What is immutable infrastructure?

Immutability is the inability to be changed. We have already looked at Docker and how a Docker container is an immutable instance of an image. However, what about the hardware that the Docker server runs on? Immutable infrastructure gives us the same benefits—we have a known state and that state is consistent across our estate. Traditionally, the software would be upgraded on an application server, but this process was often problematic. The software update process would sometimes not go to plan, leaving the operator with the arduous task of trying to roll it back. We would also experience situations where the application servers would be in different states, requiring different processes to upgrade each of them. The update process may be okay if you only have two application servers, but what if you have 200 of them? The cognitive load becomes so much to bear that the administration is distributed across a team or multiple teams, and then we need to start...