Add defensive sequence number validation in replicated event persistence#2815
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Add defensive sequence number validation in replicated event persistence#2815He-Pin wants to merge 1 commit intoapache:mainfrom
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Replace the FIXME (akka#29259) comment in handleExternalReplicatedEventPersist with a proper defensive validation of the replica sequence number before updating seenPerReplica. The validation logs a warning when the incoming event's originSequenceNr does not match the expected next sequence number for that replica. This covers the gap scenario where events from a replica may arrive out of order via the replication stream (onReplicatedEvent path). The event is still persisted for backward compatibility — rejecting it could stall the replication stream. Key design decisions (confirmed by cross-review from GPT-5.4 and Sonnet 4.6): - Only gap detection (seqNr > expected) can fire from current callers; onPublishedEvent filters both duplicates and gaps before calling. onReplicatedEvent filters duplicates via alreadySeen() but allows gaps. - Uses != check (not separate < and > branches) to avoid dead code. - Warning message includes the advancing seqNr to help operators diagnose potential event loss. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Motivation
When
handleExternalReplicatedEventPersistreceives a replicated event, it updatesseenPerReplicawith the event'soriginSequenceNrwithout validating it matches the expected next sequence number. This was flagged as a FIXME referencing akka#29259.If events arrive out of order (e.g., seqNr 7 arrives when expecting 6),
seenPerReplicaadvances past the gap, and the missed event (seqNr 6) will later be filtered as "already seen" — permanently lost.Modification
originSequenceNr != expectedSeqNrfor the replicaonPublishedEvent: Rejects both duplicates and gaps before calling — only exact-match seqNr passesonReplicatedEvent: Filters duplicates viaalreadySeen(), but gaps may pass throughDesign Decision: Why
!=instead of separate<and>branchesAnalysis confirmed that:
< expectedSeqNr(duplicate) branch would be dead code — callers already filter duplicates> expectedSeqNr(gap) scenario can actually fire!=check avoids dead code while remaining defensive against future caller changesResult
References