@Override
  public RecordWriter<ImmutableBytesWritable, Mutation> getRecordWriter(TaskAttemptContext context)
    throws IOException, InterruptedException {
    Configuration conf = context.getConfiguration();
    return new MultiTableRecordWriter(HBaseConfiguration.create(conf),
      conf.getBoolean(WAL_PROPERTY, WAL_ON));
  }
Writes an action (Put or Delete) to the specified table. the table being updated. the update,
either a put or a delete. if the action is not a put or a delete.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/MultiTableOutputFormat.java