# Deployments and ReplicaSets — Kubernetes

Source: https://www.skillbyai.com/en/kubernetes/w-deploy

> Keep N copies running.

## The standard for stateless services

A **Deployment** declares a pod template and a replica count. It creates a **ReplicaSet**, which keeps exactly that many pods running (replacing any that fail). Changing the template (a new image tag, new environment variables) makes the Deployment create a new ReplicaSet and shift pods to it gradually, keeping old ReplicaSets for **rollback** (`kubectl rollout undo`). The `selector` must match the template's labels; it is how the Deployment knows which pods it owns.

## Commands for managing a Deployment

Common day-to-day commands; they need a cluster. Not run here.

```bash
kubectl apply -f deployment.yaml
kubectl get deploy,rs,pods -l app=web
kubectl set image deployment/web web=ghcr.io/example/web:1.5.2   # triggers a rollout
kubectl rollout status deployment/web
kubectl rollout history deployment/web
kubectl rollout undo deployment/web                                # back to the previous ReplicaSet
kubectl scale deployment/web --replicas=5
```

## Pin image tags

Use explicit versions or digests, never latest; otherwise "what is running" is unknowable and rollbacks are unreliable.

**Quiz:** What does a Deployment create to manage its pods?

- [ ] A StatefulSet
- [x] A ReplicaSet for each version of the pod template
- [ ] A new cluster
- [ ] A Service automatically

*Answer:* A ReplicaSet for each version of the pod template. ReplicaSets keep the replica count; Deployments manage versions.