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Date: Sun, 22 Jan 2017 07:17:26 +0000 (UTC)
From: "Allan Yang (JIRA)" <jira@apache.org>
To: dev@hbase.apache.org
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Subject: [jira] [Created] (HBASE-17506) started mvcc transaction is not
 completed in branch-1
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archived-at: Sun, 22 Jan 2017 07:17:32 -0000

Allan Yang created HBASE-17506:
----------------------------------

             Summary: started mvcc transaction is not completed in branch-1
                 Key: HBASE-17506
                 URL: https://issues.apache.org/jira/browse/HBASE-17506
             Project: HBase
          Issue Type: Bug
    Affects Versions: 1.4.0
            Reporter: Allan Yang
            Assignee: Allan Yang


In {{doMiniBatchMutation}}, if it is in replay and if the the nonce of the mutation is different, we append them to a different wal.  But, after HBASE-14465, we start a mvcc transition in the ringbuffer's append thread.  So, every time we append a wal entry, we started a mvcc transition, but we didn't complete the mvcc transition anywhere. This can block other transition of this region.
{code}
// txid should always increase, so having the one from the last call is ok.
// we use HLogKey here instead of WALKey directly to support legacy coprocessors.
            walKey = new ReplayHLogKey(this.getRegionInfo().getEncodedNameAsBytes(),
              this.htableDescriptor.getTableName(), now, m.getClusterIds(),
              currentNonceGroup, currentNonce, mvcc);
            txid = this.wal.append(this.htableDescriptor,  this.getRegionInfo(),  walKey,
              walEdit, true);
            walEdit = new WALEdit(cellCount, isInReplay);
            walKey = null;
{code}

Looked at master branch, there is no such problem. It has a method named{{appendCurrentNonces}} :
{code}
 private void appendCurrentNonces(final Mutation mutation, final boolean replay,
       final WALEdit walEdit, final long now, final long currentNonceGroup, final long currentNonce)
   throws IOException {
     if (walEdit.isEmpty()) return;
     if (!replay) throw new IOException("Multiple nonces per batch and not in replay");
     WALKey walKey = new WALKey(this.getRegionInfo().getEncodedNameAsBytes(),
         this.htableDescriptor.getTableName(), now, mutation.getClusterIds(),
         currentNonceGroup, currentNonce, mvcc, this.getReplicationScope());
     this.wal.append(this.getRegionInfo(), walKey, walEdit, true);
     // Complete the mvcc transaction started down in append else it will block others
     this.mvcc.complete(walKey.getWriteEntry());
   }
{code}

Yes, the easiest way to fix branch-1 is to complete the writeEntry like master branch do. But is it really fine to do this?

1. Question 1:
complete the mvcc transition before waiting sync will create a disturbance of data visibility.

2.Question 2:
In what circumstance will there be different nonce and nonce group in a single wal entry? Nonce are used in append/increment. But in {{batchMuate}} ,we treat them differently and append one wal entry for each of them. So I think no test can reach this code path,  that maybe why no one has found this bug(Please tell me if I'm wrong).


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