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From "Himanshu Mishra (Jira)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-22687) Query hangs indefinitely if LLAP daemon registers after the query is submitted
Date Thu, 02 Jan 2020 21:01:00 GMT

    [ https://issues.apache.org/jira/browse/HIVE-22687?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17007080#comment-17007080
] 

Himanshu Mishra commented on HIVE-22687:
----------------------------------------

Thanks [~gopalv] for looking into this.

The patch reorders registration of {{slot-000}} and {{worker-000}} zookeeper nodes, ensuring
that slot zk node must be present when we are registering worker zk node.

On client side (Tez AM), we listen only on worker zk node and then schedule any queued task
in {{LlapTaskSchedulerService}}. To run the task we get list of all the running LLAP daemon
instances via {{LlapZookeeperRegistryImpl#getAllInstancesOrdered}}, which returns only those
worker instances whose slot zk node is already present. With the patch we'll always have associated
slot zk node present for the newly registered worker zk node, and this worker (LLAP Instance)
will definitely be returned by this method. This client side filtering was causing the issue
in first place because we were reaching this method even before the slot zk node got registered.

> Query hangs indefinitely if LLAP daemon registers after the query is submitted
> ------------------------------------------------------------------------------
>
>                 Key: HIVE-22687
>                 URL: https://issues.apache.org/jira/browse/HIVE-22687
>             Project: Hive
>          Issue Type: Bug
>          Components: llap
>    Affects Versions: 3.1.0
>            Reporter: Himanshu Mishra
>            Assignee: Himanshu Mishra
>            Priority: Major
>         Attachments: HIVE-22687.01.patch, HIVE-22687.02.patch
>
>
> If a query is submitted and no LLAP daemon is running, it waits for 1 minute and times
out with error {{SERVICE_UNAVAILABLE}}.
> While waiting, if a new LLAP Daemon starts, then the timeout is cancelled, and the tasks
do not get scheduled as well. As a result, the query hangs indefinitely.
> This is due to the race condition where LLAP Daemon first registers the LLAP instance
at {{.../workers/worker-0000}}, and afterwards registers {{.../workers/slot-0000}}. In the
gap between two, Tez AM gets notified of worker zk node and while processing it checks if
slot zk node is present, if not it rejects the LLAP Daemon. Error in Tez AM is:
> {code:java}
> [INFO] [LlapScheduler] |impl.LlapZookeeperRegistryImpl|: Unknown slot for 8ebfdc45-0382-4757-9416-52898885af90{code}



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