Create a Deployment

Pods are ephemeral units in Kubernetes and should not be manually configured. Instead, workload resources like a Deployment should be used. In this example, we will create such a Deployment and use it with a Service.

Create Basic Setup

We will adapt the very first example that the documentation of Deployment provides us with. Instead of using an nginx container, we will use the specialized container proksch/showenv that shows additional information, like the hostname or the defined environment variables.

Create a new file deployment.yml and save it with the following content.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: showenv-deployment
  labels:
    app: showenv
spec:
  replicas: 3
  selector:
    matchLabels:
      app: showenv
  template:
    metadata:
      labels:
        app: showenv
    spec:
      containers:
      - name: showenv
        image: proksch/showenv
        ports:
        - containerPort: 8080

When compared to the simple-pod.yml example, two things stick out:

The Pods are no longer directly configured, instead, a Deployment just provides a template spec that defines how Pods can be created on demand.
A Deployment has a replication factor that determines how many Pods need to be spawned simultaneously.

Apart form these two difference, the definition of the Deployment looks very similar to the previous example. To make the example easier to inspect, we also add a Service and an Ingress.

Append the following configuration to the deployment.yml file.

...
---
apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: showenv
  ports:
    - port: 1234
      targetPort: 8080
---
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: my-ingress
spec:
  defaultBackend:
    service:
      name: my-service
      port:
        number: 1234

Apply the configuration and list the pods in your cluster. Once all Pods are in the state Running, create a tunnel for the Ingress and open localhost in your browser. Refreshing the browser will show you that a Service is a load balancer that alternates between all three instances.

$ kubectl apply -f deployment.yml
deployment.apps/showenv-deployment created
service/my-service created
$ kubectl get pods
NAME                                  READY   STATUS              RESTARTS   AGE
showenv-deployment-79dbf5666c-65rfw   1/1     Running             0          3s
showenv-deployment-79dbf5666c-gggnj   0/1     ContainerCreating   0          3s
showenv-deployment-79dbf5666c-kst52   0/1     ContainerCreating   0          3s
$ minikube tunnel

Kill one of the Pods to put the self-healing capabilities of Kubernetes to a test. When you inspect the status of your Pods afterwards, you will see that Kubernetes has automatically scheduled the creation of a new container to get back to the desired state.

$ kubectl delete pod showenv-deployment-79dbf5666c-65rfw
pod "showenv-deployment-79dbf5666c-65rfw" deleted
$ kubectl get pods
NAME                                  READY   STATUS              RESTARTS   AGE
showenv-deployment-79dbf5666c-4d74f   0/1     ContainerCreating   0          1s
showenv-deployment-79dbf5666c-gggnj   1/1     Running             0          21m
showenv-deployment-79dbf5666c-kst52   1/1     Running             0          21m

Scale the Deployment

Your Deployment has a hard-coded number of replicas, however, this number might be too small in practice. Assume that the service receives a higher load at the moment, so we want to increate the replication factor to 10. Adjust the deployment.yml file accordingly.

  replicas: 10

Re-apply the config and check the status of your Pods. You will see that Kubernetes has realized that the current state lacks 7 containers and starts to create them.

$ kubectl apply -f deployment.yml
$ kubectl get pods
NAME                                  READY   STATUS              RESTARTS   AGE
showenv-deployment-79dbf5666c-57nx6   0/1     ContainerCreating   0          4s
showenv-deployment-79dbf5666c-5kx6g   0/1     ContainerCreating   0          4s
showenv-deployment-79dbf5666c-65rfw   1/1     Running             0          7m42s
showenv-deployment-79dbf5666c-fc4j6   1/1     Running             0          4s
showenv-deployment-79dbf5666c-gggnj   1/1     Running             0          7m42s
showenv-deployment-79dbf5666c-jsfbs   1/1     Running             0          4s
showenv-deployment-79dbf5666c-kst52   1/1     Running             0          7m42s
showenv-deployment-79dbf5666c-w45l9   0/1     ContainerCreating   0          4s
showenv-deployment-79dbf5666c-whnsd   0/1     ContainerCreating   0          4s
showenv-deployment-79dbf5666c-wrlkt   0/1     ContainerCreating   0          4s

Your laptop has likely less than 10 cores, it might take some time before all containers are started and the system is stabil. Refresh the website to see that the website alternates between the 10 instances. Scaling is also possible in the other direction. Reduce your replicas back to 3 and re-apply the config. When you check the status of your Pods again, you will see that all excess containers were deleted.

To finish the exercise, delete the resources from the cluster (kubectl delete -f deployment.yml).