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From "Jeffrey Zhong (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-8365) Duplicated ZK notifications cause Master abort (or other unknown issues)
Date Wed, 17 Apr 2013 20:57:15 GMT
Jeffrey Zhong created HBASE-8365:
------------------------------------

             Summary: Duplicated ZK notifications cause Master abort (or other unknown issues)
                 Key: HBASE-8365
                 URL: https://issues.apache.org/jira/browse/HBASE-8365
             Project: HBase
          Issue Type: Bug
    Affects Versions: 0.94.6
            Reporter: Jeffrey Zhong


The duplicated ZK notifications should happen in trunk as well. Since the way we handle ZK
notifications is different in trunk, we don't see the issue there. I'll explain later.

The issue is causing TestMetaReaderEditor.testRetrying flaky with error message {code}reader:
count=2, t=null{code} A related link is at https://builds.apache.org/job/HBase-0.94/941/testReport/junit/org.apache.hadoop.hbase.catalog/TestMetaReaderEditor/testRetrying/

The test case failure is due to an IllegalStateException and master is aborted so the rest
test cases also failed after testRetrying.

Below are steps why the issue is happening(region fa0e7a5590feb69bd065fbc99c228b36 is in interests):

1) Got first notification event RS_ZK_REGION_FAILED_OPEN at 2013-04-04 17:39:01,197
{code} DEBUG [pool-1-thread-1-EventThread] master.AssignmentManager(744): Handling transition=RS_ZK_REGION_FAILED_OPEN,
server=janus.apache.org,42093,1365097126155, region=fa0e7a5590feb69bd065fbc99c228b36{code}

In the step, AM tries to open the region on another RS in a separate thread

2) Got second notification event RS_ZK_REGION_FAILED_OPEN at 2013-04-04 17:39:01,200 
{code}DEBUG [pool-1-thread-1-EventThread] master.AssignmentManager(744): Handling transition=RS_ZK_REGION_FAILED_OPEN,
server=janus.apache.org,42093,1365097126155, region=fa0e7a5590feb69bd065fbc99c228b36{code}

3) Later got opening notification event result from the step 1 at 2013-04-04 17:39:01,288

{code} DEBUG [pool-1-thread-1-EventThread] master.AssignmentManager(744): Handling transition=RS_ZK_REGION_OPENING,
server=janus.apache.org,54833,1365097126175, region=fa0e7a5590feb69bd065fbc99c228b36{code}

Step 2 ClosedRegionHandler throws IllegalStateException because "Cannot transit it to OFFLINE"(state
is in opening from notification 3) and abort Master. This could happen in 0.94 because we
handle notifications using executorService which opens the door to handle events out of order
through receive them in order of updates.

I've confirmed that we don't have duplicated AM listeners and both events triggered by same
ZK data of exact same version. The issue can be reproduced once by running testRetrying test
case 20 times in a loop.

There are several issues for the failure:

1) duplicated ZK notifications. Since ZK watcher is one time trigger, the duplicated notification
should not happen from the same data of the same version in the first place

2) ZooKeeper watcher handling is wrong in both 0.94 and trunk as following:
a) 0.94 handle notifications in async way which may lead to handle notifications out of order
of the events happened
b) In trunk, we handle ZK notifications synchronously which slows down other components such
as SSH, LogSplitting etc. because we have a single notification queue
c) In trunk & 0.94, we could use stale event data because we have a long listener list.
ZK node state could have changed at the time when handling the event. If a listener needs
to act upon latest state, it should re-fetch the data to verify if the data triggered the
handler hasn't changed.

Suggestions:
For 0.94, we can bandit the CloseRegionHandler to pass in the expected ZK data version to
skip event handling on stale data with min impact.

For trunk, I'll open an improvement JIRA on ZK notification handling to provide more parallelism
to handle unrelated notifications.

For the duplicated ZK notifications, we need bring some ZK experters to take a look at this.

Please let me know what you think or any better idea.
Thanks!


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