Andor Molnar created ZOOKEEPER-3182:
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Summary: Race condition when follower syncing with leader and
starting to server 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
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.
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 the capture the experiences and to put the
unit test on Ignore list, because currenty 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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