09_ReplicaSet.md

What is ReplicaSet?

A ReplicaSet (RS) is a Kubernetes object that ensures a specified number of identical Pods are always running.

Think of it as the auto-healing layer that keeps your application running even if Pods crash or nodes fail.

Example: If you say "I want 3 replicas", ReplicaSet always maintains exactly 3 Pods.

• If 1 Pod dies → ReplicaSet creates a new one
• If 1 Pod is added manually → ReplicaSet deletes the extra Pod

Key Characteristics

1. Ensures desired number of replicas

2. Uses label selectors to identify Pods

• ReplicaSet manages Pods using labels and match selectors.

selector:
    matchLabels:
      app: nginx

• Any Pod with this label becomes part of the ReplicaSet.

3. Supports rolling updates (when used via Deployment)

While ReplicaSets can be used directly, Deployments use ReplicaSets to:

• Rollout new versions
• Roll back
• Update pods safely You rarely create ReplicaSets manually— Deployment is preferred.

4. Self-healing mechanism

When a Pod crashes or a node goes down:

• ReplicaSet recreates the missing Pod automatically
• Gives you high availability.

YAML Definition Example

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