replica_set.md

ReplicaSet

• ReplicaSet ensures that a specified number of pod replicas are running at any given time.
• It is recommended to use Deployments instead of directly using ReplicaSets, as they help manage ReplicaSets and provide declarative updates to Pods.

Check number of replica sets in the default namespace

show

kubectl get replicasets
# OR
kubectl get rs

Create a replica set named replica-set-demo using a pod named ngnix using a nginx image and labeled as tier=frontend with a single replica.

show

cat << EOF > replica-set-demo.yaml
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: replica-set-demo
spec:
  replicas: 1
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: nginx
        image: nginx
EOF

kubectl apply -f replica-set-demo.yaml

Scale up the replica-set-demo from 1 replica to 2 replicas

show

kubectl scale replicaset replica-set-demo --replicas=2

OR

Edit the replica set definition file replica-set-demo.yaml and apply kubectl apply -f replica-set-demo.yaml

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: replica-set-demo
spec:
  replicas: 2 # update this
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: nginx
        image: nginx
EOF

Scale up the replica-set-demo from 2 replicas to 1 replica

show

kubectl scale replicaset replica-set-demo --replicas=1

OR

Edit the replica set definition file replica-set-demo.yaml and apply kubectl apply -f replica-set-demo.yaml

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: replica-set-demo
spec:
  replicas: 1 # update this
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: nginx
        image: nginx
EOF

Create a replica set using the below definition and fix any issues.

apiVersion: v1
kind: ReplicaSet
metadata:
  name: replicaset-1
spec:
  replicas: 1
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
      - name: nginx
        image: nginx

show

Check the apiVersion using kubectl explain replicasets which is apps/v1.

Update the version and apply again.

Create a replica set using the below definition and fix any issues.

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: replicaset-2
spec:
  replicas: 1
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: nginx
    spec:
      containers:
      - name: nginx
        image: nginx

show

The replica set selector field tier: frontend does not match the pod labels tier: nginx. Correct either of them and reapply.

Clean up

kubectl delete replicaset replica-set-demo replicaset-1 replicaset-2
rm replica-set-demo.yaml