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From "Allan Yang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-18168) NoSuchElementException when rolling the log
Date Tue, 06 Jun 2017 08:46:18 GMT
Allan Yang created HBASE-18168:

             Summary: NoSuchElementException when rolling the log
                 Key: HBASE-18168
                 URL: https://issues.apache.org/jira/browse/HBASE-18168
             Project: HBase
          Issue Type: Bug
    Affects Versions: 1.1.11
            Reporter: Allan Yang
            Assignee: Allan Yang

Today, one of our server aborted due to the following log.
2017-06-06 05:38:47,142 ERROR [regionserver/xxxx.logRoller] regionserver.LogRoller: Log rolling
        at java.util.concurrent.ConcurrentSkipListMap$Iter.advance(ConcurrentSkipListMap.java:2224)
        at java.util.concurrent.ConcurrentSkipListMap$ValueIterator.next(ConcurrentSkipListMap.java:2253)
        at java.util.Collections.min(Collections.java:628)
        at org.apache.hadoop.hbase.regionserver.wal.FSHLog.findEligibleMemstoresToFlush(FSHLog.java:861)
        at org.apache.hadoop.hbase.regionserver.wal.FSHLog.findRegionsToForceFlush(FSHLog.java:886)
        at org.apache.hadoop.hbase.regionserver.wal.FSHLog.rollWriter(FSHLog.java:728)
        at org.apache.hadoop.hbase.regionserver.LogRoller.run(LogRoller.java:137)
        at java.lang.Thread.run(Thread.java:756)
2017-06-06 05:38:47,142 FATAL [regionserver/xxxx.logRoller] regionserver.HRegionServer: ABORTING
region server xxxx: Log rolling failed

The code is here: 
private byte[][] findEligibleMemstoresToFlush(Map<byte[], Long> regionsSequenceNums)
    List<byte[]> regionsToFlush = null;
    // Keeping the old behavior of iterating unflushedSeqNums under oldestSeqNumsLock.
    synchronized (regionSequenceIdLock) {
      for (Map.Entry<byte[], Long> e: regionsSequenceNums.entrySet()) {
        ConcurrentMap<byte[], Long> m =
        if (m == null) {
        long unFlushedVal = Collections.min(m.values()); //The exception is thrown here
The map 'm' is empty is the only reason I can think of why NoSuchElementException is thrown.
I then looked up all code related to the update of 'oldestUnflushedStoreSequenceIds'. All
update to 'oldestUnflushedStoreSequenceIds' is guarded by the synchronization of 'regionSequenceIdLock'
except here:
private ConcurrentMap<byte[], Long> getOrCreateOldestUnflushedStoreSequenceIdsOfRegion(
      byte[] encodedRegionName) {
    oldestUnflushedStoreSequenceIdsOfRegion =
        new ConcurrentSkipListMap<byte[], Long>(Bytes.BYTES_COMPARATOR);
    ConcurrentMap<byte[], Long> alreadyPut =
          oldestUnflushedStoreSequenceIdsOfRegion); // Here, a empty map may put to 'oldestUnflushedStoreSequenceIds'
with no synchronization
    return alreadyPut == null ? oldestUnflushedStoreSequenceIdsOfRegion : alreadyPut;

It should be a very rare bug. But it can lead to server abort. It only exists in branch-1.1.

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