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From "stack (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5494) Introduce a zk hosted table-wide read/write lock so only one table operation at a time
Date Fri, 23 Mar 2012 18:03:28 GMT

    [ 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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