Kubernetes Operators

Chapter 3. Operators at the Kubernetes Interface

Operators extend two key Kubernetes concepts: resources and controllers. The Kubernetes API includes a mechanism, the CRD, for defining new resources. This chapter examines the Kubernetes objects Operators build on to add new capabilities to a cluster. It will help you understand how Operators fit into the Kubernetes architecture and why it is valuable to make an application a Kubernetes native.

Standard Scaling: The ReplicaSet Resource

Looking at a standard resource, the ReplicaSet, gives a sense of how resources comprise the application management database at the heart of Kubernetes. Like any other resource in the Kubernetes API, the ReplicaSet is a collection of API objects. The ReplicaSet primarily collects pod objects forming a list of the running replicas of an application. The specification of another object type defines the number of those replicas that should be maintained on the cluster. A third object spec points to a template for creating new pods when there are fewer running than desired. There are more objects collected in a ReplicaSet, but these three types define the basic state of a scalable set of pods running on the cluster. Here, we can see these three key pieces for the staticweb ReplicaSet from Chapter 1 (the Selector, Replicas, and Pod Template fields):

$ kubectl describe replicaset/staticweb-69ccd6d6c Name: staticweb-69ccd6d6c Namespace: default Selector: pod-template-hash=69ccd6d6c,run=staticweb Labels: ...