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From "Brian Nixon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-3240) Close socket on Learner shutdown to avoid dangling socket
Date Sat, 12 Jan 2019 00:39:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-3240?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16740916#comment-16740916

Brian Nixon commented on ZOOKEEPER-3240:

[~hanm] could it be that the unclosed/unreaped Socket on the Learner side is still maintaining
its end of the tcp connection correctly via the protocol so the Leader is unable to sense
the change in Learner status through the status of the network connection? I confess that
I'm not as knowledgeable about the workings of Socket as I need to be to confirm this theory.

> Close socket on Learner shutdown to avoid dangling socket
> ---------------------------------------------------------
>                 Key: ZOOKEEPER-3240
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3240
>             Project: ZooKeeper
>          Issue Type: Improvement
>          Components: server
>    Affects Versions: 3.6.0
>            Reporter: Brian Nixon
>            Priority: Minor
>              Labels: pull-request-available
>          Time Spent: 50m
>  Remaining Estimate: 0h
> There was a Learner that had two connections to the Leader after that Learner hit an
unexpected exception during flush txn to disk, which will shutdown previous follower instance
and restart a new one.
> {quote}2018-10-26 02:31:35,568 ERROR [SyncThread:3:ZooKeeperCriticalThread@48] - Severe
unrecoverable error, from thread : SyncThread:3
> java.io.IOException: Input/output error
>         at java.base/sun.nio.ch.FileDispatcherImpl.force0(Native Method)
>         at java.base/sun.nio.ch.FileDispatcherImpl.force(FileDispatcherImpl.java:72)
>         at java.base/sun.nio.ch.FileChannelImpl.force(FileChannelImpl.java:395)
>         at org.apache.zookeeper.server.persistence.FileTxnLog.commit(FileTxnLog.java:457)
>         at org.apache.zookeeper.server.persistence.FileTxnSnapLog.commit(FileTxnSnapLog.java:548)
>         at org.apache.zookeeper.server.ZKDatabase.commit(ZKDatabase.java:769)
>         at org.apache.zookeeper.server.SyncRequestProcessor.flush(SyncRequestProcessor.java:246)
>         at org.apache.zookeeper.server.SyncRequestProcessor.run(SyncRequestProcessor.java:172)
> 2018-10-26 02:31:35,568 INFO  [SyncThread:3:ZooKeeperServerListenerImpl@42] - Thread
SyncThread:3 exits, error code 1
> 2018-10-26 02:31:35,568 INFO [SyncThread:3:SyncRequestProcessor@234] - SyncRequestProcessor
> It is supposed to close the previous socket, but it doesn't seem to be done anywhere
in the code. This leaves the socket open with no one reading from it, and caused the queue
full and blocked on sender.
> Since the LearnerHandler didn't shutdown gracefully, the learner queue size keeps growing,
the JVM heap size on leader keeps growing and added pressure to the GC, and cause high GC
time and latency in the quorum.
> The simple fix is to gracefully shutdown the socket.

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