Replicaset

Replicaset is a controller that ensures that specified no. of pod replicas are running at any given time. It will autoheal / restart if any pod dies.

Creating Replicasets

rs.yml

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-replicaset # name of replicaset 
spec:
  replicas: 3
  selector: # how does replicaset knows which are my pods ? ( here whichever has lable set to apps: nginx ) # based on this u know how many replicas of this replicaset are up whether to increase/decrease 
    matchLabels:
      apps: nginx
  template: # how does pod looks like whose replicaset are supposed to be created/ started
    metadata:
      labels:
        apps: nginx # Pod that's getting started will has this label # Should be unique to the pod of this replicaset
    spec:
      containers:
        - name: nginx
          image: nginx:latest
          ports:
          - containerPort: 80

kubectl apply -f rs.yml

kubectl get replicaset # This will return replicaset
kubectl get pods # Pods are automatically started

kubectl delete  pod nginx-replicaset-vdstl # Even if u delete one of the pods, replicaset will start another pod in order to maintain 3 replicas

# Deleting replicaset
kubectl delete rs nginx-replicaset # ( replicaset_name )

Deployment

It manages bunch of replicasets.

• An application needs to be redeployed again and again. Whenever a new update happens, it starts 1 pod of new type after the newly started pod is healthy it stops 1 pod of old type . This way it drains old pods and starts newer gradually. That’s called rolling updates

  At the time of rolling update, If new pod fails to start it still is running older pods, it will not drain until newer pods are up & ready

• It provides scaling , rollback to older version. .

Creating Deployment

Deployment orchestrates replicaset which in turn orchestrates pods.

deployment.yml

# Same as rs.yml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment # name of replicaset 
spec:
  replicas: 3
  selector: # how does replicaset knows which are my pods ? ( here whichever has lable set to apps: nginx ) # based on this u know how many replicas of this replicaset are up whether to increase/decrease 
    matchLabels:
      apps: nginx
  template: # how does pod looks like whose replicaset are supposed to be created/ started
    metadata:
      labels:
        apps: nginx # Pod that's getting started will has this label # Should be unique to the pod of this replicaset
    spec:
      containers:
        - name: nginx
          image: nginx:latest
          ports:
          - containerPort: 80

kubectl apply -f deployment.yml

kubectl get deployment # This will return deployment that u created

kubectl get replicaset # replicaset will automatically be created

kubectl get pods # replicaset will start pods

• Check the history of deployment

kubectl rollout history deployment/nginx-deployment