Bound update waits by queue progress rather than by ping - #57
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An update waiting behind its document's queue was given a budget of `ping * 5 * queueDepth * 1.5`, fixed at enqueue. Ping measures a round trip to the database. The cost it was standing in for is a transaction retrying under optimistic versioning against every other writer of the same document, which ping says nothing about, so under contention the estimate was systematically short and a busy queue failed as though it were broken. The number could only ever be right for one machine. Worse, the budget was enforced with `withTimeout`, so exceeding it threw a `TimeoutCancellationException` out of a suspend function that had not been cancelled. That marks the caller's own coroutine cancelled rather than failed, so the caller stopped silently instead of seeing something it could catch. The wait is now made of windows, and the queue's completion counter answers the question elapsed time cannot: whether anything finished. - A queue that completes nothing for one window has an item that has taken longer than any single update may. That is a fault at any depth on any machine, because the bound is on one item rather than on the queue: `UpdateQueueStalledException`. - A queue that keeps completing but never reaches this caller within the ceiling is receiving work faster than it can finish it. A different fault with a different remedy, so a different exception: `UpdateQueueTooDeepException`. - A queue that refuses or abandons an update because it is shutting down reports `UpdateQueueShutdownException`. Six sites previously handed the caller a raw `CancellationException` or a bare `IllegalStateException`. None of the three is a `CancellationException`, all three are `DocumentUpdateException`, and all three leave the outcome unknown rather than failed: only the waiting is abandoned, the queue still owns the request. The KDoc on both `DocCache.update` overloads says so, since retrying a non-idempotent update after one of them can apply it twice. The single-update bound is derived from the retry policy that sets it, `MongoTransactions.MAX_RETRY_BUDGET_MS`, plus room for the round trips that budget only covers the waiting between. Ping is used for the round trips, which is what it measures. A number chosen independently of the retry constants stops being right the moment either changes. `onUpdateQueueStalled` and `onUpdateQueueTooDeep` export both faults to metrics receivers, with default bodies so existing implementers are unaffected. Neither counts towards `onDatabaseUpdateFail`: a queue fault is not a database update failure, and shutdown under load would otherwise spike that metric on every clean teardown. Co-Authored-By: Claude Code <[email protected]>
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WalkthroughChangesThe update system distinguishes stalled queues, excessively deep queues, and shutdown outcomes. It monitors queue progress across wait windows, preserves queued work after waits expire, and adds backend timing limits, metrics callbacks, and integration tests. Update queue liveness
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🟡 Moderate · up to A completed update can be reported as stalled, leaving its outcome uncertain and potentially prompting a duplicate non-idempotent retry. This should be fixed before merge. Sequence Diagram(s)sequenceDiagram
participant DatabaseService
participant UpdateQueueManager
participant UpdateQueue
participant MongoDatabaseService
DatabaseService->>UpdateQueueManager: enqueueUpdate
UpdateQueueManager->>UpdateQueue: accept update and return QueuedUpdate
DatabaseService->>UpdateQueue: await queued result and completed count
UpdateQueue->>MongoDatabaseService: process document update
MongoDatabaseService-->>UpdateQueue: complete or fail update
UpdateQueue-->>DatabaseService: update result or queue exception
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Inline comments:
In
`@core-api/src/main/kotlin/com/jakemoore/datakache/core/connections/queues/UpdateQueue.kt`:
- Around line 177-185: Update the queue launch flow around the returned Job and
its CompletableDeferred so cancellation before the launched coroutine enters the
try block still completes any unresolved request exceptionally with
UpdateQueueShutdownException. Attach a completion handler to the Job, guard
against already-completed deferreds, and preserve the existing
CancellationException handling and cooperative rethrow behavior.
In
`@core-api/src/main/kotlin/com/jakemoore/datakache/core/connections/queues/UpdateQueueManager.kt`:
- Around line 66-74: Serialize queue admission and shutdown using queueMutex: in
enqueueUpdate, hold the mutex across the shutdown check and
getOrCreateQueue/getQueue decision, and in shutdown, hold it while transitioning
shutdown state and snapshotting/clearing queues. Ensure no queue can be created
after shutdown snapshots the map, while preserving the existing exceptional
QueuedUpdate behavior for rejected updates.
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core-api/src/main/kotlin/com/jakemoore/datakache/api/cache/DocCache.ktcore-api/src/main/kotlin/com/jakemoore/datakache/api/exception/update/DocumentUpdateExceptions.ktcore-api/src/main/kotlin/com/jakemoore/datakache/api/metrics/MetricsReceiverPartial.ktcore-api/src/main/kotlin/com/jakemoore/datakache/api/metrics/receiver/DatabaseReceiver.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/DatabaseService.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/mongo/MongoDatabaseService.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/mongo/MongoTransactions.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/queues/UpdateQueue.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/queues/UpdateQueueManager.ktcore-api/src/test/kotlin/com/jakemoore/datakache/test/integration/transactions/TestUpdateQueueLiveness.kt
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Both leave the caller holding a deferred that no longer has anything behind it, which is the failure this branch exists to remove. A backpressure retry runs in a coroutine launched on the queue's scope. A coroutine cancelled before its body starts never runs the body at all, so the catch that completes the caller's deferred is never reached. The caller then waits a full stall window and is told the queue is stalled, when the truth is that it shut down. A completion handler on the job completes anything left outstanding; completing is idempotent, so it is a no-op whenever the body did run. `UpdateQueueManager.enqueueUpdate` read its shutdown flag outside the mutex that `shutdown` uses to snapshot and clear the queue map, so a queue could be created after the snapshot was taken. Such a queue is in no snapshot, is never shut down, and leaves a processing coroutine running an executor against a service that has stopped. The flag is now read again inside that mutex, which orders creation against the snapshot: either the queue is in it, or admission is refused. The lock-free lookup is unchanged, since an entry found there was published before any snapshot. Each queue is still shut down outside the lock, because that waits on in-flight work and the idle sweep holds the same lock. Co-Authored-By: Claude Code <[email protected]>
A queue is only reachable through the manager's map, so the sweep taking one out of it makes that queue invisible to `shutdown`'s snapshot and leaves it entirely dependent on the sweep to stop it. The sweep does that in a coroutine launched on the manager's job, and a coroutine cancelled before its body starts never runs the body. A shutdown landing in the window between the sweep reading `isActive` and that coroutine starting therefore stopped nothing: the queue was in no snapshot, its processing coroutine kept running on its own scope, and no reference to it remained. This is the same failure the previous commit closed for queue creation, reached through retirement instead, so the claim that no queue survives shutdown was only true of the path that commit changed. A retiring queue is now claimed in a second set before it is removed from the map, so it is always in one of the two, and shutdown drains both. Shutting a queue down twice is a no-op, so the ordinary path where the sweep does complete is unaffected. The comment on the manager shutdown test claimed it pinned the create-versus-snapshot ordering. It does not: it passes on the steady state alone, since admission is refused once the flag is set whatever the ordering. Reaching that window means cancelling a coroutine between two adjacent statements, so it has no test, and the comment now says so. Co-Authored-By: Claude Code <[email protected]>
Both places the idle sweep shuts a queue down caught `Exception`, which on the JVM includes `CancellationException`. Cancelling the sweep is how teardown is supposed to end, so every clean shutdown logged an error for work that was doing exactly what it was told. Rethrown rather than logged, so the coroutine ends cancelled rather than completed. Nothing is dropped by ending the sweep early: a queue it has not reached yet is still in the queue map, and one it has claimed is in the retiring set, so shutdown finds either. Co-Authored-By: Claude Code <[email protected]>
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Inline comments:
In
`@core-api/src/main/kotlin/com/jakemoore/datakache/core/connections/queues/UpdateQueue.kt`:
- Line 242: Update processUpdateRequest so completedCount is incremented
immediately before each terminal completion of a processed request, including
the cancellation path, and remove the outer finally-based increment. Ensure a
request that is re-completed by the outer catch is counted only once.
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📒 Files selected for processing (10)
core-api/src/main/kotlin/com/jakemoore/datakache/api/cache/DocCache.ktcore-api/src/main/kotlin/com/jakemoore/datakache/api/exception/update/DocumentUpdateExceptions.ktcore-api/src/main/kotlin/com/jakemoore/datakache/api/metrics/MetricsReceiverPartial.ktcore-api/src/main/kotlin/com/jakemoore/datakache/api/metrics/receiver/DatabaseReceiver.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/DatabaseService.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/mongo/MongoDatabaseService.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/mongo/MongoTransactions.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/queues/UpdateQueue.ktcore-api/src/main/kotlin/com/jakemoore/datakache/core/connections/queues/UpdateQueueManager.ktcore-api/src/test/kotlin/com/jakemoore/datakache/test/integration/transactions/TestUpdateQueueLiveness.kt
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The progress counter was incremented in the loop's `finally`, which runs after the request's deferred has already been completed. A wait expiring in that window read the counter as unchanged and reported the queue stalled, when it had in fact just finished the very update being waited on. Reporting a successful update as a fault is the failure this branch exists to remove, reached from the other side. The misreading is not confined to the waiter whose request it was. Every caller behind the same queue reads the same counter, and their deferreds are not completed, so nothing else corrects them. Every terminal completion of a processed request now goes through one place that increments first and completes second. Routing them through one function is deliberate: counting only the successful path would report a queue of failing updates as wedged, and a grep for the bare completion now returns nothing. The completed check keeps the count at one per request, since a cancelled executor is completed by `processUpdateRequest` and then again by the loop that called it. The ordering itself has no test, since observing it means catching the queue between two adjacent statements. The new case covers the mistake this refactor could have made instead: thirty updates that fail slowly, and a waiter behind them that must still receive its own failure rather than a stall. Co-Authored-By: Claude Code <[email protected]>
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The problem
An update waiting behind its document's queue was given a budget of
ping * 5 * queueDepth * 1.5,fixed at enqueue and enforced with
withTimeout. Two things were wrong with it.Ping is the wrong instrument. It measures a round trip to the database. The cost it was standing
in for is a transaction retrying under optimistic versioning against every other writer of the same
document, which ping says nothing about. Under contention the estimate was systematically short, so
a queue that was merely busy failed as though it were broken. The number could only ever be made
right for one machine at a time.
The failure arrived as a cancellation.
withTimeoutthrowsTimeoutCancellationExceptionout ofa suspend function that had not been cancelled. That marks the caller's own coroutine cancelled
rather than failed, so the caller stopped silently instead of seeing something it could catch.
This is what failed the 0.4.6 publish run: a test waited behind a queue that was working perfectly
well and was told it had timed out.
The fix
The wait is now made of windows, and the queue's completion counter answers the question elapsed
time cannot: whether anything finished.
UpdateQueueStalledExceptionUpdateQueueTooDeepExceptionUpdateQueueShutdownExceptionA stall is a fault at any depth on any machine, because the bound is on one item rather than on the
queue. Being outpaced is a different fault with a different remedy, so it is a different exception:
write to that document less, rather than look for a deadlock.
The single-update bound comes from the retry policy that sets it,
MongoTransactions .MAX_RETRY_BUDGET_MS, plus room for the round trips that budget only covers the waiting between.Ping is used there, which is the one thing it does measure. A bound chosen independently of the retry
constants stops being right the moment either constant changes.
Behaviour changes for consumers
CancellationException. All three extendDocumentUpdateException, so they arrive as aFailurerather than cancelling the caller.still owns the request and may complete it afterwards. Retrying a non-idempotent update after one
of them can apply it twice. Both
DocCache.updateoverloads say so.CancellationExceptionor a bareIllegalStateExceptionnow reportUpdateQueueShutdownException. Those are: the already-shutdownflag and the closed-channel send in
UpdateQueue.enqueueUpdate, both exits fromhandleBackpressure, the forced-shutdown drain loop, andUpdateQueueManager's shutdownshort-circuit.
onUpdateQueueStalledandonUpdateQueueTooDeepare new onDatabaseReceiver, with defaultbodies, so existing implementers are unaffected. Neither counts towards
onDatabaseUpdateFail: aqueue fault is not a database update failure, and teardown under load would otherwise spike that
metric on every clean shutdown.
Tests
Eight cases in
TestUpdateQueueLiveness, covering each state above and each way a shutdown can endan update: in flight, still queued, refused at enqueue, and refused under backpressure. Each was
checked against a negative control that reverts the fix it covers, and each control failed exactly
the intended test and no other.
Summary by CodeRabbit
New Features
Bug Fixes