ReplicaSet

A ReplicaSet (RS) in Kubernetes ensures that a specified number of identical Pods are running at any given time.
It’s one of the key controllers that manages Pod replication and availability.

⚙️ Definition

A ReplicaSet is a Kubernetes controller that maintains a stable set of running Pods — it automatically adds or removes Pods to match the desired count.

🧩 Key Functions

• Ensures high availability — keeps a fixed number of replicas (Pods) running.
• If a Pod crashes or a node fails → the ReplicaSet creates a new Pod automatically.
• If there are too many Pods → it terminates extras to maintain the desired number.

🧱 Core Components

A ReplicaSet has three main parts:

1. Selector — identifies which Pods it manages (via labels).
2. Replicas — the desired number of Pod copies.
3. Pod Template — describes how to create new Pods.

🧾 Example YAML

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

Explanation:

• replicas: 3 → ensures 3 Pods are running.
• selector.matchLabels → ties the ReplicaSet to Pods with label app: myapp.
• template → defines the Pod configuration.

🔄 How It Works

1. You apply the ReplicaSet manifest.
2. The ReplicaSet controller checks how many Pods match its selector.
3. If:
   • Fewer Pods exist → it creates new Pods.
   • More Pods exist → it deletes extras.
4. It continuously monitors and self-heals the desired state.

🧠 Important Notes

• You rarely create ReplicaSets directly — instead, you use a Deployment, which manages ReplicaSets for rolling updates and rollbacks.
• ReplicaSet is the modern replacement for the older ReplicationController.
• If you delete a ReplicaSet, its Pods are deleted too (unless you set orphan deletion policy).

📊 Use Cases

• Ensuring constant availability of stateless Pods.
• Maintaining identical copies of an application across nodes.
• Forming the base for Deployments, which handle versioning and updates.