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From "Phabricator (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5344) [89-fb] Scan unassigned region directory on master failover
Date Tue, 14 Feb 2012 22:40:02 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5344?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13208089#comment-13208089

Phabricator commented on HBASE-5344:

mbautin has commented on the revision "[jira] [HBASE-5344] [89-fb] Scan unassigned region
directory on master failover".

  The bigger context of our current/planned changes in 89-fb master is as follows. In 89-fb,
region assignments happen as responses to RS -> master RPC, and RSs communicate success
of region open operations back to the master through ZK. The master then writes the new assignments
to META. ZK is the only piece in the picture that could be considered a trusted highly-available
source of truth for the region assignment, if only it had all assignments. Currently the region
assignment can be obtained from the combination of META and ZK's UNASSIGNED directory. We
have a plan to move towards always having the full assignment in ZK (the UNASSIGNED directory
will change its meaning then) to help guarantee that we never have a duplicate assignment
and to have only one source of truth for assignment. We will also keep writing the region
assignment to META for client backward-compatibility. Even though the master failover fix
does not depend on those planned changes, I thought it would b
 e useful to mention them here.

  Contacting all regionservers directly to get the region assignment is probably useful as
a sanity-check, but it is not scalable, and is subject to unpredictable timeouts in the worst
case. We would like to rely on ZK and (for now) on META instead to recover the region assignment
on master startup/failure. Also, by the way, we are planning to unify master startup on a
fresh cluster start and failover and everything in between, and use the same logic to build
a coherent picture of region assignment.


> [89-fb] Scan unassigned region directory on master failover
> -----------------------------------------------------------
>                 Key: HBASE-5344
>                 URL: https://issues.apache.org/jira/browse/HBASE-5344
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Mikhail Bautin
>            Assignee: Mikhail Bautin
>         Attachments: D1605.1.patch
> In case the master dies after a regionserver writes region state as OPENED or CLOSED
in ZK but before the update is received by master and written to meta, the new master that
comes up has to pick up the region state from ZK and write it to meta. Otherwise we can get
multiply-assigned regions.

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