Deployments

A Deployment is the primary way to run stateless applications in Kubernetes. It declaratively describes a desired number of pod replicas and the pod template to use, and it manages rolling updates and rollbacks automatically.

Why Use Deployments?

If you create a bare pod and the node it runs on fails, the pod is gone. A Deployment creates a ReplicaSet behind the scenes, which in turn ensures the specified number of pod replicas are running at all times. If a pod crashes or a node disappears, the ReplicaSet controller creates a replacement.

Writing a Deployment Manifest

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web-app
  labels:
    app: web-app
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web-app
  template:
    metadata:
      labels:
        app: web-app
    spec:
      containers:
        - name: web
          image: my-registry/web-app:1.0.0
          ports:
            - containerPort: 3000
          resources:
            requests:
              cpu: "200m"
              memory: "256Mi"
            limits:
              cpu: "500m"
              memory: "512Mi"

Key fields:

• spec.replicas — how many pod copies should always be running
• spec.selector.matchLabels — how the Deployment finds the pods it owns
• spec.template — the pod template; must match the selector labels

Deployment Strategy

By default, Kubernetes uses a RollingUpdate strategy, which replaces old pods with new ones incrementally. You can tune two parameters:

• maxUnavailable — how many pods may be unavailable during the update (default 25%)
• maxSurge — how many extra pods may be created above the desired count during the update (default 25%)

The alternative strategy is Recreate, which terminates all pods before starting new ones — useful when the application cannot run two versions simultaneously.

Interacting with Deployments

# Apply or update a deployment
kubectl apply -f deployment.yaml

# Check rollout status
kubectl rollout status deployment/web-app

# Scale the deployment
kubectl scale deployment web-app --replicas=5

# Update the container image
kubectl set image deployment/web-app web=my-registry/web-app:2.0.0

# Rollback to the previous revision
kubectl rollout undo deployment/web-app

# View rollout history
kubectl rollout history deployment/web-app

ReplicaSets

A Deployment creates and manages ReplicaSets. Each time you update the pod template (e.g., change the image), a new ReplicaSet is created and the old one is scaled down. This is how rollbacks work — Kubernetes keeps a configurable history of old ReplicaSets and can restore any of them.

Deployment vs. Other Workload Types

Deployments are intended for stateless applications. Kubernetes also provides: