Learning Kubernetes: ReplicationController vs ReplicaSet

As I dive deeper into Kubernetes, I’ve been exploring the concepts of ReplicationControllers and ReplicaSets—two core constructs used to ensure high availability and scalability of pods in a Kubernetes cluster.

In this post, I’ll share what I learned, along with sample YAML definitions and key commands. Hopefully, it helps anyone else navigating this space too!

What is a ReplicationController?

A ReplicationController (RC) ensures that a specified number of pod replicas are running at any given time. While it’s considered a bit old-school now (ReplicaSet and Deployments have largely replaced it), it’s still a fundamental concept worth understanding.

Here’s a sample YAML to create a ReplicationController:

apiVersion: v1
kind: ReplicationController
metadata: 
  name: myrc
spec: 
  replicas: 5
  template:
    metadata:
      labels: 
        app: web
    spec:
      containers:
      - name: c1
        image: nginx

Basic ReplicationController Commands

List all RCs:

kubectl get rc or kubectl get replicationcontroller

Delete an RC:

kubectl delete rc

Scale the RC:

kubectl scale --replicas= rc/

What is ReplicaSet?

A ReplicaSet (RS) is the next-gen version of RC, introduced with better support for label selectors, including set-based ones. It ensures that a specified number of pod replicas are maintained and is mostly used under the hood by Deployments.

Example 1: Equality-Based Selector:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: mywebapp
spec:
  replicas: 5
  selector:
    matchLabels: 
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: c1
        image: nginx

Example 2: Set-Based Selector:

apiVersion: apps/v1
kind: ReplicaSet
metadata: 
  name: webdata
spec:
  replicas: 4
  selector:
    matchExpressions:
    - key: app
      operator: In
      values:
        - nginx
        - web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: c1
        image: nginx

Basic Replica Set Commands:

List all ReplicaSets

kubectl get rs

This shows all the ReplicaSets in your current namespace.

Describe a ReplicaSet

kubectl describe rs

Provides detailed information including events, pod template, and selector.

Delete a ReplicaSet

kubectl delete rs

Deletes the ReplicaSet

Scale a ReplicaSet

kubectl scale rs/ --replicas=3

Adjusts the number of desired pod replicas.

Edit a ReplicaSet

kubectl edit rs

Opens the ReplicaSet manifest in your default editor so you can make changes on the fly.

Apply from a YAML file

kubectl apply -f webrs.yml

Creates or updates a ReplicaSet from a manifest file.

View pods managed by a ReplicaSet

kubectl get pods -l app=web

Use label selectors to filter pods created by the ReplicaSet.

Key difference between ReplicationController and ReplicaSet:

After getting comfortable with Replication Controllers and Replica Sets, I’ve realized they’re just the beginning. Now, I’m excited to dive into Deployments—the real game-changer for managing apps in Kubernetes!