Minimum of three is generally for one of two reasons:
1) to ensure you can deliberately take one offline for maintenance and still have redundancy in case a single node goes offline
2) in some systems an odd number of nodes is needed to ensure no ties in leader elections or decision votes. Three is the smallest odd number that has any redundancy.
1) I have taken a node in maintenance mode many times deliberately by launching second node and draining first. Its not an issue at all with just one node. For redundancy, is there ever a case where single node could go down in practice. Nodes are basically EC2 instance and they could run for years without going down.
> Amazon EC2 also sends a notification in your email
Interesting. I had seen EC2 instances running for multiple years and never saw this so I assumed that this isn't possible. I know for a fact that GCP has live migration where while the service would be degraded for few seconds, you don't need to do anything so I assumed AWS also had something similar.
1) to ensure you can deliberately take one offline for maintenance and still have redundancy in case a single node goes offline
2) in some systems an odd number of nodes is needed to ensure no ties in leader elections or decision votes. Three is the smallest odd number that has any redundancy.