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Posted to dev@zookeeper.apache.org by "Michael K. Edwards (JIRA)" <ji...@apache.org> on 2018/11/22 00:37:00 UTC
[jira] [Commented] (ZOOKEEPER-3182) Race condition when follower
syncing with leader and starting to serve requests
[ https://issues.apache.org/jira/browse/ZOOKEEPER-3182?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16695423#comment-16695423 ]
Michael K. Edwards commented on ZOOKEEPER-3182:
-----------------------------------------------
Is this fix needed for 3.5.5?
> Race condition when follower syncing with leader and starting to serve requests
> -------------------------------------------------------------------------------
>
> Key: ZOOKEEPER-3182
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3182
> Project: ZooKeeper
> Issue Type: Bug
> Components: server
> Affects Versions: 3.6.0
> Reporter: Andor Molnar
> Priority: Critical
>
> This issue is probably introduced by ZOOKEEPER-2024 where 2 seperate queues have been implemented in CommitProcessor to improve performance. [~abrahamfine] 's analysis is accurate on GitHub: https://github.com/apache/zookeeper/pull/300
> He was trying to introduce synchronization between Learner.syncWithLeader() and CommitProcessor to wait for in-flight requests to be committed before accepting client requests.
> In the affected unit test ({{testNodeDataChanged}}) there's a race between reconnecting client's setWatches request and updates coming from the leader according to the following logs:
> {noformat}
> 2018-10-25 13:59:58,556 [myid:] - DEBUG [FollowerRequestProcessor:1:CommitProcessor@424] - Processing request:: sessionid:0x10005d8fc4d0000 type:setWatches cxid:0x3 zxid:0xfffffffffffffffe txntype:unknown reqpath:n/a
> 2018-10-25 13:59:58,556 [myid:] - DEBUG [CommitProcWorkThread-1:FinalRequestProcessor@91] - Processing request:: sessionid:0x10005d8fc4d0000 type:setWatches cxid:0x3 zxid:0xfffffffffffffffe txntype:unknown reqpath:n/a
> ...
> 2018-10-25 13:59:58,557 [myid:] - DEBUG [CommitProcWorkThread-1:FinalRequestProcessor@91] - Processing request:: sessionid:0x20005d8f8a40000 type:delete cxid:0x1 zxid:0x100000004 txntype:2 reqpath:n/a
> ...
> 2018-10-25 13:59:58,561 [myid:] - DEBUG [CommitProcWorkThread-1:FinalRequestProcessor@91] - Processing request:: sessionid:0x20005d8f8a40000 type:create cxid:0x2 zxid:0x100000005 txntype:1 reqpath:n/a
> 2018-10-25 13:59:58,561 [myid:127.0.0.1:11231] - DEBUG [main-SendThread(127.0.0.1:11231):ClientCnxn$SendThread@864] - Got WatchedEvent state:SyncConnected type:NodeDeleted path:/test-changed for sessionid 0x10005d8fc4d0000
> {noformat}
> {{setWatches}} request is processed before {{delete}} and {{create}}, hence the client receives NodeDeleted event.
> In the working scenario it looks like:
> {noformat}
> 2018-10-25 14:04:55,247 [myid:] - DEBUG [CommitProcWorkThread-1:FinalRequestProcessor@91] - Processing request:: sessionid:0x20005dd88110000 type:delete cxid:
> 0x1 zxid:0x100000004 txntype:2 reqpath:n/a
> 2018-10-25 14:04:55,249 [myid:] - DEBUG [CommitProcWorkThread-1:FinalRequestProcessor@91] - Processing request:: sessionid:0x20005dd88110000 type:create cxid:
> 0x2 zxid:0x100000005 txntype:1 reqpath:n/a
> ...
> 2018-10-25 14:04:56,314 [myid:] - DEBUG [FollowerRequestProcessor:1:CommitProcessor@424] - Processing request:: sessionid:0x10005dd88110000 type:setWatches cxid:0x3 zxid:0xfffffffffffffffe txntype:unknown reqpath:n/a
> 2018-10-25 14:04:56,315 [myid:] - DEBUG [CommitProcWorkThread-1:FinalRequestProcessor@91] - Processing request:: sessionid:0x10005dd88110000 type:setWatches cxid:0x3 zxid:0xfffffffffffffffe txntype:unknown reqpath:n/a
> ...
> 2018-10-25 14:04:56,316 [myid:127.0.0.1:11231] - DEBUG [main-SendThread(127.0.0.1:11231):ClientCnxn$SendThread@842] - Got notification sessionid:0x10005dd88110000
> 2018-10-25 14:04:56,316 [myid:127.0.0.1:11231] - DEBUG [main-SendThread(127.0.0.1:11231):ClientCnxn$SendThread@864] - Got WatchedEvent state:SyncConnected type:NodeDataChanged path:/test-changed for sessionid 0x10005dd88110000
> {noformat}
> {{delete}} and {{create}} requests happen way before {{setWatches}} comes in (even before the client connection is established) and client receives NodeDataChanged event only.
> Abe's approach unfortunately raises the following concerns:
> - modifies CommitProcessor's code which might affect performance and correctness ([~shralex] raised on ZOOKEEPER-2807),
> - we experienced deadlocks while testing the patch: https://github.com/apache/zookeeper/pull/300
> As a consequence I raised this Jira to capture the experiences and to put the unit test on Ignore list, because currently I'm not sure about whether this is a real issue or a non-backward compatible change in 3.6 with the gain of a huge performance improvement.
> Either way I don't want this flaky test to influence contributions, so I'll mark as Ignored on trunk until the issue is resolved.
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