ReplicationController and ReplicaSet

ReplicationController (RC)

A ReplicationController (RC) is a legacy Kubernetes resource that ensures a specified number of Pod replicas are running at any given time. If a Pod fails or is deleted, the RC automatically creates a replacement.

Key Features

• Ensures high availability of Pods.
• Supports scaling Pods up or down.
• Performs self-healing of failed Pods.
• Works only with equality-based selectors (e.g., app=nginx).

Note: ReplicaSet has largely replaced ReplicationController, but RC is still supported for backward compatibility.

Example: ReplicationController YAML

apiVersion: v1
kind: ReplicationController
metadata:
  name: nginx-rc
spec:
  replicas: 3
  selector:
    app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx
          image: nginx:latest
          ports:
            - containerPort: 80

ReplicaSet (RS)

A ReplicaSet (RS) is the next-generation controller that replaces ReplicationController. It provides the same functionality but supports set-based selectors, making it more flexible.

Key Features

• Ensures desired number of Pods are running.
• Supports both equality-based and set-based selectors (e.g., app in (frontend, backend)).
• Typically used by Deployments, rather than being created directly.

Example: ReplicaSet YAML

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-rs
spec:
  replicas: 3
  selector:
    matchExpressions:
      - key: app
        operator: In
        values:
          - nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx
          image: nginx:latest
          ports:
            - containerPort: 80

kubectl Commands for RC and RS

Create

kubectl apply -f nginx-rc.yaml
kubectl apply -f nginx-rs.yaml

Get

kubectl get rc
kubectl get rs
kubectl get pods

Describe

kubectl describe rc nginx-rc
kubectl describe rs nginx-rs

Scale

# Scale ReplicationController
kubectl scale rc nginx-rc --replicas=5

# Scale ReplicaSet
kubectl scale rs nginx-rs --replicas=5

Update

kubectl edit rc nginx-rc
kubectl edit rs nginx-rs

Delete

kubectl delete rc nginx-rc
kubectl delete rs nginx-rs

Debugging Pods under RC/RS

kubectl logs <pod-name>
kubectl exec -it <pod-name> -- /bin/sh

Difference Between RC and RS

Feature	ReplicationController	ReplicaSet
Status	Legacy	Modern, preferred
Selector Type	Equality-based only	Equality + Set-based
Usage	Rarely used today	Typically managed by Deployments
Scalability	Supported	Supported
Self-Healing	Supported	Supported