Replica Set Arbiter

On this page

An arbiter does not have a copy of data set and cannot become a primary. Replica sets may have arbiters to add a vote in elections of for primary. Arbiters always have exactly 1 election vote, and thus allow replica sets to have an uneven number of voting members without the overhead of an additional member that replicates data.


Do not run an arbiter on systems that also host the primary or the secondary members of the replica set.

Only add an arbiter to sets with even numbers of voting members. If you add an arbiter to a set with an odd number of voting members, the set may suffer from tied elections. To add an arbiter, see Add an Arbiter to Replica Set.

For replica sets with an arbiter, replica set protocol version 1 (pv1) increases the likelihood of rollback of w:1 writes compared to replica set protocol version 0 (pv0). See Replica Set Protocol Versions.


For example, in the following replica set, an arbiter allows the set to have an odd number of votes for elections:

Diagram of a four member replica set plus an arbiter for odd number of votes.



When running with authorization, arbiters exchange credentials with other members of the set to authenticate. MongoDB encrypts the authentication process. The MongoDB authentication exchange is cryptographically secure.

Arbiters use keyfiles to authenticate to the replica set.


The only communication between arbiters and other set members are: votes during elections, heartbeats, and configuration data. These exchanges are not encrypted.

However, if your MongoDB deployment uses TLS/SSL, MongoDB will encrypt all communication between replica set members. See Configure mongod and mongos for TLS/SSL for more information.

As with all MongoDB components, run arbiters in trusted network environments.