Controllers

• Controllers in k8s control/maintain state of k8s objects

ReplicaSet

• Refer Here for ReplicaSet official docs
• ReplicaSet is controller which maintains count of Pods as Desired State

RS-Activity1 Create 3 nginx pods

• Refer Here for the nginx rs manifest without selector
  
• temporary workaround for adding selectors. Refer Here for the changes added
• Now apply the manifest
  
• Lets change the replica count
  
• We can increase (scale out) as well decrease (scale in) the replica count
  

RS-Activity2 Create 5 Pods with jenkins and alpine in one Pod

• Refer Here for the manifest
• Apply the manifest
  
  
• Get events from describe rs
  
• Now delete a pod manually
  

Labels

• Refer Here for official docs
• Labels are key value pairs that can be attached as metadata to k8s objects.
• Labels help in selecting/querying/filtering objects
• Labels can be selected using
  • equality based Refer Here
  • set based Refer Here

Label Activity 1. Create a nginx pod with label

• Lets create a nginx pod with label app: nginx
• Refer Here for the pod spec with labels
  
• Lets run some other pods using declartative
  
  
• Selectors
  
  
• Create 5 pods with label app=jenkins
  
• Now run the replicaset with 5 replicas

---
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: jenkins-rs
spec:
  minReadySeconds: 5
  replicas: 5
  selector:
    matchLabels:
      app: jenkins
  template:
    metadata:
      name: jenkins
      labels:
        app: jenkins
    spec:
      containers:
        - name: jenkins
          image: jenkins/jenkins:lts-jdk11
          ports:
            - containerPort: 8080
        - name: alpine
          image: alpine:3
          args:
            - sleep
            - 1d

• Jenkins rs didnt create any pod as there were 5 pods matching label selector. we had deleted one pod which lead to creation of jenkins pod from the template section in above manifest
  
• ReplicationController only allows equality based selectors where as ReplicaSet supports set based selectors also
• Sample

---
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: setbased
  labels:
    purpose: understanding
    concept: setbased
spec:
  minReadySeconds: 2
  replicas: 3
  selector:
    matchExpressions:
      - key: app
        operator: In
        values:
          - nginx
          - web
      - key: env
        operator: NotIn
        values:
          - prod
          - uat
      - key: failing
        operator: DoesNotExist
        values:
  template:
    metadata:
      name: nginx
      labels:
        app: nginx
        env: dev
    spec:
      containers:
        - name: nginx
          image: nginx:1.23
          ports:
            - containerPort: 80

Exercise

• Write a manifest to create
  • nginx replication controller with 3 pods

Next Steps

• Service
• InitContainers
• Health Probes
• Managed K8s