From running a container to keeping it running

In the module on containerization we ran containers with Docker, one command at a time, and watched each one start, answer a request, and stop. That way of working is perfect for a developer's laptop and hopeless for a production system, because it depends on a person being present at every moment something changes. If a container crashes at three in the morning, nobody restarts it, and if traffic doubles at noon, nobody starts a second copy, so the service is down, or slow, until someone notices.

Kubernetes removes the person from that loop by changing what we tell the system. Instead of issuing commands such as "start this container", we describe the state we want, for example "two copies of this image should always be running and reachable on this port", and Kubernetes compares that description with what is actually running and acts whenever the two differ.…