exo/docs/architecture.md

EXO Architecture overview

EXO uses an Event Sourcing architecture, and Erlang-style message passing. To facilitate this, we've written a channel library extending anyio channels with inspiration from tokio::sync::mpsc.

Each logical module - designed to be functional independently of the others - communicates with the rest of the system by sending messages on topics.

Systems

There are currently 5 major systems:

• Master

  Executes placement and orders events through a single writer

• Worker

  Schedules work on a node, gathers system information, etc.#

• Runner

  Executes inference jobs (for now) in an isolated process from the worker for fault-tolerance.

• API

  Runs a python webserver for exposing state and commands to client applications

• Election

  Implements a distributed algorithm for master election in unstable networking conditions

Topics

There are currently 5 topics:

• Commands

  The API and Worker instruct the master when the event log isn't sufficient. Namely placement and catchup requests go through Commands atm.

• Local Events

  All nodes write events here, the master reads those events and orders them

• Global Events

  The master writes events here, all nodes read from this topic and fold the produced events into their State

• Election Messages

  Before establishing a cluster, nodes communicate here to negotiate a master node.

• Connection Messages

  The networking system write mdns-discovered hardware connections here.

Event Sourcing

Lots has been written about event sourcing, but it lets us centralize faulty connections and message ACKing with the following model.

Whenever a device produces side effects, it captures those side effects in an Event. Events are then "applied" to their model of State, which is globally distributed across the cluster. Whenever a command is received, it is combined with state to produce side effects, captured in yet more events. The rule of thumb is "Events are past tense, Commands are imperative". Telling a node to perform some action like "place this model" or "Give me a copy of the event log" is represented by a command (The worker's Tasks are also commands), while "this node is using 300GB of ram" is an event. Notably, Events SHOULD never cause side effects on their own. There are a few exceptions to this, we're working out the specifics of generalizing the distributed event sourcing model to make it better suit our needs

Purity

A significant goal of the current design is to make data flow explicit. Classes should either represent simple data (CamelCaseModels typically, and TaggedModels for unions) or active Systems (Erlang Actors), with all transformations of that data being "referentially transparent" - destructure and construct new data, don't mutate in place. We have had varying degrees of success with this, and are still exploring where purity makes sense.