Deployments and ReplicaSets

Key points

• A Deployment owns ReplicaSets, and a ReplicaSet owns pods. You edit the Deployment and never touch the rest.
• replicas: 3 means three pods, always. Kill one and a replacement is scheduled within seconds.
• Change anything in the pod template and the Deployment creates a new ReplicaSet, then shifts pods across to it.
• The selector must match the template labels, and it is immutable once created.
• kubectl scale is fine for an emergency, but the real replica count belongs in YAML in git.

Example

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
        - name: app
          image: ghcr.io/acme/web:1.4.2
          ports:
            - containerPort: 8080

You manage Deployments, Deployments manage ReplicaSets, ReplicaSets manage pods.