@Override
  public int getReplicationNum(ConsensusGroupId groupId) {
    RaftGroupId raftGroupId = Utils.fromConsensusGroupIdToRaftGroupId(groupId);
    try {
      return server.get().getDivision(raftGroupId).getGroup().getPeers().size();
    } catch (IOException e) {
      return 0;
    }
  }
returns the known leader to the given group. NOTICE: if the local peer isn't a member of given
group, getLeader will return null.

@return null if local peer isn't in group, otherwise group leader.
iotdb-core/consensus/src/main/java/org/apache/iotdb/consensus/ratis/RatisConsensus.java