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From hanm <...@git.apache.org>
Subject [GitHub] zookeeper pull request #112: ZOOKEEPER-2355:Ephemeral node is never deleted ...
Date Tue, 13 Jun 2017 20:52:38 GMT
Github user hanm commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/112#discussion_r121793647
  
    --- Diff: src/java/main/org/apache/zookeeper/server/quorum/Learner.java ---
    @@ -390,6 +391,7 @@ else if (qp.getType() == Leader.SNAP) {
                                 + Long.toHexString(qp.getZxid()));
                         System.exit(13);
                     }
    +                zk.getZKDatabase().setlastProcessedZxid(qp.getZxid());
    --- End diff --
    
    The fix looks good to me.
    
    I think we should also set the zxid extracted from the current proposal packet after each
proposal is [committed](https://github.com/arshadmohammad/zookeeper/blob/master/src/java/main/org/apache/zookeeper/server/quorum/Learner.java#L460).
Otherwise the follower will have a lagged view of the committed transactions, because with
the fix in this patch, we will never do setlastProcessedZxid during a DIFF sync. For example
imagine a case like this:
    * Follower has its latest zxid with value a before DIFF SYNC happens.
    * Leader send over proposals with zxids value b, c, d. 
    * Follower received and applied proposals b and c. Before follower had a chance to get
hands on d, network partition happens.
    * Now partition healed, follower will do a DIFF think again. Because the zk database would
not be reloaded from logs (it's already initialized), follower has a skewed view of the world
- it thinks it only has tnx a, but in fact it has a, b, and c. So rather asking b, c, and
d, the follower could just ask d.
    
    Anyway I think it is an optimization that might worth doing - it is not functional critical
because the idempotent nature of applying transactions.


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