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https://issues.apache.org/jira/browse/HBASE-18158?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16036815#comment-16036815
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Anastasia Braginsky commented on HBASE-18158:
---------------------------------------------
I understand that the concern is that the first compaction thread got to be
interrupted, but didn't really stopped yet and the second one succeed in CAS on
inMemoryFlushInProgress and is already starting. I also understand that two
those threads have different VersionedList of segments in pipeline, meaning
also different VersionedList objects. If the first compaction thread still
succeeds to do the swap on the pipeline (compaction), the second one will not
do anything due to different version number. So I still do not see how NPE can
happen. Would be happy for more clarifications...
> Two running in-memory compaction threads may lose data
> ------------------------------------------------------
>
> Key: HBASE-18158
> URL: https://issues.apache.org/jira/browse/HBASE-18158
> Project: HBase
> Issue Type: Bug
> Affects Versions: 2.0.0
> Reporter: Chia-Ping Tsai
> Assignee: Chia-Ping Tsai
> Fix For: 2.0.0
>
> Attachments: HBASE-18158.v0.patch
>
>
> {code:title=CompactingMemStore.java|borderStyle=solid}
> private void stopCompaction() {
> if (inMemoryFlushInProgress.get()) {
> compactor.stop();
> inMemoryFlushInProgress.set(false);
> }
> }
> {code}
> The stopCompaction() set inMemoryFlushInProgress to false so there may be two
> in-memory compaction threads which execute simultaneously. If there are two
> running InMemoryFlushRunnable, the later InMemoryFlushRunnable may change the
> versionedList.
> {code:title=MemStoreCompactor.java|borderStyle=solid}
> public boolean start() throws IOException {
> if (!compactingMemStore.hasImmutableSegments()) { // no compaction on
> empty pipeline
> return false;
> }
> // get a snapshot of the list of the segments from the pipeline,
> // this local copy of the list is marked with specific version
> versionedList = compactingMemStore.getImmutableSegments();
> {code}
> And the first InMemoryFlushRunnable will use the chagned versionedList to
> remove the corresponding segments.
> {code:title=MemStoreCompactor#doCompaction|borderStyle=solid}
> if (!isInterrupted.get()) {
> if (resultSwapped = compactingMemStore.swapCompactedSegments(
> versionedList, result, (action==Action.MERGE))) {
> // update the wal so it can be truncated and not get too long
> compactingMemStore.updateLowestUnflushedSequenceIdInWAL(true); //
> only if greater
> }
> }
> {code}
> In conclusion, first InMemoryFlushRunnable will remove the worng segment. And
> the later InMemoryFlushRunnable may introduce NPE because first
> InMemoryFlushRunnable set versionedList to null after compaction.
> {code}
> Exception in thread
> "RpcServer.default.FPBQ.Fifo.handler=3,queue=0,port=45712-inmemoryCompactions-1496563908038"
> java.lang.NullPointerException
> at
> org.apache.hadoop.hbase.regionserver.CompactionPipeline.swap(CompactionPipeline.java:119)
> at
> org.apache.hadoop.hbase.regionserver.CompactingMemStore.swapCompactedSegments(CompactingMemStore.java:283)
> at
> org.apache.hadoop.hbase.regionserver.MemStoreCompactor.doCompaction(MemStoreCompactor.java:212)
> at
> org.apache.hadoop.hbase.regionserver.MemStoreCompactor.start(MemStoreCompactor.java:122)
> at
> org.apache.hadoop.hbase.regionserver.CompactingMemStore.flushInMemory(CompactingMemStore.java:388)
> at
> org.apache.hadoop.hbase.regionserver.CompactingMemStore$InMemoryFlushRunnable.run(CompactingMemStore.java:500)
> at
> java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
> {code}
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