@Override
  public void clear() {
    nodesLock.lock();
    try {
      for (WALNode walNode : identifier2Nodes.values()) {
        walNode.close();
      }
      identifier2Nodes.clear();
    } finally {
      nodesLock.unlock();
    }
  }
This strategy creates one wal node for each unique identifier. In other words, each identifier
(like data region) has its own wal node.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/allocation/FirstCreateStrategy.java