Skip oversized /sync event replay and refresh watched channels - #6652
Skip oversized /sync event replay and refresh watched channels#6652aleksandar-apostolov wants to merge 2 commits into
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A /sync response can sit below the backend cap and still be too expensive to replay: every event goes through the state and persistence pipeline, holding it long enough to slow down the requests that also need it. Above 250 events, skip the replay and refresh the actively watched channels with queryChannels instead, which carries the latest channel state, messages, members and read state and re-registers the watch. The existing HTTP 400 too-many-events branch now takes the same path instead of refreshing nothing. Channels that are not actively watched wait for a future explicit query. The limit stays internal - it is a performance guard, not a knob.
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Walkthrough
ChangesSync recovery
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🟠 High · up to The PR skips replay for oversized sync payloads and advances the sync checkpoint after a fallback refresh that may fail or omit channels. This could permanently skip events and leave watched channels stale, so merge should be blocked until checkpoint advancement is gated on complete recovery or reliable retry state. Sequence Diagram(s)sequenceDiagram
participant SyncManager
participant SyncAPI
participant queryChannelsInternal
participant ChannelLogic
participant Persistence
SyncManager->>SyncAPI: Request /sync
SyncAPI-->>SyncManager: Return events or too-many-events error
SyncManager->>queryChannelsInternal: Refresh watched channels in batches
queryChannelsInternal-->>SyncManager: Return channel results
SyncManager->>ChannelLogic: Apply channel updates
ChannelLogic->>Persistence: Persist refreshed channels
SyncManager->>SyncManager: Advance sync timestamp
Suggested reviewers: Poem
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Actionable comments posted: 1
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Inline comments:
In
`@stream-chat-android-client/src/main/java/io/getstream/chat/android/client/internal/state/sync/internal/SyncManager.kt`:
- Around line 342-347: Update SyncManager at
stream-chat-android-client/src/main/java/io/getstream/chat/android/client/internal/state/sync/internal/SyncManager.kt
lines 342-347 and 398-401 so updateLastSyncedDate runs only when
refreshActiveWatchedChannels confirms every targeted watched channel was
refreshed; retain the prior checkpoint for failed batches or partial responses,
preserving idempotent retry behavior and adding deterministic coverage for both
cases.
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stream-chat-android-client/src/main/java/io/getstream/chat/android/client/internal/state/sync/internal/SyncManager.ktstream-chat-android-client/src/test/java/io/getstream/chat/android/client/internal/state/internal/SyncManagerTest.kt
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SDK Size Comparison 📏
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The oversized-payload and too-many-events paths advanced lastSyncedAt even when the queryChannels fallback failed, so the events they skipped could never be replayed and the watched channels were left stale with no way back. refreshActiveWatchedChannels now reports failure, and both callers keep the previous checkpoint on it - the next reconnect asks for the same range and retries. Having nothing to refresh stays a success: with no watched channels nothing visible can go stale, and failing there would re-fetch the payload on every reconnect forever. A cid the server does not return is not a failure either, since a deleted or invisible channel would pin the checkpoint permanently.
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andremion
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andremion
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Nice, the skip logic reads clearly. One concern on the 400 branch and a scope question on the fallback, both inline.
| // succeeded, otherwise the skipped events would become unrecoverable. | ||
| logger.e { "[performSync] failed (too many events to sync): $result" } | ||
| val refresh = refreshActiveWatchedChannels() | ||
| if (refresh !is Result.Success) { |
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This can leave a permanent 400 stuck. Before, the branch always advanced lastSyncedAt, so the next /sync asked for a fresh range. Now if the refresh fails nothing moves, and since isTooManyEventsToSyncError() is just 400 plus validation error, the same request keeps failing on every reconnect. I ran three reconnects locally with a failing queryChannelsInternal and lastSyncedAt never moved.
The hold-on-failure rule makes sense in skipEventReplay, where you can ask for the payload again. Here the server already refused it, so could this branch keep the unconditional updateLastSyncedDate(now())? Happy to be told otherwise.
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Answering the "happy to be told otherwise": I don't think unconditional advance is right either, though the stuck-400 concern is real.
iOS holds the checkpoint on fallback failure in this same 400 path. syncMissingEvents routes the 400 into handleSyncEventReplayFallback (SyncRepository.swift:417), and on .failure that completes without ever calling updateLastSyncAt (:459-461). So the hold isn't an Android deviation.
What keeps it from wedging over there is two things we don't have:
maximumDaysSinceLastSync = 30: whenlastSyncAtis older than 30 days it skips/syncaltogether and advances the checkpoint unconditionally (SyncRepository.swift:291-304). That's the backstop.SyncError.failedFetchingChannels.shouldRetry == true, so the operation retries twice within the same reconnect before giving up (:18-19,SyncOperations.swift:20).
On Android there's no equivalent. Every lastSyncedAt write is on the /sync path, and syncMaxThreshold only gates the channel/message/reaction retries (lines 768-930), never the checkpoint. So I'd rather add the max-age backstop than make this branch advance unconditionally — otherwise we diverge from the implementation we're porting and still have no floor under a stuck checkpoint.
One thing that makes your concern worse than it looks: isTooManyEventsToSyncError() is just 400 + serverErrorCode == VALIDATION_ERROR (NetworkError.kt:36). Any /sync validation error — bad cid, malformed date — now holds the checkpoint, not only genuinely oversized ranges.
| logger.e { "[skipEventReplay] fallback refresh failed, keeping lastSyncedAt: ${refresh.errorOrNull()}" } | ||
| return emptySet() | ||
| } | ||
| val skippedLatestEvent = sortedEvents.last() |
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Not blocking: the replay path calls updateAllReadStateForDate for each MarkAllReadEvent, this one drops them. Looks low impact since markedAllReadAt is only stored and compared, but a quick scan of sortedEvents before discarding would keep the two paths the same. Worth it?
| val updatedCids = result.value | ||
| logger.v { "[restoreActiveChannels] standardCids.size: ${result.value.size}" } | ||
| updateActiveChannels(recoverAll, updatedCids + groupedHandledCids) | ||
| updateActiveChannels(recoverAll, updatedCids + groupedHandledCids + alreadyRefreshedCids) |
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alreadyRefreshedCids only gets to updateActiveChannels. rewatchTrackedWatchedChannels() on line 497 still goes over the whole tracked set and does one queryChannel per cid, so on the grouped path the channels the fallback just refreshed get fetched again one by one. The cidsToExclude param on refreshActiveWatchedChannels is also unused right now.
Could alreadyRefreshedCids be passed there too?
| val online = clientState.isOnline | ||
| val watchedCids = buildSet { | ||
| addAll(stateRegistry.getTrackedWatchedChannels()) | ||
| logicRegistry.getActiveChannelsLogic().mapTo(this) { it.cid } |
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getActiveChannelsLogic() is channels.values (LogicRegistry.kt:300), and that cache gets an entry for every channel in every queryChannels result (QueryChannelsStateLogic.kt:184). So this is every channel the list has paged through, not the ones on screen, and there is no cap. With 300 cids I get 10 sequential queryChannelsInternal calls, each with watch=true and a full state write.
That can cost more than the 251 events it avoids, and it re-watches channels the user is not viewing. Any reason not to cap it at 30 like updateActiveChannels does, or limit it to getTrackedWatchedChannels() plus channels with recoveryNeeded?
| foundChannels.forEach { channel -> | ||
| ChannelId.fromTypeAndId(channel.type, channel.id) | ||
| ?.let(logicRegistry::channel) | ||
| ?.updateDataForChannel(channel, channel.messages.size) |
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Small thing: shouldRefreshMessages stays false here, so this takes the upsertMessages branch (ChannelLogicImpl.kt:346), which adds and updates but never removes. A message hard-deleted during the skipped range stays in local state, and the checkpoint has already moved past the message.deleted event. rewatchTrackedWatchedChannels sets shouldRefresh = true for this reason.
A full replace resets scroll position though, so not obvious either way. Which do you prefer?
| private val userPresence: Boolean, | ||
| private val isAutomaticSyncOnReconnectEnabled: Boolean, | ||
| private val syncMaxThreshold: TimeDuration, | ||
| private val eventReplayMaxCount: Int = SYNC_EVENT_REPLAY_MAX_COUNT, |
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Small thing: only tests pass this. Could it get @VisibleForTesting like performSync?
| verify(chatClient).queryChannelsInternal(capture()) | ||
| val filter = firstValue.filter as InFilterObject | ||
| assertEquals(setOf(watchedChannel.cid), filter.values) | ||
| assertEquals(true, firstValue.watch) |
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Small thing: state and watch both default to true on QueryChannelsRequest, so these two pass whatever the fallback does. If they matter here, maybe set them explicitly in the request?
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Most of what I had queued up is already covered by @andremion's review, so I've trimmed this down to what isn't.
Two inline notes: the fallback being inert at cold start (non-blocking, and probably an iOS-side design question), and the batch loop not breaking or rolling back on a failed request.
Plus a reply on the 400-branch thread — iOS holds the checkpoint there too, so I think the fix is the max-age backstop we're missing rather than an unconditional advance.
| * | ||
| * @return The channel ids that were refreshed, empty when the refresh failed. | ||
| */ | ||
| private suspend fun skipEventReplay(sortedEvents: List<ChatEvent>): Set<String> { |
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Not blocking, and I suspect this is an iOS-side design question rather than something to fix here.
The fallback is inert in exactly the case that triggers it. The "no active cids found" branch above is only reached when getActiveChannelsLogic() is empty — so when the fallback re-reads that same source it contributes nothing by construction, and watchedCids reduces to getTrackedWatchedChannels(), itself empty before any screen has subscribed. Cold start after a long offline stretch therefore gives Success(emptySet()): the checkpoint jumps to the newest skipped event and nothing is refreshed.
The impact is mild — those channels have no state objects and aren't on screen, so the cost is a stale Room cache until the next query refreshes them. But it does mean the KDoc's "the actively watched channels are refreshed with queryChannels instead" doesn't hold for the main trigger of an oversized payload, which is worth saying out loud somewhere.
iOS is inert here too: refreshActiveWatchedChannels collects from the same kind of weak live-controller set (SyncRepository.swift:320-323), so this is a shared gap rather than an Android deviation. Probably one to raise with them rather than diverge on quietly.
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| val failed = AtomicReference<Error>() | ||
| val refreshedCids = mutableSetOf<String>() | ||
| watchedCids.chunked(QUERY_CHANNELS_MAX_LIMIT).forEach { batch -> |
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A failed batch neither breaks the loop nor rolls back. The remaining ceil(n/30) requests still fire after a known failure, failed.set() keeps only the last error, and — because the function then returns Result.Failure — the batches that did succeed are dropped from the return value, so restoreActiveChannels re-queries them.
iOS breaks on the first error instead of recursing into the remaining batches: startWatchingChannelsInBatches returns .failure(.failedFetchingChannels) immediately (SyncRepository.swift:478-482). Doing the same here avoids both the wasted requests and the redundant re-query — in the path this PR exists to make cheaper.
| // succeeded, otherwise the skipped events would become unrecoverable. | ||
| logger.e { "[performSync] failed (too many events to sync): $result" } | ||
| val refresh = refreshActiveWatchedChannels() | ||
| if (refresh !is Result.Success) { |
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Answering the "happy to be told otherwise": I don't think unconditional advance is right either, though the stuck-400 concern is real.
iOS holds the checkpoint on fallback failure in this same 400 path. syncMissingEvents routes the 400 into handleSyncEventReplayFallback (SyncRepository.swift:417), and on .failure that completes without ever calling updateLastSyncAt (:459-461). So the hold isn't an Android deviation.
What keeps it from wedging over there is two things we don't have:
maximumDaysSinceLastSync = 30: whenlastSyncAtis older than 30 days it skips/syncaltogether and advances the checkpoint unconditionally (SyncRepository.swift:291-304). That's the backstop.SyncError.failedFetchingChannels.shouldRetry == true, so the operation retries twice within the same reconnect before giving up (:18-19,SyncOperations.swift:20).
On Android there's no equivalent. Every lastSyncedAt write is on the /sync path, and syncMaxThreshold only gates the channel/message/reaction retries (lines 768-930), never the checkpoint. So I'd rather add the max-age backstop than make this branch advance unconditionally — otherwise we diverge from the implementation we're porting and still have no floor under a stuck checkpoint.
One thing that makes your concern worse than it looks: isTooManyEventsToSyncError() is just 400 + serverErrorCode == VALIDATION_ERROR (NetworkError.kt:36). Any /sync validation error — bad cid, malformed date — now holds the checkpoint, not only genuinely oversized ranges.
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Goal
Closes AND-1196
Every event returned by
/syncon reconnect is replayed through the shared state and persistence pipeline. A payload can sit below the backend cap and still be expensive enough to hold that pipeline, delaying the other requests that need it. The existing HTTP 400 "too many events" branch advances the last-synced timestamp and refreshes nothing, leaving visible channels stale.Implementation
SyncManagerskips event replay when a/syncpayload exceedsSYNC_EVENT_REPLAY_MAX_COUNT(250), refreshes the actively watched channels viaqueryChannelsinstead, then moves the last-synced timestamp to the newest skipped event.isTooManyEventsToSyncError()(HTTP 400) branch now takes the same fallback before its existingupdateLastSyncedDate(now()).refreshActiveWatchedChannels(cidsToExclude)— tracked watched cids plus active channel logic cids, chunked by 30 (thequeryChannelspage cap),state/watchon,presencefollowinguserPresence. Logs any cid it could not refresh instead of truncating silently.performSyncreturns the refreshed cids;restoreActiveChannels(alreadyRefreshedCids)excludes them so recovery does not query the same channels twice.apiCheckpasses.🎨 UI Changes
No visual changes — reconnect recovery behavior only.
Testing
:stream-chat-android-client:testDebugUnitTest --tests "*SyncManagerTest"— new cases: oversized payload bails out and refreshes, payload size equal to the limit still replays, HTTP 400 branch refreshes, fallback batches 35 cids into 30 + 5.:stream-chat-android-client:testDebugUnitTest,detekt,spotlessCheck,apiCheckgreen.compileDebugUnitTestKotlingreen across all modules.☑️ Contributor Checklist
developSummary by CodeRabbit
Bug Fixes
Tests