Controllers in K8s

CONTROLLERS

Controllers are the brain behind the k8s. They are the processes that monitor the k8s objects and respond accordingly.

Replication Controller:

The replication controller helps us to run multiple instances of a single pod in a k8s cluster, thus providing High Availability.

They can also replace a Single failed pod, thus provide HA even without multiple instance of PODs.

It also helps in Load Balancing and Scaling. Another reason we need replicaiton controller is to create mulltiple pods and to share the load between them. At first it will increase the number of pods in the same node when demand increases, say when the user is increasing. After the node has reached its bottleneck , we will create additional pods in another new nodes. Thus the replication controller spans across multiple nodes in the cluster.

Replication Controller Vs Replica Set:

They both have the same purpose, but they are not the same.

Replica Set is the new technology for replication setup in k8s, the features that we said about Replicaton Controller is also applicable to Replica Set too.

Replication Controller definition: rc-definition.yml

apiVersion: v1
kind: ReplicationController
metadata:
  name: myapp-rc
  labels:
    app: myapp
    type: front-end
spec:
  template:
    metadata:
      name: myapp-pod
      labels:
        app: myapp
        type: front-end
      spec:
        containers:
          - name: nginx-image
            image: nginx
  replicas: 3

# kubectl create -f rc-definition.yml
# kubectl get replicationcontroller
# kubectl get pods

Replica Set Definition : replicaset-definition.yml

The mager difference between the Replication Controller and the Replica Set is there is an another child (selector: )for the component spec: in Replica Set

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: myapp-replicaset
  labels:
    app: myapp
    type: front-end
spec:
  template:
    metadata:
      name: myapp-pod
      labels:
        app: myapp
        type: front-end
      spec:
        containers:
          - name: nginx-image
            image: nginx
  replicas: 3
  selector:
    matchLabels:
      type: front-end

# kubectl create -f replicaset-definition.yml
# kubectl get replicaset
# kubectl get pods
# kubectl delete replicaset myapp-replicaset

Scale the Replica Set:

First need to edit the “replicas: 3” section and change that to “replicas: 6” in replicaset-definition.yml

Now execute the below commands to scale the replica sets to 6 replicas

#  kubectl replace -f replicaset-definition.yml
    OR
# kubectl scale --replicas=6 -f replicaset-definition.yml
    OR
# kubectl scale --replicas=6 replicaset myapp-replicaset
Where ( replicaset -> Type ) & ( myapp-replicaset -> Name of replicaset )

NB: The value in the replicaset-definition.yml file will remain 3 , even after you update the replicas value to 6  with the “kubectl scale” command method using replicaset-definition.yml file.