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Replica Set Arbiter
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In some circumstances (such as you have a primary and a secondary but
cost constraints prohibit adding another secondary), you may choose to
add an arbiter to your replica set. An arbiter does not have a copy
of the data set and cannot become a primary. However, an arbiter
participates in elections for primary.
By default an arbiter has priority 0. An arbiter has exactly 1
election vote.
Important
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.
Release Version Considerations
Arbiters are not supported with quarterly rapid releases releases. If your deployment includes arbiters, only use LTS releases.
Example
For example, in the following replica set with a 2 data bearing members (the primary and a secondary), an arbiter allows the set to have an odd number of votes to break a tie:
Performance Issues with PSA replica sets
If you are using a three-member primary-secondary-arbiter (PSA)
architecture, the write concern "majority" can cause
performance issues if a secondary is unavailable or lagging. See
Mitigate Performance Issues with PSA Replica Set for advice on how to mitigate these
issues.
Replica Set Protocol Version and Arbiter
For the following MongoDB versions, pv1 increases the likelihood
of w:1 rollbacks compared to pv0
(no longer supported in MongoDB 4.0+) for replica sets with arbiters:
MongoDB 3.4.1
MongoDB 3.4.0
MongoDB 3.2.11 or earlier
Security
Authentication
When running with authorization, arbiters exchange credentials with
other members of the set to authenticate. MongoDB encrypts the
authentication process, and 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.
Communication
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.