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  1. HBase
  2. HBASE-14004

[Replication] Inconsistency between Memstore and WAL may result in data in remote cluster that is not in the origin

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      Now when replicating a wal file which is still opened for write, we will get its committed length from the WAL instance in the same RS to prevent replicating uncommit WALEdit.

      This is very important if you use AsyncFSWAL, as we use fan-out in AsyncFSWAL. The data written to DN will be visible immediately as all DNs think it is the end of a pipeline, although the client has not received an ack, and also NN may truncate the file if the client crashes at the same time.
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      Now when replicating a wal file which is still opened for write, we will get its committed length from the WAL instance in the same RS to prevent replicating uncommit WALEdit. This is very important if you use AsyncFSWAL, as we use fan-out in AsyncFSWAL. The data written to DN will be visible immediately as all DNs think it is the end of a pipeline, although the client has not received an ack, and also NN may truncate the file if the client crashes at the same time.

      Description

      Looks like the current write path can cause inconsistency between memstore/hfile and WAL which cause the slave cluster has more data than the master cluster.

      The simplified write path looks like:
      1. insert record into Memstore
      2. write record to WAL
      3. sync WAL
      4. rollback Memstore if 3 fails

      It's possible that the HDFS sync RPC call fails, but the data is already (may partially) transported to the DNs which finally get persisted. As a result, the handler will rollback the Memstore and the later flushed HFile will also skip this record.

      ==================================

      This is a long lived issue. The above problem is solved by write path reorder, as now we will sync wal first before modifying memstore. But the problem may still exists as replication thread may read the new data before we return from hflush. See this document for more details:

      https://docs.google.com/document/d/11AyWtGhItQs6vsLRIx32PwTxmBY3libXwGXI25obVEY/edit#

      So we need to keep a sync length in WAL and tell replication wal reader this is limit when you read this wal file.

        Attachments

        1. HBASE-14004-v3.patch
          79 kB
          Duo Zhang
        2. HBASE-14004-v2.patch
          86 kB
          Duo Zhang
        3. HBASE-14004-v2.patch
          86 kB
          Duo Zhang
        4. HBASE-14004-v1.patch
          85 kB
          Duo Zhang
        5. HBASE-14004.patch
          84 kB
          Duo Zhang

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              • Assignee:
                zhangduo Duo Zhang
                Reporter:
                heliangliang He Liangliang
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