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The classifier's inputs are the actual denials from the log attached to issue #1947: owned titles on an inactive account, a Plus title no longer in the catalog, and an unreleased preorder Audible has no audio for. The backoff tests also caught a real overflow: first * 2^n exceeds a TimeSpan long before the cap matters. Co-authored-by: rmcrackan <rmcrackan@gmail.com>
54 lines
2.4 KiB
C#
54 lines
2.4 KiB
C#
using DataLayer;
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using System;
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using System.Collections.Generic;
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namespace ApplicationServices;
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/// <summary>
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/// How long to leave a title alone after a download attempt Audible refused, so a scheduled run stops asking
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/// for the same license every time.
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/// <para>
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/// The wait doubles with each failure in a row, up to a cap, and every kind of failure has a finite cap: a
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/// title held back is always attempted again eventually. Audible never distinguishes "you will never have
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/// rights to this" from "not right now", so nothing here may be permanent.
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/// </para>
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/// </summary>
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public static class DownloadRetryBackoff
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{
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private static readonly Dictionary<DownloadFailureKind, (TimeSpan First, TimeSpan Max)> schedule = new()
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{
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// An eligibility refusal changes only when the account or the catalog changes. A day matches the
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// advice Libation already gives for a Plus title ("try again in 1 to 2 days"), which is the most
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// common refusal that clears by itself.
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[DownloadFailureKind.LicenseDenied] = (TimeSpan.FromDays(1), TimeSpan.FromDays(30)),
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// A preorder becomes downloadable on its release date, which nobody can predict from the error, so
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// keep checking within a week.
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[DownloadFailureKind.AssetUnavailable] = (TimeSpan.FromHours(6), TimeSpan.FromDays(7)),
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// Short: an outage that has passed should not delay a title any longer than it has to.
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[DownloadFailureKind.ServiceInterruption] = (TimeSpan.FromHours(1), TimeSpan.FromHours(12)),
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};
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/// <summary>How long to wait after the <paramref name="consecutiveFailures"/>th failure in a row.</summary>
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public static TimeSpan GetWait(DownloadFailureKind kind, int consecutiveFailures)
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{
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var (first, max) = schedule.TryGetValue(kind, out var found)
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? found
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: schedule[DownloadFailureKind.ServiceInterruption];
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// Doubled one step at a time and stopped at the cap: first * 2^n overflows a TimeSpan long before a
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// plausible failure count, let alone the absurd ones a corrupt row could hold.
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var doublings = Math.Max(0, consecutiveFailures - 1);
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var wait = first;
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for (var i = 0; i < doublings && wait < max; i++)
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wait += wait;
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return wait > max ? max : wait;
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}
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/// <summary>When a title becomes eligible for another automatic attempt.</summary>
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public static DateTimeOffset GetRetryAfter(DownloadFailureKind kind, int consecutiveFailures, DateTimeOffset failedAt)
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=> failedAt + GetWait(kind, consecutiveFailures);
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}
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