@Override
  public void serialize(IWALByteBufferView buffer) {
    buffer.put(type.getCode());
  }
This entry class provides a signal to help wal buffer dealing with some special cases.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/buffer/WALSignalEntry.java