Replica Set Arbiter

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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 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.

To add an arbiter, see Add an Arbiter to Replica Set.


In general, avoid deploying more than one arbiter per replica set.

For the following MongoDB versions, pv1 increases the likelihood of w:1 rollbacks compared to pv0 for replica sets with arbiters:

  • MongoDB 3.4.1
  • MongoDB 3.4.0
  • MongoDB 3.2.11 or earlier

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 via keyfiles. MongoDB encrypts the authentication process. The MongoDB authentication exchange is cryptographically secure.

Because arbiters do not store data, they do not possess the internal table of user and role mappings used for authentication. Thus, the only way to log on to an arbiter with authorization active is to use the localhost exception.


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.