I am playing with multi node docker swarm in cloud. I setup 4 nodes swarm where 2 manager (1 primary and the other one is reachable manager ) and 2 worker nodes. While I am reading docs, I found out that we have to choose odd number of manager nodes like 1,3.... Not sure what is the technical restriction behind this decision.
This is related to how consensus across managers is determined when maintaining cluster consistency during an outage. See Raft consensus in swarm mode.
The algorithm used to derive consensus for a cluster of N nodes requires (N/2)+1 of them to agree. For a cluster of 2 managers you would actually be reducing reliability because if either of them goes down the other would be unable to do anything. In general, having an even number of managers provides no benefit over having one less.