I think I already have the answer, but I'm a developer so thought I'd check here first.
There are AWS regions with more than 2 zone, but some with only 2. E.g. Sydney.
Now if I were to deploy a mongodb replica set cluster across only 2 availability zone, I am getting the impression that it is NOT possible for the cluster to fail over automatically.
Reason being if one of the two zones went offline and take out the majority of your nodes, the rest of the nodes in the other zone won't elect a new primary? It'll be up to someone manually going into one of the nodes and promoting it as a primary.
Or am I missing something.