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https://issues.apache.org/jira/browse/RATIS-2547?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Tsz-wo Sze reassigned RATIS-2547:
---------------------------------
Component/s: Leader
(was: server)
Assignee: Yongzao Dan
> Advance repliedIndex before completing held write replies in ReplyFlusher
> -------------------------------------------------------------------------
>
> Key: RATIS-2547
> URL: https://issues.apache.org/jira/browse/RATIS-2547
> Project: Ratis
> Issue Type: Bug
> Components: Leader
> Reporter: Yongzao Dan
> Assignee: Yongzao Dan
> Priority: Major
> Time Spent: 20m
> Remaining Estimate: 0h
>
> When `raft.server.read.option` uses `REPLIED_INDEX`, `ReplyFlusher` batches
> write replies and advances the leader's replied index. The current
> implementation invokes each held reply callback while computing the max
> flushed log index. That means a write reply can be completed and returned to
> the client before `ReplyFlusher` advances `repliedIndex` to include the
> corresponding log index.
> This ordering creates a race for read-after-write workloads. A client may
> observe a successful write reply, then issue a linearizable read routed
> through `REPLIED_INDEX` while the leader's `repliedIndex` still lags behind
> the completed write. The read can wait unnecessarily or time out even though
> the write reply has already been released.
> Proposed fix:
> - Store the log index separately from the reply completion callback in
> `ReplyFlusher`.
> - Compute the max flushed log index without completing replies.
> - Advance `repliedIndex` first.
> - Complete the held reply callbacks only after `repliedIndex` has been
> updated.
> - Make `LeaderTracer.removePendingRequest` tolerate a null pending request,
> since some code paths can remove no pending request while still completing
> cached replies.
> Test plan:
> - Add `TestReplyFlusher#testAdvanceRepliedIndexBeforeCompletingReplies`.
> - Exercise the replied-index follower read path via
> `TestLinearizableReadRepliedIndexWithGrpc#testFollowerLinearizableReadParallel`.
> - Run `mvn -pl ratis-server,ratis-test -am
> -Dtest=TestReplyFlusher,TestLinearizableReadRepliedIndexWithGrpc test`.
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