ReplicaSets

When you run a Pod, it is a single instance. If that Pod fails, it’s gone. To keep multiple copies of your application running and ensure they stay running, you need a controller. The simplest controller is a ReplicaSet.

What Is a ReplicaSet?

A ReplicaSet ensures that a specified number of identical Pods are running at all times. If a Pod fails, the ReplicaSet creates a new one. If there are too many, it terminates extra ones.

A ReplicaSet maintains a stable set of replica Pods running at any given time.

ReplicaSet YAML Example

Here’s a ReplicaSet that maintains three Nginx Pods:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-rs
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:latest
        ports:
        - containerPort: 80

replicas: 3 – desired number of Pods.
selector – identifies which Pods belong to this ReplicaSet.
template – the Pod specification to create new Pods.

Creating and Inspecting a ReplicaSet

Save the YAML as nginx-rs.yaml and apply:

kubectl apply -f nginx-rs.yaml

View the ReplicaSet:

kubectl get replicasets

View the Pods it created:

kubectl get pods -l app=nginx

Scaling a ReplicaSet

You can scale the ReplicaSet imperatively:

kubectl scale replicaset nginx-rs --replicas=5

Or edit the YAML and reapply.

Deleting a ReplicaSet

Deleting the ReplicaSet also deletes its Pods:

kubectl delete replicaset nginx-rs

Two Minute Drill

ReplicaSet ensures a specified number of identical Pods run at all times.
Define replicas, selector, and pod template in YAML.
Scale with kubectl scale replicaset name --replicas=N.
Deleting a ReplicaSet deletes its Pods.

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