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Dechen Tshering
Dechen Tshering

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Learning Kubernetes: ReplicationController vs ReplicaSet

#kubernetes #devops #yaml #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
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Basic ReplicationController Commands

List all RCs: 

kubectl get rc or kubectl get replicationcontroller
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Delete an RC: 

kubectl delete rc <rc-name>
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Scale the RC: 

kubectl scale --replicas=<number> rc/<rc-name>
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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
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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
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Basic Replica Set Commands:

List all ReplicaSets 

kubectl get rs
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This shows all the ReplicaSets in your current namespace.

Describe a ReplicaSet 

kubectl describe rs <replicaset-name>
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Provides detailed information including events, pod template, and selector.

Delete a ReplicaSet 

kubectl delete rs <replicaset-name>
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Deletes the ReplicaSet

Scale a ReplicaSet 

kubectl scale rs/<replicaset-name> --replicas=3
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Adjusts the number of desired pod replicas.

Edit a ReplicaSet 

kubectl edit rs <replicaset-name>
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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
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Creates or updates a ReplicaSet from a manifest file.

View pods managed by a ReplicaSet 

kubectl get pods -l app=web
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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!

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