Controller pattern in k8s operators?

The controller pattern has been summarized in these 3 sentences:

A controller tracks at least one Kubernetes resource type. These objects have a spec field that represents the desired state. The controller(s) for that resource are responsible for making the current state come closer to that desired state.

Ref: https://kubernetes.io/docs/concepts/architecture/controller/#controller-pattern

Basically, the Kubernetes controllers watches for the respective resource in a control loop. Once, it find a resource, it reads the desired state from the spec and do some work to make the cluster state same as the desired state.

For example, you have created a Deployment where you have specified in the spec that you want 1 pod that runs your application. Now, Deployment controller sees this and create 1 pod in the cluster to match your desired state. Now, if you update the Deployment spec and say that you now want 2 pods. The Deployment controller will see this change as it is always watching for Deployment and create another pod in the cluster to match the desired state.

You can find more details about these in the following resources:

• Inside of Kubernetes Controller
• A deep dive into Kubernetes controllers

The Job controller is an example of a Kubernetes built-in controller. Built-in controllers manage state by interacting with the cluster API server.

Ref: https://kubernetes.io/docs/concepts/architecture/controller/#control-via-api-server

In my understanding, you create a xxx-operator, it means you add a Kind for your k8s, the kubectl explain <kind-name> can show the Kind msg you've given.

So, use the xx-operator, you can set your app with a simpler yaml. You can also use tools like Kustomize or Helm based on this way.

e.g. :

1. run this, at first:

   kubectl explain ZookeeperCluster
   

   you will get an err.

2. you can install a helm chart pravega/zookeeper-operator:

   helm install --create-namespace -n op-pravega-zk -- zookeeper-operator 
   pravega/zookeeper-operator
   

   you will get out msg after installed but it's helpless:

   NAME: zookeeper-operator
   LAST DEPLOYED: Thu Nov 25 11:20:31 2021
   NAMESPACE: op-pravega-zk
   STATUS: deployed
   REVISION: 1
   TEST SUITE: None
   

3. but, now, if you run this again:

   kubectl explain ZookeeperCluster
   

   you will get these:

   KIND:     ZookeeperCluster
   VERSION:  zookeeper.pravega.io/v1beta1
   
   DESCRIPTION:
       ZookeeperCluster is the Schema for the zookeeperclusters API
   
   FIELDS:
   apiVersion   <string>
       APIVersion defines the versioned schema of this representation of an
       object. Servers should convert recognized schemas to the latest internal
       value, and may reject unrecognized values. More info:
       https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#resources
   
   kind <string>
       Kind is a string value representing the REST resource this object
       represents. Servers may infer this from the endpoint the client submits
       requests to. Cannot be updated. In CamelCase. More info:
       https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#types-kinds
   
   metadata     <Object>
       Standard object's metadata. More info:
       https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#metadata
   
   spec <Object>
       ZookeeperClusterSpec defines the desired state of ZookeeperCluster
   
   status       <Object>
       ZookeeperClusterStatus defines the observed state of ZookeeperCluster
   
   

4. now, you can use it like this yaml ( ... means ellipsis) :

   apiVersion: "zookeeper.pravega.io/v1beta1"
   kind: "ZookeeperCluster"
   metadata:
     name: zookeeper
     namespace: pravega-zk
   ...
   spec:
     replicas: 3
     image:
       repository: pravega/zookeeper
     ...
     pod:
       serviceAccountName: zookeeper
     storageType: persistence
     persistence:
       reclaimPolicy: Delete
       spec:
         storageClassName: local-hostpath
     ...
   

   or like this ( a ZookeeperCluster Kind in this helm chart ) :

   helm install --create-namespace -n pravega-zk --set persistence.storageClassName=local-hostpath -- zookeeper pravega/zookeeper
   

other e.g. like:

2. Create a Zookeeper cluster.

   kubectl create --namespace zookeeper -f - <<EOF
   apiVersion: zookeeper.pravega.io/v1beta1
   kind: ZookeeperCluster
   metadata:
       name: zookeeper
       namespace: zookeeper
   spec:
       replicas: 1
   EOF
   

Ref: https://banzaicloud.com/docs/supertubes/kafka-operator/install-kafka-operator/

btw, after you install a operator, you can use kubectl describe clusterrole zookeeper-operator to show some message, then run kubectl api-resources to find it, then you may find the name of this kind ....