@Override
  protected void serializeStateData(ProcedureStateSerializer serializer) throws IOException {
    super.serializeStateData(serializer);
    serializer.serialize(RecoverStandbyStateData.newBuilder().setSerial(serial).build());
  }
The procedure for replaying all the remote wals for transitting a sync replication peer from
STANDBY to DOWNGRADE_ACTIVE.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/replication/RecoverStandbyProcedure.java