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Priority 0 Replica Set Members

A priority 0 member is a member that cannot become primary and cannot trigger elections.

Other than the aforementioned restrictions, secondaries that have priority 0 function as normal secondaries: they maintains a copy of the data set, accept read operations, and vote in elections.

Configure a secondary to have priority 0 to prevent it from becoming primary, which is particularly useful in multi-data center deployments.

For example, in the following diagram, one data center hosts the primary and a secondary. A second data center hosts a secondary with priority 0. Only the members in data center 1 can become primary.

Diagram of a 3 member replica set distributed across two data centers. Replica set includes a priority 0 member.

Priority 0 Members as Standbys

A secondary with priority 0 can function as a standby. In some replica sets, it might not be possible to add a new member in a reasonable amount of time. A standby member keeps a current copy of the data to be able to replace an unavailable member.

In many cases, you need not set standby to priority 0. However, in replica sets with varied hardware or geographic distribution, a priority 0 standby ensures that only certain members become primary.

A priority 0 standby may also be valuable for some members of a set with different hardware or workload profiles. In these cases, deploy a member with priority 0 so it can’t become primary. Also consider using an hidden member for this purpose.

If your set already has seven voting members, also configure the member as non-voting.

Failover Considerations

When configuring a secondary to have priority 0, consider potential failover patterns, including all possible network partitions. Always ensure that your main data center contains both a quorum of voting members and members that are eligible to be primary.

Example

To configure a secondary to have priority 0, see Prevent Secondary from Becoming Primary.