@Override
  protected Writer createCombinedWriter(Writer localWriter, Writer remoteWriter) {
    // put remote writer first as usually it will cost more time to finish, so we write to it first
    return CombinedWriter.create(remoteWriter, localWriter);
  }
This method gets the pipeline for the current WAL.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/FSHLog.java