@Override
  public DataSet read(ConsensusGroupId groupId, IConsensusRequest request)
      throws ConsensusException {
    return Optional.ofNullable(stateMachineMap.get(groupId))
        .orElseThrow(() -> new ConsensusGroupNotExistException(groupId))
        .read(request);
  }
A simple consensus implementation, which can be used when replicaNum is 1.

<p>Notice: The stateMachine needs to implement WAL itself to ensure recovery after a restart
iotdb-core/consensus/src/main/java/org/apache/iotdb/consensus/simple/SimpleConsensus.java