code: protected void handleCheckpoint(Tuple checkpointTuple, Action action, long txid) {
          collector.emit(CheckpointSpout.CHECKPOINT_STREAM_ID, checkpointTuple, new Values(txid, action));
          collector.ack(checkpointTuple);
      }
docs: Forwards the checkpoint tuple downstream. Sub-classes can override
  with the logic for handling checkpoint tuple.
  
  @param checkpointTuple the checkpoint tuple
  @param action          the action (prepare, commit, rollback or initstate)
  @param txid            the transaction id.