An operator is a domain specific controller.
basically, a kubernetes operator is the name of a pattern that consists of a kubernetes controller that adds new objects to the Kubernetes API, in order to configure and manage an application, such as Prometheus or etcd.
- Controller == Works on vanilla K8s resources
- Operator == a Controller that adds custom resources (CRDs) required for it’s operation
Controllers are the core abstraction used to build Kubernetes. Once you’ve declared the desired state of your cluster using the API server, controllers ensure that the cluster’s current state matches the desired state by continuously watching the state of the API server and reacting to any changes. Controllers operate using a simple loop that continuously checks the current state of the cluster against the desired state of the cluster. If there are any differences, controllers perform tasks to make the current state match the desired state. In pseudo-code:
while true:
X = currentState()
Y = desiredState()
if X == Y:
return # Do nothing
else:
do(tasks to get to Y)For example, when you create a new Pod using the API server, the Kubernetes scheduler (a controller) notices the change and makes a decision about where to place the Pod in the cluster. It then writes that state change using the API server (backed by etcd). The kubelet (a controller) then notices that new change and sets up the required networking functionality to make the Pod reachable within the cluster. Here, two separate controllers react to two separate state changes to make the reality of the cluster match the intention of the user.