[philips]

I don't think stateful applications require the use of something like an Operator. It really just comes down to where the state lives. For example if you are wanting to run your database on top of an EBS, SAN, or something like that it is no problems to just throw up a StatefulSet and go for it.

However, if you start to think about things like orchestrating scaling of databases that have a administrative tool like Cassandra, Vitess, RethinkDB, read-replicated Postgres, etc you need to have some glue/gel that integrates those admin tools with the Kubernetes APIs. And that is what a minimal Operator should do.

The other thing Operators can do is glue existing software to the Kubernetes APIs. Which is what the Prometheus Operator does. Prometheus has its own configuration system for finding monitoring targets; instead of forcing the user to drop down into that different format the Operator adapts Kubernetes concepts like label queries and generates the equivalent Prometheus config.

Overall I don't think this is required for every application. Static databases persisting to shared storage, stateless applications, or cluster wide daemons all fit nicely in Kubernetes abstractions before Operators. But, there is a class of clustered applications that are served well by this pattern.

[tristanz]

I think philips is exactly right. The whole design of Kubernetes is geared toward allowing users write their own controllers for advanced use cases. You could view something like Jenkins or Vitess as controllers because they spawn Kubernetes pods on demand. The beauty is Kubernetes gives you great primitives, so you often will be controlling these objects, not the underlying pods. Of course many simple applications don't need a controller, although I suspect more and more simple use cases will be managed by an external controller like Helm which orchestrates the lifecycle of applications.

The questions is: Why isn't this just called a controller? What's this new term Operator?

[philips]

Controller wasn't quite the term to capture the combination of an application specific controller and third party resource to manage a collection of user created application instances.

So, we arrived at Operator. It felt like a good term that we could put after X that encapsulates the intent of the pattern. It helps you operate instances of an application.