Hi
I am aware of that rule but in a slightly different context:
MongoDB write concern is an acknowledgment mechanism that developers can add to write operations.
Higher levels of acknowledgment produce a stronger durability guarantee. Durability means that the write has propagated to the number of replica set member nodes specified in the write concern.Majority here is defined as a simple majority of replica set members. So divide by two, and round up.
Taken from M103: Basic Cluster Administration
I was under the same assumption as you. But I found nowhere a statement that an election is blocked with an even number of members and I doubt that it would.
Michael