@Override
    public void postAppend(long entryLen, long elapsedTimeMillis, WALKey logKey, WALEdit logEdit)
      throws IOException {
      for (int k = 0; k < 4; k++) {
        if (!walKeysCorrect[k]) {
          walKeysCorrect[k] = Arrays.equals(SimpleRegionObserver.WAL_EXTENDED_ATTRIBUTE_BYTES,
            logKey.getExtendedAttribute(Integer.toString(k + 1)));
        }
      }
    }
Tests overriding compaction handling via coprocessor hooks
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/TestRegionObserverInterface.java