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ReplicaSets

In this section, we will take a look at the below

  • Replication Controller
  • ReplicaSet

Controllers are brain behind kubernetes

What is a Replica and Why do we need a replication controller?

rc

rc1

Difference between ReplicaSet and Replication Controller

  • Replication Controller is the older technology that is being replaced by a ReplicaSet.
  • ReplicaSet is the new way to setup replication.

Creating a Replication Controller

Replication Controller Definition File

rc2

    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-container
           image: nginx
     replicas: 3
  • To Create the replication controller
    $ kubectl create -f rc-definition.yaml
    
  • To list all the replication controllers
    $ kubectl get replicationcontroller
    
  • To list pods that are launch by the replication controller
    $ kubectl get pods
    
    rc3

Creating a ReplicaSet

ReplicaSet Definition File

rs

    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-container
           image: nginx
     replicas: 3
     selector:
       matchLabels:
        type: front-end

ReplicaSet requires a selector definition when compare to Replicaton Controller.

  • To Create the replicaset

    $ kubectl create -f replicaset-definition.yaml
    
  • To list all the replicaset

    $ kubectl get replicaset
    
  • To list pods that are launch by the replicaset

    $ kubectl get pods
    

    rs1

Labels and Selectors

What is the deal with Labels and Selectors? Why do we label pods and objects in kubernetes?

labels

How to scale replicaset

  • There are multiple ways to scale replicaset
    • First way is to update the number of replicas in the replicaset-definition.yaml definition file. E.g replicas: 6 and then run
   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-container
          image: nginx
    replicas: 6
    selector:
      matchLabels:
       type: front-end
$ kubectl apply -f replicaset-definition.yaml
  • Second way is to use kubectl scale command.
$ kubectl scale --replicas=6 -f replicaset-definition.yaml
  • Third way is to use kubectl scale command with type and name
$ kubectl scale --replicas=6 replicaset myapp-replicaset

rs2

K8s Reference Docs: