ReplicaSet

In Kubernetes, a ReplicaSet ensures that a specified number of pod replicas are running at any given time. If a pod crashes or is deleted, the ReplicaSet automatically creates a new pod to replace it, maintaining the desired number of replicas.

🚨 Note: Similar to Pods, you will almost never deploy/manage a ReplicaSet directly. Instead you will define a Deployment (see next lesson) and Kubernetes will create/manage the underlying ReplicaSets.

Hands-On: Working with ReplicaSets

We will create and examine a ReplicaSet to understand its behavior.

1. Create a Namespace for the Examples

First, we'll create a namespace for these examples and set it as the default.

# task 01-create-namespace
# - Create a namespace for these examples and set as default.
kubectl apply -f Namespace.yaml
kubens 04--replicaset

2. Apply a ReplicaSet Configuration

Next, we'll apply a ReplicaSet configuration containing 3 replicas of a pod containing an nginx container image.

# task 03-better-apply
# - Apply the ReplicaSet configuration.
kubectl apply -f ReplicaSet.nginx-better.yaml

You will notice that within the ReplicaSet definition we have a field spec.template. This contains all the same fields that our Pod spec from the previous lesson contained.

The key attributes the replica set (beyond the pod spec) are:

• spec.replicas: Determines how many copies of the pod should be run. If any pod fails, the ReplicaSet controller will create a new pod to maintain the desired state.
• spec.selector: Uses matchLabels to identify the pods that belong to this ReplicaSet. Only pods with the label app: nginx-better will be managed by this ReplicaSet.

# ReplicaSet.nginx-better.yaml
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-better
  namespace: 04--replicaset
  labels:
    app: nginx-better
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx-better
  template:
    metadata:
      labels:
        app: nginx-better
    spec:
      containers:
        - name: nginx
          image: cgr.dev/chainguard/nginx:latest
          ports:
            - containerPort: 8080
              protocol: TCP
          readinessProbe:
            httpGet:
              path: /
              port: 8080
          resources:
            limits:
              memory: "50Mi"
            requests:
              memory: "50Mi"
              cpu: "250m"
          securityContext:
            allowPrivilegeEscalation: false
            privileged: false
      securityContext:
        seccompProfile:
          type: RuntimeDefault
        runAsUser: 1001
        runAsGroup: 1001
        runAsNonRoot: true

3. Delete the Pods!

One of the key features of ReplicaSets is their ability to maintain the desired number of replicas. Deleting all pods will demonstrate this as the ReplicaSet will automatically recreate the deleted pods.

# task 04-delete-pods
# - Delete all the pods (they will be automatically recreated)
kubectl delete pods --all
watch "kubectl get pods"

4. Delete the Namespace to Clean Up

Finally, clean up by deleting the namespace, which will also delete all resources within it.

# task 05-delete-namespace
# - Delete the namespace to clean up
kubectl delete -f Namespace.yaml