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From "Alex Feinberg (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, 04 May 2012 02:16:48 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5494?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13268042#comment-13268042
] 

Alex Feinberg commented on HBASE-5494:
--------------------------------------

Re: "One thing we'd like to prevent is a table being disabled while splits (or merges) are
going on. How hard would it be to add this facility (in another jira?). One way of doing it
would be that a regionserver before splitting, it'd take out the table lock. That woul prevent
the table from being disabled. But what about the case if two regionservers try to split a
region from the same table at the one time? Or, what if the regionserver dies mid-split; the
lock will be stuck in place."

This is an interesting question. I think one approach may be to create a region level lock
manager, and to convert the table-level lock manager to support read-write locks. Schema modifications
(create/disable/alter/delete/) would acquire a table-wide read lock (as now). For splits and
merges, region servers would acquire a table wide _read lock_ (to allow two regionserves to
split differnet regions of a table at the same time, but prevent schema modifications during
a split/merge), and a write lock (i.e., a usual lock) over the regions that are being split
(I'm not even sure if this step is even needed at this point).

We also need a way to handle stuck locks (currently DistributedLock uses persistent ZNodes)
after crashes with minimal (if any) manual intervention (key thing being that whatever schema-modification
was started prior to the crash is safely rolled back -- which may be non-trivial, as I would
guess it would more complex than just keeping a txn id in the log and then reading through
the HLog for META). 
                
> 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
>         Attachments: D2997.3.patch, D2997.4.patch
>
>
> 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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