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From "Jimmy Xiang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8545) Meta stuck in transition when it is assigned to a just restarted dead region sever
Date Fri, 17 May 2013 22:13:16 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8545?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13661089#comment-13661089
] 

Jimmy Xiang commented on HBASE-8545:
------------------------------------

[~ram_krish], thanks for the review.

In patch v2, we just fail the region open, but don't change the znode to FAILED_OPEN. So the
SSH of the dead server will pick it up and assign it.  If we move it to FAILED_OPEN, AM could
be a little fast in assigning it.  However, this will be racing with SSH. It also violates
our assumption that only the target server can transition the znode. I think for now we can
keep it simple and not to make it FAILED_OPEN because this scenario is rare.


                
> Meta stuck in transition when it is assigned to a just restarted dead region sever 
> -----------------------------------------------------------------------------------
>
>                 Key: HBASE-8545
>                 URL: https://issues.apache.org/jira/browse/HBASE-8545
>             Project: HBase
>          Issue Type: Bug
>          Components: Region Assignment
>            Reporter: Jimmy Xiang
>            Assignee: Jimmy Xiang
>         Attachments: trunk-8545.patch, trunk-8545_v2.patch
>
>
> Support the meta region server is down, and the SSH tries to re-assign it.  This could
happen:
> 1. AM plans to assign meta to a region server (R_old);
> 2. Now R_old is dead, the new region server (R_new) starts up on the same host, port,
but gets a different start code;
> 3. AM sends the open region request to R_new and the Meta is opened on it;
> 4. AM gets ZK event, but it is from a different region server instance (R_new), not the
expected one (R_old), so it sends a close region request to R_new;
> 5. Now, the meta is stuck in transition and won't be assigned.
> This won't happen to a user region since the SSH for R_old will find out the user region
stuck in transition and re-assign it.  For meta, it is a little different.  AM checks if a
dead region server carries the meta based on the ZK info, which is changed to the new region
server R_new at step 3 by the open region handler.
> The fix I was thinking about is:
> 1. In checking if a region server carries a region, uses the region transition information
if it exists (which is the source of truth, to master), if not, checks the ZK data as before;
> 2. In open region handler, when transition assign zk node from offline to opening, make
sure the current region server is the expected one (ZK#transitionNode, existing code doesn't
check the target server name).

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