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What is a Deployment?

A Deployment is a higher-level abstraction that manages a set of replicas of the application, just like a ReplicaSet, but with more features like rolling updates and rollbacks. 
Deployment
  └── ReplicaSet
        ├── Pod
        ├── Pod
        └── Pod
For example, if you want to upgrade 3 application instances one by one without downtime, this is where Deployment comes into play. This upgrade process is called a rolling update.
deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: app-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: backend
  template:
    metadata:
      name: app-pod
      labels:
        app: backend
    spec:
      containers:
        - name: backend-container
          image: backend:1.0

kubectl get deployments
kubectl create deployment <deployment-name> --image=<image-name> --replicas=<replicas>
kubectl create deployment backend --image=web:1.0 --replicas=3

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Rollout and Rollback

Rollout — A process of updating the application to a new version by gradually replacing old pods with new ones. Rollback — A process of reverting to a previous version if something goes wrong during the rollout. When you first create a deployment, it triggers a rollout. Each subsequent update triggers another rollout. If something goes wrong, you can rollback. 
# Get the status of a rollout
kubectl rollout status deployment/<deployment-name>

# Get the revision history of a rollout
kubectl rollout history deployment/<deployment-name>

# Check a specific revision
kubectl rollout history deployment/<deployment-name> --revision=<revision-number>
kubectl rollout history deployment/app-deploy --revision=1

# Rolling update
kubectl set image deployment/<deployment-name> <container-name>=<new-image>
kubectl set image deployment/app-deployment backend-container=backend:2.0

# Save the command used to create/update against the revision number
kubectl set image deployment/<deployment-name> <container-name>=<new-image> --record

# Rollback (undo)
kubectl rollout undo deployment/<deployment-name>

# Rollback to a specific revision
kubectl rollout undo deployment/<deployment-name> --to-revision=<revision-number>
kubectl rollout undo deployment/app-deploy --to-revision=1

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Deployment Strategies

All existing pods are terminated before new pods are created with the new configuration. This causes downtime during the update.
deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: app-deployment
spec:
  replicas: 3
  strategy:
    type: Recreate
  selector:
    matchLabels:
      app: backend
  template:
    metadata:
      name: app-pod
      labels:
        app: backend
    spec:
      containers:
        - name: backend-container
          image: backend:1.0

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