What is a Deployment?

Learn that a Deployment manages Pods.

What is a Deployment?

A Deployment is the standard way to run stateless applications. You give the desired pod template and replica count; the Deployment creates a ReplicaSet, and the ReplicaSet creates the pods. When you change the image, the Deployment opens a new ReplicaSet and gradually replaces the old pods with new ones.

Deployment (web)
 └── ReplicaSet (web-7c5b9f6d8)
      ├── Pod web-7c5b9f6d8-abcde
      ├── Pod web-7c5b9f6d8-fghij
      └── Pod web-7c5b9f6d8-klmno

Seeing the hierarchy

To list the three levels together and see the ownership chain:

$ kubectl get deploy,rs,pods -l app=web
NAME                  READY   UP-TO-DATE   AVAILABLE
deployment.apps/web   3/3     3            3
NAME                             DESIRED   CURRENT   READY
replicaset.apps/web-7c5b9f6d8    3         3         3
$ kubectl get pod web-7c5b9f6d8-abcde -o jsonpath='{.metadata.ownerReferences[0].kind}'
ReplicaSet

Exercise Create a deployment

Write two commands: create an api deployment with 2 replicas using the api:1.0 image; then list the deployments, ReplicaSets and pods labelled app=api in one command.

Show example solution

kubectl create deployment api --image=api:1.0 --replicas=2
kubectl get deploy,rs,pods -l app=api

Test yourself

In the pod name web-7c5b9f6d8-abcde, what does 7c5b9f6d8 show?
- The node name
- The image version
- A random number
- The ReplicaSet ID derived from a hash of the pod template
Show answer

The ReplicaSet ID derived from a hash of the pod template — When the template changes, a new hash means a new ReplicaSet.

List the deployments, ReplicaSets and pods together.

kubectl get deploy,____,pods -l app=web

Show answer

kubectl get deploy,rs,pods -l app=web

Match.

Deployment

Manages versions and updates
ReplicaSet

Keeps the replica count
Pod

Runs the containers
ownerReferences

Ownership chain

Next lesson: Deployment YAML

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