Kubernetes - ReplicationController

  A ReplicationController ensures that a specified number of pod replicas are running at any one time. In other words, a ReplicationController makes sure that a pod or a homogeneous set of pods is always up and available.

How a ReplicationController Works

If there are too many pods, the ReplicationController terminates the extra pods. If there are too few, the ReplicationController starts more pods. Unlike manually created pods, the pods maintained by a ReplicationController are automatically replaced if they fail, are deleted, or are terminated. For example, your pods are re-created on a node after disruptive maintenance such as a kernel upgrade. For this reason, you should use a ReplicationController even if your application requires only a single pod.


LAB


1. Replication Controller YAML file


# nginx-rc.yaml  

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



*******************************************************************

# 2. Create and display


kubectl create -f nginx-rc.yaml

kubectl get po -o wide

kubectl get po -l app=nginx-app

kubectl get rc nginx-rc

kubectl describe rc nginx-rc


*******************************************************************

# 3. Reschedule


kubectl get po -o wide --watch

kubectl get po -o wide

kubectl get nodes


*******************************************************************

# 4. Scaling up cluster


kubectl scale rc nginx-rc --replicas=5

kubectl get rc nginx-rc

kubectl get po -o wide


*******************************************************************

# 5. Scalling down


kubectl scale rc nginx-rc --replicas=3

kubectl get rc nginx-rc

kubectl get po -o wide


*******************************************************************

# 6. Cleanup


kubectl delete -f nginx-rc.yaml

kubectl get rc

kubectl get po -l app=nginx-app

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