Adjust Priority for Replica Set Member¶

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Overview
Considerations
Procedure

Overview¶

The priority settings of replica set members affect both the timing and the outcome of elections for primary. Higher-priority members are more likely to call elections, and are more likely to win. Use this setting to ensure that some members are more likely to become primary and that others can never become primary.

The value of the member’s priority setting determines the member’s priority in elections. The higher the number, the higher the priority.

Considerations¶

To modify priorities, you update the members array in the replica configuration object. The array index begins with 0. Do not confuse this index value with the value of the replica set member’s members[n]._id field in the array.

The value of priority can be any floating point (i.e. decimal) number between 0 and 1000. The default value for the priority field is 1.

To block a member from seeking election as primary, assign it a priority of 0. Hidden members and delayed members have priority set to 0.

Changed in version 3.6: Starting in MongoDB 3.6, arbiters have priority 0. When you upgrade a replica set to MongoDB 3.6, if the existing configuration has an arbiter with priority 1, MongoDB 3.6 reconfigures the arbiter to have priority 0.

Adjust priority settings during a scheduled maintenance window. Reconfiguring priority can force the current primary to step down, leading to an election. Before an election, the primary closes all open client connections.

Priority and Votes¶

members[n].priority and members[n].votes have the following relationship:

Non-voting (i.e. votes is 0) members must have priority of 0.
Members with priority greater than 0 cannot have 0 votes.

As such, increasing a non-voting member’s priority requires setting votes to 1 and increases the number of voting replica set members. Before increasing the priority of a non-voting member, consider the following:

MongoDB replica sets can have no more than 7 voting members. If the replica set already has 7 voting members, you cannot modify the priority of any remaining members in the replica set to be greater than 0.
Starting in MongoDB 4.4, replica reconfiguration can add or remove no more than one voting member at a time. To change multiple non-voting members to have a priority greater than 0, issue a series of replSetReconfig or rs.reconfig() operations to modify one member at a time. See Reconfiguration Can Add or Remove No More than One Voting Member at a Time for more information.

Procedure¶

Warning

The rs.reconfig() shell method can force the current primary to step down, which causes an election. When the primary steps down, the mongod closes all client connections. While this typically takes 10-20 seconds, try to make these changes during scheduled maintenance periods.
Avoid reconfiguring replica sets that contain members of different MongoDB versions as validation rules may differ across MongoDB versions.

1

Copy the replica set configuration to a variable.¶

In the mongo shell, use rs.conf() to retrieve the replica set configuration and assign it to a variable. For example:

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cfg = rs.conf()

2

Change each member’s priority value.¶

Change each member’s members[n].priority value, as configured in the members array.

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cfg.members[0].priority = 0.5
cfg.members[1].priority = 2
cfg.members[2].priority = 2

This sequence of operations modifies the value of cfg to set the priority for the first three members defined in the members array.

3

Assign the replica set the new configuration.¶

Use rs.reconfig() to apply the new configuration.

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rs.reconfig(cfg)

This operation updates the configuration of the replica set using the configuration defined by the value of cfg.

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