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ReplicaSets

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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?

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

    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
  

  

Creating a ReplicaSet

ReplicaSet Definition File

    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 Replication 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
  

  

Labels and Selectors

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

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
    

    

K8s Reference Docs:

• https://kubernetes.io/docs/concepts/workloads/controllers/replicaset/
• https://kubernetes.io/docs/concepts/workloads/controllers/replicationcontroller/