16. Controller - Replication Controller

Summary

A replication controller in Kubernetes is a resource object that helps ensure a specified number of pod replicas are running at all times. It monitors the state of the pods and automatically replaces any that have failed or been terminated, thus maintaining the desired number of replicas. It also allows for scaling up or down the number of replicas as needed to handle changes in application demand.

Basic Configuration

• selector: It defines pods in the replication controller range

• replicas: It defines the desired number of pods that would run.

Principle of Replication Controller operation

Replication Controller definition

apiVersion: v1
kind: ReplicationController        
metadata:
  name: rc-nginx
spec:
  replicas: 3            # Desired number of pods
  selector:
    app: webui           #
  template: 
    metadata:
      name: nginx-pod
      labels:
        app: webui       #
  spec:
    containers:
      - name: nginx-container
        image: nginx:1.14

Example

Basic

1. Enforce replication controller yaml

    $ kubectl create -f re-nginx.yaml  
   

   

2. Check pods status

    $ kubectl get pods -o wide  
   

   

    $ kubectl get pods --show-labes
   

   

3. Check the replicationcontrollers status

    $ kubectl get replicationcontrollers
   

   

    $ kubectl get rc -o wide
   

   

4. Check the describe information

    $ kubectl describe rc rc-nginx
   

   

Change replicas number

1. open rc-nginx template for modifying

    $ kubectl edit re re-nginx
   

   

2. Change replicas number as 5

    ...
    spec:
      replicas: 5  # Change 3 to 5
    ...
   

   

3. Check the pods status

    $ kubectl get pods -o wide  
   

   

    $ kubectl get pods --show-labels
   

   

    $ kubectl describe rc rc-nginx
   

   

Modify configuration by CLI command

1. Change replicas number to 2 by CLI command

    $ kubectl scale re re-nginx --replicas=2
   

   

2. Chech the pods status

    $ kubectl get pods -o wide
   

   

3. Change replicas number to 4 by CLI command again

    $ kubectl scale re re-nginx --replicas=4
   

   

4. Chech the pods status again

    $ kubectl get pods -o wide
   

   

Delete pods made by replication controller

1. Delete pods

    $ kubectl delete re rc-nginx
   

   

2. Check pods

    $ kubectl get pods
   

   

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