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Don't unconditionally send consumer snapshot when becoming leader #4965

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Jan 18, 2024
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Don't unconditionally send consumer snapshot when becoming leader #4965

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10 changes: 0 additions & 10 deletions server/jetstream_cluster.go
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Original file line number Diff line number Diff line change
Expand Up @@ -4660,16 +4660,6 @@ func (js *jetStream) monitorConsumer(o *consumer, ca *consumerAssignment) {
js.setConsumerAssignmentRecovering(ca)
}

// Synchronize everyone to our state.
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Should we remove all together or adjust to make sure our upper state is correct?

It is there for a reason :)

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Do you remember what that reason was? I can't find anything written down as to why, the best I can find is from the commit message in cc77d66 which says:

Also sync other consumers when taking over as leader but no need to process snapshots when we are in fact the leader.

As far as I can tell everything should work without this because the Raft machinery would be keeping the ConsumerStore up-to-date on the replicas in the usual way anyway, whereas sending snapshots from the upper layer directly opens up a potential for a race between the committed Raft node state and the applied upper layer state, but I'm not sure if I'm missing something here.

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We were experiencing state skew, and I wanted us synched. But could have been from a prior bug that has since been corrected.

if isLeader && n != nil {
// Only send out if we have state.
if _, _, applied := n.Progress(); applied > 0 {
if snap, err := o.store.EncodedState(); err == nil {
n.SendSnapshot(snap)
}
}
}

// Process the change.
if err := js.processConsumerLeaderChange(o, isLeader); err == nil && isLeader {
doSnapshot(true)
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