@Override
  protected void deserializeStateData(ProcedureStateSerializer serializer) throws IOException {
    super.deserializeStateData(serializer);
    RecoverStandbyStateData data = serializer.deserialize(RecoverStandbyStateData.class);
    serial = data.getSerial();
  }
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