Replica Set Elections¶

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Replica sets use elections to determine which set member will become primary. Elections occur after initiating a replica set, and also any time the primary becomes unavailable. The primary is the only member in the set that can accept write operations. If a primary becomes unavailable, elections allow the set to recover normal operations without manual intervention. In the following three-member replica set, the primary is unavailable. One of the remaining secondaries holds an election to elect itself as a new primary.

Diagram of an election of a new primary. In a three member replica set with two secondaries, the primary becomes unreachable. The loss of a primary triggers an election where one of the secondaries becomes the new primary

Elections are essential for independent operation of a replica set; however, elections take time to complete. While an election is in process, the replica set has no primary and cannot accept writes and all remaining members become read-only.

If a majority of the replica set is inaccessible or unavailable to the current primary, the primary will step down and become a secondary. The replica set cannot accept writes after this occurs, but remaining members can continue to serve read queries if such queries are configured to run on secondaries.

Factors and Conditions that Affect Elections¶

Replication Election Protocol¶

New in version 3.2: MongoDB introduces a version 1 of the replication protocol (protocolVersion: 1) to reduce replica set failover time and accelerates the detection of multiple simultaneous primaries. New replica sets will, by default, use protocolVersion: 1. Previous versions of MongoDB use version 0 of the protocol.

Heartbeats¶

Replica set members send heartbeats (pings) to each other every two seconds. If a heartbeat does not return within 10 seconds, the other members mark the delinquent member as inaccessible.

Member Priority¶

After a replica set has a stable primary, the election algorithm will make a “best-effort” attempt to have the secondary with the highest priority available call an election. Member priority affects both the timing and the outcome of elections; secondaries with higher priority call elections relatively sooner than secondaries with lower priority, and are also more likely to win. However, a lower priority instance can be elected as primary for brief periods, even if a higher priority secondary is available. Replica set members continue to call elections until the highest priority member available becomes primary.

Members with a priority value of 0 cannot become primary and do not seek election. For details, see Priority 0 Replica Set Members.

Loss of a Data Center¶

With a distributed replica set, the loss of a data center may affect the ability of the remaining members in other data center or data centers to elect a primary.

If possible, distribute the replica set members across data centers to maximize the likelihood that even with a loss of a data center, one of the remaining replica set members can become the new primary.

Network Partition¶

A network partition may segregate a primary into a partition with a minority of nodes. When the primary detects that it can only see a minority of nodes in the replica set, the primary steps down as primary and becomes a secondary. Independently, a member in the partition that can communicate with a majority of the nodes (including itself) holds an election to become the new primary.

Vetoes in Elections¶

Changed in version 3.2: The protocolVersion: 1 obviates the need for vetos. The following veto discussion applies to replica sets that use the older protocolVersion: 0.

For replica sets using protocolVersion: 0, all members of a replica set can veto an election, including non-voting members. A member will veto an election:

If the member seeking an election is not a member of the voter’s set.
If the current primary has more recent operations (i.e. a higher optime) than the member seeking election, from the perspective of another voting member.
If the current primary has the same or more recent operations (i.e. a higher or equal optime) than the member seeking election.
If a priority 0 member [1] is the most current member at the time of the election. In this case, another eligible member of the set will catch up to the state of the priority 0 member member and then attempt to become primary.
If the member seeking an election has a lower priority than another member in the set that is also eligible for election.

[1]	Hidden and delayed imply priority 0 configuration.

Voting Members¶

The replica set member configuration setting members[n].votes and member state determine whether a member votes in an election.

All replica set members that have their members[n].votes setting equal to 1 vote in elections. To exclude a member from voting in an election, change the value of the member’s members[n].votes configuration to 0.
- Non-voting members must have priority of 0.
- Members with priority greater than 0 cannot have 0 votes.
Only voting members in the following states are eligible to vote:

Non-Voting Members¶

Although non-voting members do not vote in elections, these members hold copies of the replica set’s data and can accept read operations from client applications.

Because a replica set can have up to 50 members, but only 7 voting members, non-voting members allow a replica set to have more than seven members.

Non-voting members must have priority of 0.

For instance, the following nine-member replica set has seven voting members and two non-voting members.

Diagram of a 9 member replica set with the maximum of 7 voting members.

A non-voting member has both votes and priority equal to 0:

{
   "_id" : <num>,
   "host" : <hostname:port>,
   "arbiterOnly" : false,
   "buildIndexes" : true,
   "hidden" : false,
   "priority" : 0,
   "tags" : {

   },
   "slaveDelay" : NumberLong(0),
   "votes" : 0
}

Important

Do not alter the number of votes to control which members will become primary. Instead, modify the members[n].priority option. Only alter the number of votes in exceptional cases. For example, to permit more than seven members.

To configure a non-voting member, see Configure Non-Voting Replica Set Member.

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