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https://issues.apache.org/jira/browse/ZOOKEEPER-962?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12977879#action_12977879
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Camille Fournier commented on ZOOKEEPER-962:
--------------------------------------------
Waiting for committedRequests to be empty gives correct behavior (actually, you
have to wait for all the queues in commit processor to be empty) , the thread
that sends commits to the follower is the same as the thread that sends
UPTODATE and the same thread processes commit requests as the UPTODATE. But as
soon as I can give you this patch, you'll have to take a look and tell me what
you think.
I have testcases, yes. They are painful and due to the nature of the bug they
don't always fail even if the bug exists, but I've run them enough locally to
be relatively confident that they cover the error well enough that my fixes are
at least correct, if not perfect. My actual fixes may not be ideal, but at
least the tests should allow people to try other approaches and see how they
work.
> leader/follower coherence issue when follower is receiving a DIFF
> -----------------------------------------------------------------
>
> Key: ZOOKEEPER-962
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-962
> Project: ZooKeeper
> Issue Type: Bug
> Components: server
> Affects Versions: 3.3.2
> Reporter: Camille Fournier
> Priority: Critical
> Fix For: 3.3.3, 3.4.0
>
>
> From mailing list:
> It seems like we rely on the LearnerHandler thread startup to capture all of
> the missing committed
> transactions in the SNAP or DIFF, but I don't see anything (especially in the
> DIFF case) that
> is preventing us for committing more transactions before we actually start
> forwarding updates
> to the new follower.
> Let me explain using my example from ZOOKEEPER-919. Assume we have quorum
> already, so the
> leader can be processing transactions while my follower is starting up.
> I'm a follower at zxid N-5, the leader is at N. I send my FOLLOWERINFO packet
> to the leader
> with that information. The leader gets the proposals from its committed log
> (time T1), then
> syncs on the proposal list (LearnerHandler line 267. Why? It's a copy of the
> underlying proposal
> list... this might be part of our problem). I check to see if the
> peerLastZxid is within my
> max and min committed log and it is, so I'm going to send a diff. I set the
> zxidToSend to
> be the maxCommittedLog at time T3 (we already know this is sketchy), and
> forward the proposals
> from my copied proposal list starting at the peerLastZxid+1 up to the last
> proposal transaction
> (as seen at time T1).
> After I have queued up all those diffs to send, I tell the leader to
> startFowarding updates
> to this follower (line 308).
> So, let's say that at time T2 I actually swap out the leader to the thread
> that is handling
> the various request processors, and see that I got enough votes to commit
> zxid N+1. I commit
> N+1 and so my maxCommittedLog at T3 is N+1, but this proposal is not in the
> list of proposals
> that I got back at time T1, so I don't forward this diff to the client.
> Additionally, I processed
> the commit and removed it from my leader's toBeApplied list. So when I call
> startForwarding
> for this new follower, I don't see this transaction as a transaction to be
> forwarded.
> There's one problem. Let's also imagine, however, that I commit N+1 at time
> T4. The maxCommittedLog
> value is consistent with the max of the diff packets I am going to send the
> follower. But,
> I still committed N+1 and removed it from the toBeApplied list before calling
> startFowarding
> with this follower. How does the follower get this transaction? Does it?
> To put it another way, here is the thread interaction, hopefully formatted so
> you can read
> it...
> LearnerHandlerThread
> RequestProcessorThread
> T1(LH): get list of proposals (COPY)
> T2(RPT): commit
> N+1, remove from toBeApplied
> T3(LH): get maxCommittedLog
> T4(LH): send diffs from view at T1
> T5(LH): startForwarding
> Or
> T1(LH): get list of proposals (COPY)
> T2(LH): get maxCommittedLog
> T3(RPT): commit
> N+1, remove from toBeApplied
> T4(LH): send diffs from view at T1
> T5(LH): startFowarding
> I'm trying to figure out what, if anything, keeps the requests from being
> committed, removed,
> and never seen by the follower before it fully starts up.
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