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Priority 0 Replica Set Members

A priority 0 member is a member that cannot become primary and cannot trigger elections. Priority 0 members can acknowledge write operations issued with write concern of w : <number>. For "majority" write concern, the priority 0 member must also be a voting member (i.e. members[n].votes is greater than 0) to acknowledge the write. Non-voting replica set members (i.e. members[n].votes is 0) cannot contribute to acknowledging write operations with "majority" write concern.

Other than the aforementioned restrictions, secondaries that have priority 0 function as normal secondaries: they maintain 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 one priority 0 member that cannot 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 priority 0 member 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 sets with varied hardware or geographic distribution, a priority 0 standby ensures that only qualified 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.

Priority 0 Members and Failover

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

Configuration

To configure a priority 0 member, see Prevent Secondary from Becoming Primary.