Kubernetes - ReplicaSet

 A ReplicaSet's purpose is to maintain a stable set of replica Pods running at any given time. As such, it is often used to guarantee the availability of a specified number of identical Pods.

Replication Controller(Equality based)	ReplicaSet (Set Based
It uses the operators ( =, ==, !=)	Operators ( in, notin,exists)
Example env=prod, env!=stag	Env in(prod)
Command Line kubectl get pods -l env=prod	Kubectl get pods ‘env in(prod,qa)’
Manifest selector:     app: nginx-app	selector:     matchLabels:       app: nginx-app     matchExpressions:       - {key: tier, operator: In, values: [frontend]}

LAB

# nginx-rs.yaml
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-rs
spec:
  replicas: 3
  template:
    metadata:
      name: nginx-pod
      labels:
        app: nginx-app
        tier: frontend
    spec:
      containers:
      - name: nginx-container
        image: nginx
        ports:
        - containerPort: 80
  selector:
    matchLabels:
      app: nginx-app
    matchExpressions:
      - {key: tier, operator: In, values: [frontend]}

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# 2. Create and display replicaset

kubectl create -f nginx-rs.yaml

kubectl get po -o wide

kubectl get po -l app=nginx-app

kubectl get rs nginx-rs -o wide

kubectl describe rs nginx-rs

kubectl get po -l 'tier in (frontend)'

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# 3. Automatic Pod Reschedule 

kubectl get po -o wide --watch

kubectl get po -o wide

kubectl get nodes

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# 4. Scale up pods

kubectl scale rs nginx-rs --replicas=5

kubectl get rs nginx-rs -o wide

kubectl get po -o wide

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# 5. Scale down pods

kubectl scale rs nginx-rs --replicas=3

kubectl get rs nginx-rs -o wide

kubectl get po -o wide

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# 6. Cleanup

kubectl delete -f nginx-rs.yaml

kubectl get rs

kubectl get po -l app=nginx-app