Replica Set Member States

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Each member of a replica set has a state.

Number	Name	State Description
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. Data is not available for reads from this member. 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. Eligible to vote. 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.
10	`REMOVED`	This member was once in a replica set but was subsequently removed.

States

Core States

PRIMARY: Members in PRIMARY state accept write operations. A replica set has at most one primary at a time. [1] A SECONDARY member becomes primary after an election. Members in the PRIMARY state are eligible to vote.

SECONDARY: Members in SECONDARY 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 state if the primary becomes unavailable.

ARBITER: Members in ARBITER 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 state if the set would otherwise have an even number of voting members, and could suffer from tied elections. There should only be at most one arbiter configured in any replica set. For considerations when using an arbiter, see Replica Set Arbiter.

See Replica Set Members for more information on core states.

Other States

STARTUP: Each member of a replica set starts up in STARTUP state. mongod then loads that member's replica set configuration, and transitions the member's state to STARTUP2 or ARBITER. Members in STARTUP are not eligible to vote, as they are not yet a recognized member of any replica set.

STARTUP2: Each data-bearing member of a replica set enters the STARTUP2 state as soon as mongod finishes loading that member's configuration, at which time it becomes an active member of the replica set and is eligible to vote. The member then decides whether or not to undertake an initial sync. If a member begins an initial sync, the member remains in STARTUP2 until all data is copied and all indexes are built. Afterwards, the member transitions to RECOVERING.

RECOVERING

A member of a replica set enters RECOVERING state when it is not ready to accept reads. The RECOVERING state can occur during normal operation, and doesn't necessarily reflect an error condition. Members in the RECOVERING state are eligible to vote in elections, but are not eligible to enter the PRIMARY state.

A member transitions from RECOVERING to SECONDARY after replicating enough data to guarantee a consistent view of the data for client reads. The only difference between RECOVERING and SECONDARY states is that RECOVERING prohibits client reads and SECONDARY permits them. SECONDARY state does not guarantee anything about the staleness of the data with respect to the primary.

Due to overload, a secondary may fall far enough behind the other members of the replica set such that it may need to resync with the rest of the set. When this happens, the member enters the RECOVERING state and requires manual intervention.

ROLLBACK

Whenever the replica set replaces a primary in an election, the old primary may contain documents that did not replicate to the secondary members. In this case, the old primary member reverts those writes. During rollback, the member will have ROLLBACK state. Members in the ROLLBACK state are eligible to vote in elections.

Starting in version 4.2, MongoDB kills all in-progress user operations when a member enters the ROLLBACK state.

Error States

Members in any error state can't vote.

UNKNOWN: Members that have never communicated status information to the replica set are in the UNKNOWN state.

DOWN: Members that lose their connection to the replica set are seen as DOWN by the remaining members of the set.

REMOVED: Members that are removed from the replica set enter the REMOVED state. When members enter the REMOVED state, the logs will mark this event with a replSet REMOVED message entry.

[1] In some circumstances, two nodes in a replica set may transiently believe that they are the primary, but at most, one of them will be able to complete writes with { w: "majority" } write concern. The node that can complete { w: "majority" } writes is the current primary, and the other node is a former primary that has not yet recognized its demotion, typically due to a network partition. When this occurs, clients that connect to the former primary may observe stale data despite having requested read preference primary, and new writes to the former primary will eventually roll back.

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