which entity in MongoDB manages cluster membership?

Question

Could someone please tell me which entity in MongoDB manages cluster membership?(e.g., Redis uses a gossip-based protocol to do that).

In addition, could you please correct me if I am not right for the following drawbacks (in my opinion) in MongoDB:

• There is no support for multi-document transactions.
• The load balancing strategy in MongoDB is not perfect since it identifies imbalance only based on the number of chunks in cluster shards. Therefore, it does not meet the common scene of the uneven distribution of hot spots. Moreover, MongoDB conducts chunk migration by only taking the amount of data into consideration, without considering the physical distance and other specific replica set’s load information.
• The serialization procedure, used for storing data in BSON format, can decrease the performance of the system.
• MongoDB does not support multi-master replication which affects its writing scalability especially across different data centers.
• There is not any strategy to alleviate the situations where a master node cannot efficiently handle replication to its slaves.