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https://issues.apache.org/jira/browse/HBASE-2952?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12905396#action_12905396
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Prakash Khemani commented on HBASE-2952:
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The shutdown hook prevents the zookeeper logs from getting flooded with
unnecessary "connection timed out" or such messages. If that is the case then
the shutdown hook still serves some good purpose. IMO the behavior ought to be
the following - users who properly call HTable::close on all the open Htables
should see this nice HCM shutdown hook behavior. Others who don’t call close()
will have their zk logs flooded. This goes to my earlier suggestion that
HTable::close should trigger HCM::close and there should be some kind of ref
counting in HCM.
> HConnectionManager's shutdown hook interferes with client's operations
> ----------------------------------------------------------------------
>
> Key: HBASE-2952
> URL: https://issues.apache.org/jira/browse/HBASE-2952
> Project: HBase
> Issue Type: Improvement
> Components: client
> Affects Versions: 0.20.0
> Reporter: Prakash Khemani
>
> My HBase client calls incrementColValue() in pairs. If someone kills the
> client (SIGINT or SIGTERM) I want my client's increment threads to gracefully
> exit. If a thread has already done one of the incrementColValue() then I want
> that thread to complete the other incrementColValue() and then exit.
> For this purpose I installed my own shutdownHook(). My shitdownHook() thread
> 'sugnals' all the threads in my process that it is time to exit and then
> waits for them to complete.
> The problem is that HConnectionManager's shutdownHook thread also runs and
> shuts down all connections and IPC threads.
> My increment thread keeps waiting to increment and then times out after 240s.
> Two problems with this - the incrementColValiue() didn't go through which
> will increase the chances of inconsistency in my HBase data. And it too 240s
> to exit. I am pasting some of the messages that the client thread outputs
> while it tries contact the HBase server.
> Signalled. Exiting ...
> 2010-09-01 12:11:14,769 DEBUG [HCM.shutdownHook]
> zookeeper.ZooKeeperWrapper(787):
> <localhost:/hbase,org.apache.hadoop.hbase.client.HConnectionManager>Closed
> connection with ZooKeeper; /hbase/root-region-server
> flushing after 7899
> 2010-09-01 12:11:19,669 DEBUG [Line Processing Thread 0]
> client.HConnectionManager$TableServers(903): Cache hit for row <> in
> tableName .META.: location server hadoop2205.snc3.facebook.com:60020,
> location region name .META.,,1.1028785192
> 2010-09-01 12:11:19,671 INFO [Line Processing Thread 0]
> zookeeper.ZooKeeperWrapper(206): Reconnecting to zookeeper
> 2010-09-01 12:11:19,671 DEBUG [Line Processing Thread 0]
> zookeeper.ZooKeeperWrapper(212):
> <localhost:/hbase,org.apache.hadoop.hbase.client.HConnectionManager>Connected
> to zookeeper again
> 2010-09-01 12:11:24,679 DEBUG [Line Processing Thread 0]
> client.HConnectionManager$TableServers(964): Removed .META.,,1.1028785192 for
> tableName=.META. from cache because of content_action_url_metrics,\x080r&
> B\xF7\x81_T\x07\x08\x16uOrcom.gigya 429934274290948,99999999999999
> 2010-09-01 12:11:24,680 DEBUG [Line Processing Thread 0]
> client.HConnectionManager$TableServers(857): locateRegionInMeta attempt 0 of
> 4 failed; retrying after sleep of 5000 because: The client is stopped
> 2010-09-01 12:11:24,680 DEBUG [Line Processing Thread 0]
> zookeeper.ZooKeeperWrapper(470):
> <localhost:/hbase,org.apache.hadoop.hbase.client.HConnectionManager>Trying to
> read /hbase/root-region-server
> 2010-09-01 12:11:24,681 DEBUG [Line Processing Thread 0]
> zookeeper.ZooKeeperWrapper(489):
> <localhost:/hbase,org.apache.hadoop.hbase.client.HConnectionManager>Read
> ZNode /hbase/root-region-server got 10.26.119.190:60020
> 2010-09-01 12:11:24,681 DEBUG [Line Processing Thread 0]
> client.HConnectionManager$TableServers(1116): Root region location changed.
> Sleeping.
> ===
> It might be a good idea to only run the HCM shutdown code when all the
> HTables referring to it have been closed. That way the client can control
> when the shutdown actually happens.
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