Why is the maximum number of nodes in a MongoDB Replica Set exactly 12?

Question

Sorry for that dumb question, but why exactly that number? At first I thought it must only be odd number, but actually it is 7 as the maximum voting node, and overall a maximum of 12 nodes. If you wish to run more than this, you must use the deprecated master/slave configuration.

So how to explain this number?

Answer 1

All members of a replica set maintain knowledge of the current state of each of the other members. This is the rationale for limiting the total number of nodes to twelve - more than that would introduce too much overhead in heartbeats between each pair of nodes.

Maximum of seven voting members is to avoid slowing down elections - since you need to have a consensus election to select a new primary limiting the number of nodes that need to coordinate amongst themselves will keep things faster.