04-ReplicationController&ReplicaSet.md

Replication Controller

ensures that a specified number of pods are running at any time

• if there are excess pods, they get killed and vice versa
• new pods are launched when they get fail, get deleted or terminated

creating a rc with count of 1 ensures that a pod is always available

create a manifest file for Replication Controller nginx-rc.yml

apiVersion: v1
kind: ReplicationController
metadata:
  name: nginx-rc
spec:
  replicas: 3
  selector:
    app: nginx-app
  template:  
  metadata:
    name: nginx-pod
    labels:
      app: nginx-app
    spec:
      containers:
        - name: nginx
          image: nginx
          imagePullPolicy: ifNotPresent

• kubctl create -f nginx-rc.yml

To filter the pods using lables

• kubctl get pod -l app=nginx-app

To display complete details of nginx ReplicationController

• kubectl describe rc nginx-rc

To scale up nginx app to 5 replicas

• kubectl scale rc nginx-rc --replicas=5

To scale down nginx app to 3 replicas

• kubectl scale rc nginx-rc --replicas=3

to delelte the ReplicationController created

if we delete the ReplicationController it will delete all the pods which are under its control.

• kubectl delete -f nginx-rc.yml or
• kubectl delete rc nginx-rc

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Difference between ReplicationController and ReplicaSet

• ReplicaSet is next generation ReplicationController
• ReplicaSet support set based selector whereas ReplicationController supports equality based selector
• equality based operators =, == , !=

eg: environment = production tier != frontend kubctl get pods -l environment=production

• set based operators in, notin, exist

has a advantage of having two or more options eg: environment in (production, qa) tier notin (frontend,backend) kubectl get pods -l environmentin (production)

• Difference in manifeast file

supports on older resources such as: ReplicationController, services

selector:
   app: nginx
   tier: frontend

---

supports on newer resources such as: ReplicaSet, Deployments, jobs, DaemonSet

selector:
      matchLables:
         app: nginx
         tier: frontend

---

ReplicaSet

create a manifest file for ReplicaSet nginx-rs.yml

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-rs
spec:
  replicas: 3
  selector:
    matchLables:
      app: nginx-app
  template:  
  metadata:
    name: nginx-pod
    labels:
      app: nginx-app
    spec:
      containers:
        - name: nginx
          image: nginx
          imagePullPolicy: ifNotPresent

• kubectl create -f nginx-rs.yml

Other commands are all same as ReplicationController only write rs instead of rc