ReplicationController

What is ReplicationController in Kubernetes?

Replication Controller ensure that number of specified replicas run at a time.

ReplicationControllr control POD  make sure desired number of PODs always be ready for service request.

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.

Behavior:

Keeps POD running

Gives Direct Control of PODs.

Benefits:

Restart PODs , Desired State.

Fine grained control for scaling..

In above diagram we can see replication controller with pod count 2 and selector v1 keeps 2 PODs up.

and in second diagram we can see RC keep one POD always up.

When a node fails, only pods backed by a replication controller are recreated.

In above diagram we can see POD1 dies with node-1. POD2 created new replica on node2

Create Replication Controller:

Execute below command:

kubectl apply -f https://k8s.io/examples/controllers/replication.yaml

The output is similar to this:

replicationcontroller/nginx created

Replication Controller Yaml File:

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

view raw replicacontrol.yml hosted with ❤ by GitHub

root@ip-172-31-31-126:~# kubectl describe replicationcontrollers/nginx

Delete Replication Controller:

root@ip-172-31-31-126:~# kubectl get rc ==> Display available ReplicationController

NAME    DESIRED   CURRENT   READY   AGE

nginx   3         3         3       33m

root@ip-172-31-31-126:~# kubectl delete rc nginx  ==> It will delete RC nginx.

replicationcontroller "nginx" deleted

Scale Up:

root@ip-172-31-31-126:~# kubectl scale rc --replicas=7 nginx

replicationcontroller/nginx scaled

root@ip-172-31-31-126:~# kubectl get pods

NAME          READY   STATUS    RESTARTS   AGE

nginx-7pbtj   1/1     Running   0          10s

nginx-f247s   1/1     Running   0          10s

nginx-lj2ln   1/1     Running   0          2m31s

nginx-ptv2j   1/1     Running   0          2m31s

nginx-pzjq8   1/1     Running   0          10s

nginx-skltf   1/1     Running   0          10s

nginx-xm262   1/1     Running   0          2m31s

root@ip-172-31-31-126:~# kubectl get rc

NAME    DESIRED   CURRENT   READY   AGE

nginx   7         7         7       2m37s

root@ip-172-31-31-126:~#

 Scale Down:

root@ip-172-31-31-126:~# kubectl scale rc --replicas=3 nginx

replicationcontroller/nginx scaled

root@ip-172-31-31-126:~# kubectl get rc

NAME    DESIRED   CURRENT   READY   AGE

nginx   3         3         3       3m33s

root@ip-172-31-31-126:~# kubectl get pods

NAME          READY   STATUS    RESTARTS   AGE

nginx-lj2ln   1/1     Running   0          3m39s

nginx-ptv2j   1/1     Running   0          3m39s

nginx-xm262   1/1     Running   0          3m39s

Another option we can change yml file