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