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Chapter 3.5 - ReplicaSets

Learning Objectives​

By the end of this chapter, you will be able to:

  • Understand the role of ReplicaSets
  • Create and manage ReplicaSets
  • Understand how ReplicaSets maintain the number of replicas
  • Use ReplicaSets with Deployments

What is a ReplicaSet?​

A ReplicaSet maintains a stable number of running Pod replicas. It guarantees that a specified number of identical Pods is always available.

Why ReplicaSets?​

Self-Healing​

If a Pod crashes, the ReplicaSet automatically creates a new Pod:

Scaling​

You can easily increase or decrease the number of replicas.

YAML Declaration​

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: web-replicaset
  labels:
    app: web
    tier: frontend
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
      tier: frontend
  template:
    metadata:
      labels:
        app: web
        tier: frontend
    spec:
      containers:
      - name: nginx
        image: nginx:1.20
        ports:
        - containerPort: 80

Important Sections​

  1. replicas: Desired number of Pods
  2. selector: How to identify Pods to manage
  3. template: Template for creating new Pods

Selector and Labels​

Important: The labels in selector.matchLabels must match the labels in template.metadata.labels.

Managing ReplicaSets​

Create a ReplicaSet​

# From a YAML file
kubectl apply -f replicaset.yaml

# Create and view the YAML
kubectl create replicaset web-rs --image=nginx:1.20 --replicas=3 --dry-run=client -o yaml

Scaling​

# Change the number of replicas
kubectl scale replicaset web-replicaset --replicas=5

# Or edit directly
kubectl edit replicaset web-replicaset

Delete​

# Delete the ReplicaSet (Pods too)
kubectl delete replicaset web-replicaset

# Delete only the ReplicaSet (keep Pods)
kubectl delete replicaset web-replicaset --cascade=orphan

ReplicaSets vs Deployments​

ReplicaSets (Low Level)​

  • Directly manage Pods
  • No version management
  • No rolling updates

Deployments (High Level)​

  • Manage ReplicaSets
  • Version management
  • Rolling updates and rollbacks
  • Recommended for most use cases

Summary​

In this chapter, you learned:

ReplicaSet: Maintains a stable number of identical Pods
Self-healing: Automatically creates new Pods when needed
Scaling: Easily adjustable
Selector: Identifies Pods to manage
Deployments: Use ReplicaSets with advanced features

Next Steps​

Now that you understand ReplicaSets:

Chapter 3.6: Deployments - Advanced Management
Lab 3.2: Deploying with ReplicaSets

Chapter created: December 2024