Understanding the replica set in Kubernetes

What is Replica?

Replica means copy so having exact same copy of a running pod is called as Replica. Where as ReplicaSet is a way to maintain the stable set of replica Pods running at any time. It guarantees the availability of specified number of identical Pods.

How does it work

ReplicaSet is defined with fields having a selector that specifies how to identify Pods It can acquire, a number of replicas indicating how many Pods it should be maintaining and a pod template specifying the data of of new pods which it should be maintaining. When replica-set needs to create a new pod it uses pod template to create a new pod.

ReplicaSet VS ReplicationController

ReplicaSet and Replication Controller both do the same thing. Both of them ensures that a specified number of Pods mentioned in replicas are running at any time. The only difference comes with the usage

of selectors to replicate Pods.ReplicaSet uses Set-Based selectors while Replication Controller use Equity-Based selectors.

See in below examples:

Replication Controller

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

Replica-Set

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: frontend
  labels:
    app: RS
    tier: frontend
spec:
  # modify replicas according to your case
  replicas: 3
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: redis
        image: gcr.io/google_samples/gb-frontend:v3

Save it as ReplicaSet.yaml in current directory and run below command.

kubectl apply -f ReplicaSet.yaml

Now you can check the ReplicaSet with below command:

kubectl get rs

or

kubectl get replicaset

Note: If you think this helped you and you want to learn more stuff on devops, then I would recommend joining the Kodecloud devops course and go for the complete certification path by clicking this link