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Date: Fri, 23 Mar 2012 18:03:28 +0000 (UTC)
From: "stack (Commented) (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (HBASE-5494) Introduce a zk hosted table-wide
 read/write lock so only one table operation at a time
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    [ https://issues.apache.org/jira/browse/HBASE-5494?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13236857#comment-13236857 ] 

stack commented on HBASE-5494:
------------------------------

Thinking on this, I started to sketch a scheme where /unassigned in zk became instead /regions and under /regions there'd be a subdir per table and then under each table, you'd do something like what is described here http://zookeeper.apache.org/doc/r3.1.2/recipes.html#Shared+Locks only instead of read/write we'd have a sort of exclusive-access vs non-exclusive-access.

Already the above would be a massive refactor not to mind compounding it with the fact that doing an exclusive-operation, you'll usually want to do something like close all regions on a table (disabling) and to do that you'd need to be able to do non-exclusive operations...

Let me start over and make a little grid of what operations we do not want to run concurrently.  Will post here.   Might be able to get away w/ something less disruptive and more basic.
                
> Introduce a zk hosted table-wide read/write lock so only one table operation at a time
> --------------------------------------------------------------------------------------
>
>                 Key: HBASE-5494
>                 URL: https://issues.apache.org/jira/browse/HBASE-5494
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: stack
>
> I saw this facility over in the accumulo code base.
> Currently we just try to sort out the mess when splits come in during an online schema edit; somehow we figure we can figure all possible region transition combinations and make the right call.
> We could try and narrow the number of combinations by taking out a zk table lock when doing table operations.
> For example, on split or merge, we could take a read-only lock meaning the table can't be disabled while these are running.
> We could then take a write only lock if we want to ensure the table doesn't change while disabling or enabling process is happening.
> Shouldn't be too hard to add.

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