Jeffrey Zhong created HBASE-8365:
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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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