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: Feature ReplicationController (RC) ReplicaSet (RS) API Version v1 apps/v1 Label Selector Support Equality-based only Equality & set-based Usage Legacy / older systems Modern replacement for RC Used by Deployment No Yes Flexibility Less flexible More flexible Community Support Deprecated Actively supported 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!

Apr 27, 2025 - 20:55

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:

Feature	ReplicationController (RC)	ReplicaSet (RS)
API Version	`v1`	`apps/v1`
Label Selector Support	Equality-based only	Equality & set-based
Usage	Legacy / older systems	Modern replacement for RC
Used by Deployment	No	Yes
Flexibility	Less flexible	More flexible
Community Support	Deprecated	Actively supported

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!