Kubernetes Patterns, 2nd Edition

Chapter 28. Operator

An operator is a controller that uses a CRD to encapsulate operational knowledge for a specific application in an algorithmic and automated form. The Operator pattern allows us to extend the Controller pattern from the preceding chapter for more flexibility and greater expressiveness.

Problem

You learned in Chapter 27, “Controller”, how to extend the Kubernetes platform in a simple and decoupled way. However, for extended use cases, plain custom controllers are not powerful enough, as they are limited to watching and managing Kubernetes intrinsic resources only. Moreover, sometimes we want to add new concepts to the Kubernetes platform, which requires additional domain objects. For example, let’s say we chose Prometheus as our monitoring solution and want to add it as a monitoring facility to Kubernetes in a well-defined way. Wouldn’t it be wonderful to have a Prometheus resource describing our monitoring setup and all the deployment details, similar to how we define other Kubernetes resources? Moreover, could we have resources relating to services we have to monitor (e.g., with a label selector)?

These situations are precisely the kind of use cases where CustomResourceDefinition (CRD) resources are very helpful. They allow extensions of the Kubernetes API, by adding custom resources to your Kubernetes cluster and using them as if they were native resources. Custom resources, together with a controller acting on these resources, form the Operator pattern. ...