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From "Scott Fines (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-12565) Race condition in HRegion.batchMutate() causes partial data to be written when region closes
Date Mon, 24 Nov 2014 21:56:12 GMT
Scott Fines created HBASE-12565:
-----------------------------------

             Summary: Race condition in HRegion.batchMutate()  causes partial data to be written
when region closes
                 Key: HBASE-12565
                 URL: https://issues.apache.org/jira/browse/HBASE-12565
             Project: HBase
          Issue Type: Bug
          Components: Performance, regionserver
    Affects Versions: 0.98.6
            Reporter: Scott Fines


The following sequence of events is possible to occur in HRegion's batchMutate() call:

1. caller attempts to call HRegion.batchMutate() with a batch of N>1 records
2. batchMutate acquires region lock in startRegionOperation, then calls doMiniBatchMutation()
3. doMiniBatchMutation acquires one row lock
4. Region closes
5. doMiniBatchMutation attempts to acquire second row lock.

When this happens, the lock acquisition will also attempt to acquire the region lock, which
fails (because the region is closing). At this stage, doMiniBatchMutation will stop writing
further, BUT it WILL write data for the rows whose locks have already been acquired, and advance
the index in MiniBatchOperationInProgress. Then, after it terminates successfully, batchMutate()
will loop around a second time, and attempt AGAIN to acquire the region closing lock. When
that happens, a NotServingRegionException is thrown back to the caller.

Thus, we have a race condition where partial data can be written when a region server is closing.

The main problem stems from the location of startRegionOperation() calls in batchMutate and
doMiniBatchMutation():

1. batchMutate() reacquires the region lock with each iteration of the loop, which can cause
some successful writes to occur, but then fail on others
2. getRowLock() attempts to acquire the region lock once for each row, which allows doMiniBatchMutation
to terminate early; this forces batchMutate() to use multiple iterations and results in condition
1 being hit.

There appears to be two parts to the solution as well:

1. open an internal path so that doMiniBatchMutation() can acquire row locks without checking
for region closure. This will have the added benefit of a significant performance improvement
during large batch mutations.
2. move the startRegionOperation() out of the loop in batchMutate() so that multiple iterations
of doMiniBatchMutation will not cause the operation to fail.



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