MongoDB Documentation, Release 3.0.4 States Core States PRIMARY SECONDARY ARBITER Other States STARTUP2 STARTUP Num- ber Name State Description 0 STARTUP (page 655) Not yet an active member of any set. All members start up in this state. The mongod parses the replica set configuration document (page 620) while in STARTUP (page 655). 1 PRIMARY (page 655) The member in state primary (page 564) is the only member that can accept write operations. 2 SECONDARY (page 655) A member in state secondary (page 567) is replicating the data store. Data is available for reads, although they may be stale. 3 RECOVERING (page 656) Can vote. Members either perform startup self-checks, or transition from completing a rollback (page 584) or resync (page 633). 5 STARTUP2 (page 655) The member has joined the set and is running an initial sync. 6 UNKNOWN (page 656) The member’s state, as seen from another member of the set, is not yet known. 7 ARBITER (page 655) Arbiters (page ?? ) do not replicate data and exist solely to participate in elections. 8 DOWN (page 656) The member, as seen from another member of the set, is unreachable. 9 ROLLBACK (page 656) This member is actively performing a rollback (page 584). Data is not available for reads. 10 REMOVED (page 656) This member was once in a replica set but was subsequently removed. Members in PRIMARY (page 655) state accept write operations. A replica set has at most one primary at a time. A SECONDARY (page 655) member becomes primary after an election (page 580). Members in the PRIMARY (page 655) state are eligible to vote. Members in SECONDARY (page 655) state replicate the primary’s data set and can be configured to accept read operations. Secondaries are eligible to vote in elections, and may be elected to the PRIMARY (page 655) state if the primary becomes unavailable. Members in ARBITER (page 655) state do not replicate data or accept write operations. They are eligible to vote, and exist solely to break a tie during elections. Replica sets should only have a member in the ARBITER (page 655) state if the set would otherwise have an even number of members, and could suffer from tied elec- tions. There should only be at most one arbiter configured in any replica set. See Replica Set Members (page 564) for more information on core states. Each member of a replica set starts up in STARTUP (page 655) state. mongod then loads that member’s replica set configuration, and transitions the member’s state to STARTUP2 (page 655). Members in STARTUP (page 655) are not eligible to vote, as they are not yet a recognized member of any replica set. Each member of a replica set enters the STARTUP2 (page 655) state as soon as mongod finishes loading that member’s configuration, at which time it becomes an active member of the replica set. The member then decides whether or not to undertake an initial sync. If a member begins an initial sync, the member remains in 9.4. Replication Reference 655