In a mission-critical application, each application service has to run in multiple replicas. Depending on the load, that can be as few as two to three instances and as many as dozens, hundreds, or thousands of instances. At any given time, we want to have a clear majority when it comes to all the service instances running. So, if we have three replicas, we want to have at least two of them up and running at all times. If we have 100 replicas, we can be content with a minimum of, say, 90 replicas, being available. By doing this, we can define a batch size of replicas that we may take down to upgrade. In the first case, the batch size would be 1 and in the second case, it would be 10.
When we take replicas down, Docker Swarm ...
