Tutorial: Deploying a Spring Boot app on Kubernetes

I set up this project to demonstrate how to dockerize a Spring Boot app and deploy, configure and scale it on Kubernetes. In this tutorial I'm using minikube locally, you can also read my last post on how to run Kubernetes on AWS or try hosted Kubernetes in Google Container Engine.

Prequisites

To follow along, check out this repo and make sure you have the following tools ready:

• docker - to build the docker images we want to deploy
• minikube - a local Kubernetes environment
• kubectl - the Kubernetes command line interface, on macOS you can brew install kubernetes-cli it

The Spring Boot Service

The details of this service don't matter much. I used the Spring Initializr to create a very simple Spring Boot app which answers on port 8090 to these routes:

• /hello - which returns {"greeting": "hello world"}
• /health - to report the app's health status

The app can be built with gradle clean build which results in a standalone jar named demo-service-0.0.1-SNAPSHOT.jar in build/libs. The simplest way to run the app is with java -jar build/libs/demo-service-0.0.1-SNAPSHOT.jar.

Creating a Docker image

We need a container which has a JDK. If you just create an Ubuntu image with the standard Oracle JDK installation, you will end up with and image size of about 1 GB. Not nice to work with. There are better options though: Creating a minimial JDK installation based on an AlpineLinux image. The docker/minimal-java directory contains a Dockerfile I generated taking that approach.

So we'll just create our own JDK base image first:

cd docker/minimal-java
docker build -t sbrosinski/minimal-java .

Now we create a container for our demo service inherating from that base image:

cd docker/demo-service
docker build -t sbrosinski/demo-service .

This Dockerfile is very simple and based on Spring Boot's docker intro:

FROM sbrosinski/minimal-java
VOLUME /tmp
EXPOSE 8090
ADD demo-service-0.0.1-SNAPSHOT.jar app.jar
RUN sh -c 'touch /app.jar'
ENV JAVA_OPTS=""
ENTRYPOINT [ "sh", "-c", "java $JAVA_OPTS -Djava.security.egd=file:/dev/./urandom -jar /app.jar" ]

If you do this in production you should probably add some memory limiting options to the java call.

Running the Docker image

To try it out, we can run it locally just using docker:

docker run -p 8090:8090 -t --name demo-service --rm b7i/demo-service:latest
curl http://localhost:8090/hello => {"greeting":"hello world"}
docker stop demo-service

Publishing the Docker image

Kubernetes will have to pull the docker image from a registry. For this example we can use a public repository on DockerHub. Register on docker.com to create a docker ID. You can now log into your DockerHub account from your machine with:

docker login

Push your image to DockerHub with:

docker push sbrosinski/demo-service

The image for the demo service is publicly available at https://hub.docker.com/r/sbrosinski/demo-service/.

Setting up Kubernetes

We're using the local Kubernetes cluster provided by minikube. Start your cluster with:

minikube start

You can take a look at the (still empty) Kubernetes dashboard with:

minikube dashboard        

Deploying the service to Kubernetes

To run our application on the minikube cluster we need to specify a deployment. The deployment descriptor looks like this:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: demo-service-deployment
spec:
replicas: 2 # tells deployment to run 2 pods matching the template
template: # create pods using pod definition in this template
    metadata:
    labels:
        app: demo-service
    spec:
    containers:
    - name: demo-service
        image: sbrosinski/demo-service
        ports:
        - containerPort: 8090

Create this deployment on the cluster using kubectl:

kubectl create -f deployment.yml 

You can look at the deployment with:

kubectl describe deployment demo-service-deployment

Name:  			demo-service-deployment
Namespace:     		default
CreationTimestamp:     	Fri, 18 Nov 2016 11:42:05 +0100
Labels:			app=demo-service
Selector:      		app=demo-service
Replicas:      		2 updated | 2 total | 2 available | 0 unavailable
StrategyType:  		RollingUpdate
MinReadySeconds:       	0
RollingUpdateStrategy: 	1 max unavailable, 1 max surge
OldReplicaSets:		<none>
NewReplicaSet: 		demo-service-deployment-1946011246 (2/2 replicas created)
Events:
FirstSeen    	LastSeen       	Count  	From   				SubobjectPath  	Type   		Reason 			Message
---------    	--------       	-----  	----   				-------------  	--------       	------ 			-------
1m   		1m     		1      	{deployment-controller }       			Normal 		ScalingReplicaSet      	Scaled up replica set demo-service-deployment-1946011246 to 2

Two pods have been created, a replica set, and the default rolling update strategy. You can also look at the pods with:

kubectl get pods

NAME                                       READY     STATUS    RESTARTS   AGE
demo-service-deployment-1946011246-ap47n   1/1       Running   0          3m
demo-service-deployment-1946011246-u3dcj   1/1       Running   0          3m

We can join these pods as part of a service and expose it outside of our cluster. Create a service with:

kubectl create -f service.yml

The service descriptor looks like this:

apiVersion: v1
kind: Service
metadata:
name: demo-service
spec:
ports:
    - port: 8090
    targetPort: 8090
selector:
    app: demo-service
type: NodePort

By specifying a service type of NodePort we declare to expose the service outside the cluster. Type LoadBalancewould create a load balancer (e.g. ELB on AWS, but this feature is not availabe for minikube), type ClusterIP would expose the service only within the cluster. We can look at the service details with:

kubectl describe service demo-service

Name:  			demo-service
Namespace:     	default
Labels:			<none>
Selector:      	app=demo-service
Type:  			NodePort
IP:    			10.0.0.221
Port:  			<unset>	8090/TCP
NodePort:      	<unset>	31039/TCP
Endpoints:     	172.17.0.6:8090,172.17.0.7:8090
Session Affinity:  None
No events.

To now access the service, we can use a minikube command to tell us the exact service address:

minikube service demo-service

This would open your browser and point it, for example, to http://192.168.99.100:31039. Port 31039 is the NodePort we requested and the IP address is the address of our minikube cluster. We can now access the service routes:

curl http://192.168.99.100:31039/hello => {"greeting":"hello world"}        

That's it for now. To expand on this you can try the following: Make the service use a DB and play with Kubernetes persistent volumes and service discovery.