复制集成员的状态总结

Number	Name	State Description	Vote
0	STARTUP	Not yet an active member of any set. All members start up in this state. The mongod parses the replica set configuration document while in STARTUP.
1	PRIMARY	The member in state primary is the only member that can accept write operations.	Eligible to vote.
2	SECONDARY	A member in state secondary is replicating the data store.	Eligible to vote.
3	RECOVERING	Members either perform startup self-checks, or transition from completing a rollback or resync.	Eligible to vote.
5	STARTUP2	The member has joined the set and is running an initial sync.	Eligible to vote.
6	UNKNOWN	The member's state, as seen from another member of the set, is not yet known.
7	ARBITER	Arbiters do not replicate data and exist solely to participate in elections.	Eligible to vote.
8	DOWN	The member, as seen from another member of the set, is unreachable.
9	ROLLBACK	This member is actively performing a rollback.  Data is not available for reads from this member. Starting in version 4.2, MongoDB kills all in-progress user operations when a member enters the ROLLBACK state.	Eligible to vote.
10	REMOVED	This member was once in a replica set but was subsequently removed.