[jira] [Assigned] (RATIS-2547) Advance repliedIndex before completing held write replies in ReplyFlusher

["Tsz-wo Sze (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/RATIS-2547?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

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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