K8s - ReplicaSet

19 Apr 2023 • kubernetes

About ReplicaSet

A ReplicaSet’s purpose is to maintain number of Pod available at any given time. It is used to guarantee the availability of a specified number of identical Pods.

From the above picture, if the Pod die for any reason, Kubernetes will spring up a new one Pod to ensure the Pod availability.

In short, if want to deploy a Pod in Kubernates cluster, we could wrap it in a ReplicaSet to ensure its availability in the cluster in case the Pod dies for any reason.

Creating ReplicaSet

Let’s wrap the previous Pod in ReplicaSet in the following replica-set.yaml

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: webapp
spec:
  replicas: 1
  selector:
    matchLabels:
      app: webapp
      release: "0-5"
  template: # template for the pod
    metadata:
      labels:
        app: webapp
        release: "0-5"
    spec:
      containers:
      - name: webapp
        image: richardchesterwood/k8s-fleetman-webapp-angular:release0-5

---
apiVersion: v1
kind: Service
metadata:
  name: fleetman-webapp
  namespace: default
spec:
  # This defines which pods are going to be represented by this service
  # The service becomes a network endpoint for either other services
  # or maybe external users to connect to (browser)
  selector:
    app: webapp # can be any selector according to the label defined in pod ex: myapp:webapp
    release: "0-5"
  ports:
  - name: http
    port: 80
    # the nodeport should be greater than 30000
    nodePort: 30080 
  type: NodePort

Before applying the above yaml, let’s delete the previous pods to make it clean.

# deleting all pods
kubectl delete pods --all

# creating replica-set
kubectl apply -f replicaset.yaml

Before open the app through browser, we could evaluate the ReplicaSet creation and the Service.

If the ReplicaSet creation as expected, then we can open the app through browser (http://192.168.59.100:30080) to ensure the service links to Pods.

Simulating Pod Crash

To simulate Pod crash, let’s delete the pod. The Kubernetes should recreated the a new similar pod for us.

On the detail checking of replica set shown that the pod recreated when it’s deleted.

kubectl describe replicaset {NAME_REPLICASET}
kubectl describe rs {NAME_REPLICASET}

Deleting Replica Set

If delete ReplicaSet, the corresponding pods will be deleted as well.

kubectl delete rs {NAME_REPLICA_SET}
 kubernetes

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