Kubernetes - replication controller and replicaSets

what is a replica and why do we need a replication controller?

the replication controller helps us to run multiple instances of a single pod in a Kubernetes cluster to provide high-availability, even if you have a single pod the replication controller can help, by automatically bringing up a new pod when the existing one fails. another need for the replication controller is to have multiple pods to share the load across them

replica set is the new recommended way to use a replication controller

replication controller example

# replication-controller-example.yml
apiVersion: v1
kind: ReplicationController
metadata:
  name: nginx
spec:
  replicas: 3
  selector:
    app: nginx
  template:
    metadata:
      name: nginx
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx
        ports:
        - containerPort: 80

a replica set example

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: frontend
  labels:
    app: guestbook
    tier: frontend
spec:
  # modify replicas according to your case
  replicas: 3
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: php-redis
        image: us-docker.pkg.dev/google-samples/containers/gke/gb-frontend:v5

replica set vs replication controller

in a replica set a selector is required however in a replication controller it is not the case the controller will pick up the labels that were provided under the pod template section also in a replica set situation the replica set can control pods that were already created before even not by the replica set itself as long the label are matched