Kubernetes ReplicaSets

Step-01: Introduction to ReplicaSets¶

What are ReplicaSets?
What is the advantage of using ReplicaSets?

Step-02: Create ReplicaSet¶

Step-02-01: Create ReplicaSet¶

Create ReplicaSet

# Kubernetes ReplicaSet
kubectl create -f replicaset-demo.yml

replicaset-demo.yml

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: my-helloworld-rs
  labels:
    app: my-helloworld
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-helloworld
  template:
    metadata:
      labels:
        app: my-helloworld
    spec:
      containers:
      - name: my-helloworld-app
        image: stacksimplify/kube-helloworld:1.0.0

Step-02-02: List ReplicaSets¶

Get list of ReplicaSets

# List ReplicaSets
kubectl get replicaset
kubectl get rs

Step-02-03: Describe ReplicaSet¶

Describe the newly created ReplicaSet

# Describe ReplicaSet
kubectl describe rs/<replicaset-name>

kubectl describe rs/my-helloworld-rs
[or]
kubectl describe rs my-helloworld-rs

Step-02-04: List of Pods¶

Get list of Pods

# Get list of Pods
kubectl get pods
kubectl describe pod <pod-name>

# Get list of Pods with Pod IP and Node in which it is running
kubectl get pods -o wide

Step-02-05: Verify the Owner of the Pod¶

Verify the owner reference of the pod.

Verify under "name" tag under "ownerReferences". We will find the replicaset name to which this pod belongs to.

# List Pod with Output as YAML
kubectl get pods <pod-name> -o yaml
kubectl get pods my-helloworld-rs-c8rrj -o yaml

Step-03: Expose ReplicaSet as a Service¶

Expose ReplicaSet with a service (Load Balancer Service) to access the application externally (from internet)

# Expose ReplicaSet as a Service
kubectl expose rs <ReplicaSet-Name>  --type=LoadBalancer --port=80 --target-port=8080 --name=<Service-Name-To-Be-Created>
kubectl expose rs my-helloworld-rs  --type=LoadBalancer --port=80 --target-port=8080 --name=my-helloworld-rs-service

# List Services
kubectl get service
kubectl get svc

Access the Application using External or Public IP

# Access Application
http://<External-IP-from-get-service-output>/hello
curl http://<External-IP-from-get-service-output>/hello

# Observation
1. Each time we access the application, request will be sent to different pod and pods id will be displayed for us.

Step-04: Test Replicaset Reliability or High Availability¶

Test how the high availability or reliability concept is achieved automatically in Kubernetes

Whenever a POD is accidentally terminated due to some application issue, ReplicaSet should auto-create that Pod to maintain desired number of Replicas configured to achive High Availability.

# To get Pod Name
kubectl get pods

# Delete the Pod
kubectl delete pod <Pod-Name>

# Verify the new pod got created automatically
kubectl get pods   (Verify Age and name of new pod)

Step-05: Test ReplicaSet Scalability feature¶

Test how scalability is going to seamless & quick

Update the replicas field in replicaset-demo.yml from 3 to 6.

# Before change
spec:
  replicas: 3

# After change
spec:
  replicas: 6

Update the ReplicaSet

# Apply latest changes to ReplicaSet
kubectl replace -f replicaset-demo.yml

# Verify if new pods got created
kubectl get pods -o wide

Step-06: Delete ReplicaSet & Service¶

Step-06-01: Delete ReplicaSet¶

# Delete ReplicaSet
kubectl delete rs <ReplicaSet-Name>

# Sample Commands
kubectl delete rs/my-helloworld-rs
[or]
kubectl delete rs my-helloworld-rs

# Verify if ReplicaSet got deleted
kubectl get rs

Step-06-02: Delete Service created for ReplicaSet¶

# Delete Service
kubectl delete svc <service-name>

# Sample Commands
kubectl delete svc my-helloworld-rs-service
[or]
kubectl delete svc/my-helloworld-rs-service

# Verify if Service got deleted
kubectl get svc