_card(ModelId("custom/model"))
+    worker = _worker()
+    worker._synced_custom_cards = {card.model_id: card}
+
+    await worker._sync_custom_cards_from_state()
+
+    assert deleted == [card.model_id]
+    assert worker._synced_custom_cards == {}

src/exo/worker/tests/unittests/test_worker_instance_backoff.py

@@ -0,0 +1,36 @@
+# pyright: reportPrivateUsage=false
+
+from exo.shared.types.common import ModelId, NodeId
+from exo.shared.types.state import State
+from exo.shared.types.worker.instances import InstanceId, MlxRingInstance
+from exo.shared.types.worker.runners import ShardAssignments
+from exo.utils.keyed_backoff import KeyedBackoff
+from exo.worker.main import Worker
+
+
+def _make_instance(instance_id: InstanceId) -> MlxRingInstance:
+    return MlxRingInstance(
+        instance_id=instance_id,
+        shard_assignments=ShardAssignments(
+            model_id=ModelId("test-model"),
+            node_to_runner={},
+            runner_to_shard={},
+        ),
+        hosts_by_node={NodeId("node-1"): []},
+        ephemeral_port=1,
+    )
+
+
+def test_worker_reconciles_instance_backoff_from_state() -> None:
+    live_instance_id = InstanceId("inst-live")
+    deleted_instance_id = InstanceId("inst-deleted")
+    worker = object.__new__(Worker)
+    worker.state = State(instances={live_instance_id: _make_instance(live_instance_id)})
+    worker._instance_backoff = KeyedBackoff[InstanceId]()
+    worker._instance_backoff.record_attempt(live_instance_id)
+    worker._instance_backoff.record_attempt(deleted_instance_id)
+
+    worker._reconcile_instance_backoff_once()
+
+    assert worker._instance_backoff.attempts(live_instance_id) == 1
+    assert worker._instance_backoff.attempts(deleted_instance_id) == 0

src/exo/worker/tests/unittests/test_worker_snapshot_bootstrap.py

@@ -0,0 +1,130 @@
+# pyright: reportPrivateUsage=false
+
+import hashlib
+
+import anyio
+import pytest
+import zstandard
+
+from exo.routing.event_router import EventRouter
+from exo.shared.types.commands import (
+    ForwarderCommand,
+    ForwarderDownloadCommand,
+    RequestSnapshot,
+)
+from exo.shared.types.common import NodeId, SessionId
+from exo.shared.types.events import (
+    Event,
+    GlobalForwarderEvent,
+    IndexedEvent,
+    LocalForwarderEvent,
+    TestEvent,
+    TransientEvent,
+)
+from exo.shared.types.snapshots import SnapshotChunk, SnapshotTransferId
+from exo.shared.types.state import State
+from exo.utils.channels import Receiver, Sender, channel
+from exo.worker.main import Worker
+
+
+def _snapshot_chunk(
+    state: State, *, requester_node_id: NodeId, session_id: SessionId
+) -> SnapshotChunk:
+    body = zstandard.ZstdCompressor().compress(state.model_dump_json().encode("utf-8"))
+    return SnapshotChunk.from_data(
+        data=body,
+        transfer_id=SnapshotTransferId("transfer-1"),
+        requester_node_id=requester_node_id,
+        session_id=session_id,
+        schema_version=state.schema_version,
+        last_event_applied_idx=state.last_event_applied_idx,
+        chunk_index=0,
+        total_chunks=1,
+        sha256_hex=hashlib.sha256(body).hexdigest(),
+    )
+
+
+def _worker(
+    node_id: NodeId, session_id: SessionId
+) -> tuple[
+    Worker,
+    EventRouter,
+    Receiver[ForwarderCommand],
+    Sender[SnapshotChunk],
+    Sender[IndexedEvent],
+]:
+    router_command_sender, _router_command_receiver = channel[ForwarderCommand]()
+    _global_event_sender, global_event_receiver = channel[GlobalForwarderEvent]()
+    local_event_sender, _local_event_receiver = channel[LocalForwarderEvent]()
+    event_router = EventRouter(
+        session_id=session_id,
+        command_sender=router_command_sender,
+        external_inbound=global_event_receiver,
+        external_outbound=local_event_sender,
+    )
+
+    event_sender, event_receiver = channel[IndexedEvent]()
+    local_event_output_sender, _local_event_output_receiver = channel[Event]()
+    transient_sender, transient_receiver = channel[TransientEvent]()
+    command_sender, command_receiver = channel[ForwarderCommand]()
+    download_command_sender, _download_command_receiver = channel[
+        ForwarderDownloadCommand
+    ]()
+    snapshot_sender, snapshot_receiver = channel[SnapshotChunk]()
+
+    worker = Worker(
+        node_id,
+        session_id,
+        event_router=event_router,
+        event_receiver=event_receiver,
+        event_sender=local_event_output_sender,
+        transient_event_receiver=transient_receiver,
+        transient_event_sender=transient_sender,
+        snapshot_chunk_receiver=snapshot_receiver,
+        command_sender=command_sender,
+        download_command_sender=download_command_sender,
+        api_port=52415,
+    )
+    return worker, event_router, command_receiver, snapshot_sender, event_sender
+
+
+@pytest.mark.asyncio
+async def test_worker_fetch_snapshot_applies_state_and_fast_forwards_router() -> None:
+    node_id = NodeId("worker")
+    session_id = SessionId(master_node_id=NodeId("master"), election_clock=1)
+    worker, event_router, command_receiver, snapshot_sender, _event_sender = _worker(
+        node_id, session_id
+    )
+    state = State(last_event_applied_idx=7)
+
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(worker._fetch_snapshot)
+        command = await command_receiver.receive()
+        assert isinstance(command.command, RequestSnapshot)
+        assert command.command.requester_node_id == node_id
+
+        await snapshot_sender.send(
+            _snapshot_chunk(state, requester_node_id=node_id, session_id=session_id)
+        )
+
+    assert worker.state.last_event_applied_idx == 7
+    assert event_router.event_buffer.next_idx_to_release == 8
+
+
+@pytest.mark.asyncio
+async def test_worker_event_applier_ignores_events_covered_by_snapshot() -> None:
+    node_id = NodeId("worker")
+    session_id = SessionId(master_node_id=NodeId("master"), election_clock=1)
+    worker, _event_router, _command_receiver, _snapshot_sender, event_sender = _worker(
+        node_id, session_id
+    )
+    worker.state = State(last_event_applied_idx=7)
+
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(worker._event_applier)
+        await event_sender.send(IndexedEvent(idx=7, event=TestEvent()))
+        await event_sender.send(IndexedEvent(idx=8, event=TestEvent()))
+
+        while worker.state.last_event_applied_idx != 8:
+            await anyio.sleep(0.001)
+        tg.cancel_scope.cancel()