Replication Controllers and ReplicaSets

Kubernetes controllers act as the control plane’s “brain,” continuously observing cluster objects and reconciling the current state with your declared desired state. In this lesson, we’ll deep dive into two controllers that manage Pod replicas: ReplicationController and ReplicaSet.

Why Use Replication Controllers?

Running a single Pod is risky—if it crashes, your application goes offline. A ReplicationController (RC) maintains a specified number of identical Pods, ensuring high availability and load distribution.

High availability: If one Pod fails, others continue serving traffic.

The image illustrates a high availability setup with a user accessing a node containing a replication controller managing two pods.

Redundancy and load balancing across nodes:

The image illustrates a high availability setup with two nodes, each containing a replication controller and pods, indicating redundancy and load balancing.

Scaling across nodes to meet growing demand:

The image illustrates a load balancing and scaling concept using Kubernetes, showing multiple pods distributed across nodes with a replication controller managing them.

ReplicationController vs. ReplicaSet

While both controllers maintain a set of Pod replicas, ReplicaSet (RS) is the modern API (apps/v1) and supports set-based label selectors. ReplicationController (v1) is older and has fewer selector capabilities. We recommend using ReplicaSets for new deployments.

Feature	ReplicationController (`v1`)	ReplicaSet (`apps/v1`)
API Version	v1	apps/v1
Selector Support	Equality-based labels only	Equality & set-based label match
Recommended for Usage	Deprecated	Yes

Creating a ReplicationController

Define rc-definition.yaml:

apiVersion: v1
kind: ReplicationController
metadata:
  name: myapp-rc
  labels:
    app: myapp
    tier: front-end
spec:
  replicas: 3
  template:
    metadata:
      name: myapp-pod
      labels:
        app: myapp
        tier: front-end
    spec:
      containers:
        - name: nginx-container
          image: nginx

Apply the manifest and verify:

kubectl create -f rc-definition.yaml
kubectl get replicationcontroller

Example output:

NAME       DESIRED   CURRENT   READY   AGE
myapp-rc   3         3         3       19s

List Pods managed by the RC:

kubectl get pods

NAME            READY   STATUS    RESTARTS   AGE
myapp-rc-4lvk9  1/1     Running   0          20s
myapp-rc-mc2mf  1/1     Running   0          20s
myapp-rc-px9pz  1/1     Running   0          20s

Introducing ReplicaSets

A ReplicaSet manifest closely mirrors an RC, with two key differences:

apiVersion: apps/v1
selector: Required to match Pods

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: myapp-replicaset
  labels:
    app: myapp
    tier: front-end
spec:
  replicas: 3
  selector:
    matchLabels:
      app: myapp
      tier: front-end
  template:
    metadata:
      name: myapp-pod
      labels:
        app: myapp
        tier: front-end
    spec:
      containers:
        - name: nginx-container
          image: nginx

Apply and verify:

kubectl create -f replicaset-definition.yaml
kubectl get replicaset
kubectl get pods

Labels and Selectors

Labels are key/value pairs attached to objects. A ReplicaSet’s spec.selector.matchLabels determines which Pods it manages—even pre-existing ones. Example Pod with labels:

metadata:
  name: myapp-pod
  labels:
    tier: front-end

Matching selector in a ReplicaSet:

selector:
  matchLabels:
    tier: front-end

Scaling a ReplicaSet

You can adjust replica counts by editing the YAML or using kubectl scale: Option 1: Update replicas in replicaset-definition.yaml and apply:

kubectl replace -f replicaset-definition.yaml

Option 2: Scale on the fly:

kubectl scale --replicas=6 -f replicaset-definition.yaml

Using kubectl scale updates in-memory replica counts but does not modify your source YAML file.
For automatic scaling based on metrics, see the Horizontal Pod Autoscaler.

Command Cheat Sheet

Command	Description
kubectl create -f `<file>`	Create any Kubernetes object
kubectl get `<resource>`	List resources (e.g., pods, rs)
kubectl delete `<resource>` `<name>`	Delete a resource by name
kubectl replace -f `<file>`	Update by replacing the manifest
kubectl scale —replicas=`<n>` -f `<file>`	Scale replicas from the command line

Introduction

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Docker Engine - Security

Docker Engine - Networking

Docker Engine - Storage

Docker Compose

Docker Swarm

Kubernetes

Docker Engine Enterprise

Docker Trusted Registry

Disaster Recovery

Replication Controllers and ReplicaSets

Why Use Replication Controllers?

ReplicationController vs. ReplicaSet

Creating a ReplicationController

Introducing ReplicaSets

Labels and Selectors

Scaling a ReplicaSet

Command Cheat Sheet

Links and References

Watch Video

​Why Use Replication Controllers?

​ReplicationController vs. ReplicaSet

​Creating a ReplicationController

​Introducing ReplicaSets

​Labels and Selectors

​Scaling a ReplicaSet

​Command Cheat Sheet

​Links and References

Watch Video

Why Use Replication Controllers?

ReplicationController vs. ReplicaSet

Creating a ReplicationController

Introducing ReplicaSets

Labels and Selectors

Scaling a ReplicaSet

Command Cheat Sheet

Links and References