Peter Vary commented on HIVE-14979:

[~sershe], thanks for the review!

I am not a zookeeper expert so feel free to correct me if I am wrong somewhere.

This was my reasoning:
- Ephemeral nodes kept alive until the session is alive
- The session is alive until the client sending requests, or extra PING request 
if there is no other request. 

If the HiveServer2 is down and the session timeout is not yet reached, then 
even for the ephemeral nodes the locks will be there. In HiveConf the 
SessionTimeout is set to 1200000ms which seems pretty excessive to me, but set 
by HIVE-8890 (HiveServer2 dynamic service discovery: use persistent ephemeral 
nodes curator recipe). This means the ephemeral locks could stay there after 
the crash for 20 minutes. 

For this reason I think the administrator would need this removal tool, or we 
should set the timeout to a lower value.

[~vgumashta], [~thejas]: Is it possible to lower the default of the 
HIVE_ZOOKEEPER_SESSION_TIMEOUT, or there will be performance and/or stability 
issues if we change this value?


> Removing stale Zookeeper locks at HiveServer2 initialization
> ------------------------------------------------------------
>                 Key: HIVE-14979
>                 URL: https://issues.apache.org/jira/browse/HIVE-14979
>             Project: Hive
>          Issue Type: Improvement
>          Components: Locking
>            Reporter: Peter Vary
>            Assignee: Peter Vary
>         Attachments: HIVE-14979.patch
> HiveServer2 could use Zookeeper to store token that indicate that particular 
> tables are locked with the creation of persistent Zookeeper objects. 
> A problem can occur when a HiveServer2 instance creates a lock on a table and 
> the HiveServer2 instances crashes ("Out of Memory" for example) and the locks 
> are not released in Zookeeper. This lock will then remain until it is 
> manually cleared by an admin.
> There should be a way to remove stale locks at HiveServer2 initialization, 
> helping the admins life.

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