What is a replica set and deployment?

ReplicaSet ensures a specified number of identical pods running in a cluster. It monitors the state of pods and automatically replaces those that have failed or stopped.

Deployment is built on top of ReplicaSet. It manages application updates and rollbacks, maintaining a deployment version history. It allows updates using strategies like rolling update or recreate.

# Example Deployment manifest
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  replicas: 3 # Defines the desired number of pods (via ReplicaSet)
  selector:
    matchLabels:
      app: nginx
  strategy:
    type: RollingUpdate # Update strategy
    rollingUpdate:
      maxUnavailable: 1
      maxSurge: 1
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2 # Image version to be updated
        ports:
        - containerPort: 80

In this example, Deployment creates a ReplicaSet that maintains 3 pods with the image nginx:1.14.2. When the image version in the Deployment manifest is changed, it creates a new ReplicaSet for the new version and gradually shifts traffic to it, removing old pods according to the rolling update strategy.

Comparison table:

Feature	ReplicaSet	Deployment
Guarantees number of pods	✅	✅
Application updates	❌	✅
Version rollback	❌	✅
Manages deployment history	❌	✅