Kubernetes Controllers

Objective

This lab will help you understand the role of Kubernetes controllers in managing the desired/actual state of cluster resources.

Prerequisites

• A Kubernetes cluster (Minikube, Kind, or a cloud-based Kubernetes cluster)
• kubectl installed and configured

Step 1: Understand Controllers

Kubernetes controllers are control loops that monitor the state of the cluster and make or request changes where needed. The key controllers include:

• Deployment Controller (manages ReplicaSets for stateless applications)
• ReplicaSet Controller (ensures a specified number of pod replicas)
• StatefulSet Controller (manages stateful applications)
• DaemonSet Controller (ensures a copy of a pod runs on each node)
• Job Controller (runs batch jobs to completion)
• CronJob Controller (manages periodic tasks)

Step 2: Create a ReplicaSet

A ReplicaSet ensures a specified number of pod replicas run at all times.

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: my-replicaset
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
      - name: my-container
        image: nginx
        ports:
        - containerPort: 80

kubectl apply -f replicaset.yaml

Verify the ReplicaSet.

# Observe pod lifecycle
kubectl get pods --watch

# In a second terminal, then go back to previous command
kubectl get replicaset

Delete a pod and observe self-healing.

kubectl delete pod <pod-name>
kubectl get pods

Check the ReplicaSet description to see controller activity.

kubectl describe replicaset my-replicaset

Observe Pod Events for the current namespace.

kubectl get events --sort-by=.metadata.creationTimestamp

View logs related to the ReplicaSet controller, and extract the one related to <my-namespace>/my-replicaset.

kubectl logs -n kube-system kube-controller-manager-kind-control-plane

You should see that Kubernetes automatically creates a new pod to maintain the desired state.

Step 3: Scale the ReplicaSet

Scale the ReplicaSet to 5 replicas:

kubectl scale replicaset my-replicaset --replicas=5

Check the number of pods:

kubectl get pods

Step 4: Cleanup

Delete the ReplicaSet:

kubectl delete replicaset my-replicaset

Verify resources are removed:

kubectl get all

Conclusion

You have successfully learned how Kubernetes controllers maintain the desired state of your applications. Explore other controllers like StatefulSet, DaemonSet, and Jobs to deepen your understanding!