Containers are made to fail gracefully. When your container shuts down due to a hardware or software failure, your distributed application should be able to tolerate that failure. One simple way to be able to tolerate such a failure is to make all of your application logic “stateless.”

If your application does not maintain state, then shutting it down in the middle of a computation is not a problem–you can just restart the application, restart your computation, and get the same result.

But applications need to maintain state. We need to use databases and in-memory systems to manage long-lived user sessions and other interactions. A database is not just an on-disk abstraction–a database requires an application server to be accepting network traffic. We can run those database applications within containers.

There is a fundamental tension between stateful applications and the idea that containers are meant to tolerate failure gracefully.

Saad Ali is an engineer at Google, and he returns to the show to discuss Kubernetes storage and state management. He gave a keynote at KubeCon EU, which I spoke to him about.

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