[
https://issues.apache.org/jira/browse/ZOOKEEPER-962?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12979623#action_12979623
]
Camille Fournier commented on ZOOKEEPER-962:
--------------------------------------------
I made a reviewboard for this myself acutally,
https://reviews.apache.org/r/253/, I guess it didn't get emailed to the group
properly? I'll cancle it and use yours since we've already got comments in
there.
Let me see if I understand what you're getting at:
We see snap to Z0. We start to see the toBeApplied proposals being forwarded,
and we write them to the log, but before we get the UPTODATE (and thus write
the snap file to disk), we crash. So we have a log file with Z1, but no snap
file with the older data.
Yes, you're right, that's a nasty problem.
Are you proposing for all packets to UPTODATE in all syncWithLeader scenarios,
we process them inside SyncWithLeader, or just the ones for snap?
I think I can twiddle my test to catch this error, let me look at it. I'm not
sure how much time I will have to actually make the fix you are proposing,
though, so if you have time to try to add it onto my patch please let me know.
> 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
> Assignee: Camille Fournier
> Priority: Critical
> Fix For: 3.3.3, 3.4.0
>
> Attachments: ZOOKEEPER-962.patch
>
>
> 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.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
