MongoDB allows us to set different priority levels for each member. This allows for some interesting applications and topologies to be implemented.
To change the priority after we have set up our cluster, we have to connect using the mongo shell to our primary and get the configuration object (hereby named as cfg):
> cfg = rs.conf()
Then we can change the members subdocument priority attribute to the value of our choice:
> cfg.members[0].priority = 0.778
> cfg.members[1].priority = 999.9999
Default priority is 1 for every member. Priority can be set from 0 (never become a primary) to 1000 in floating point precision.
Higher priority members will be the first to call an election when the primary steps down and the most likely to win the election.
Custom priorities should be configured with great consideration of all different network partitions. Setting priorities the wrong way may lead to elections not being able to elect a primary, thus stopping all writes to our MongoDB replica set.
If we want to prevent a secondary from becoming a primary, we can set its priority to 0 as explained in the following section.