Test Java Apps with Kubernetes

Prerequisites

• JDK 17+
• Maven 3.8+
• WSL 2
• Rancher Desktop 1.0.1+ (containerd runtime)

Build Java Artifact

• mvn clean package

Build Container Image

• nerdctl image build --namespace k8s.io -t k8s-test/test-app:latest .

Deploy to Kubernetes

• kubectl apply -f src/main/k8s/postgresql
• kubectl apply -f src/main/k8s/test-app

Upgrade App

• Build Java artifact and container image
• Restart app: kubectl rollout restart deployment/test-app

Monitor via JMX and VisualVM

• Port-forward particular pod's JMX port: kubectl port-forward <pod-name> --address 0.0.0.0 9999
• Open VisualVM and create JMX connection localhost:9999

Install Kubernetes Dashboard

Official Instructions

Helper objects are in src/main/k8s/dashboard

Important Points

Container Image

• Use fixed base image version
• Use two-stage image build to reduce image size and attack surface area
• In the build stage fetch the Spring Boot fat jar Java artifact (in this test plain copy is used for simplicity)
• Use Spring Boot layer tools to produce a layered image
• Do not run the container as root, create group and user for the app
• Externalize JVM options out of Dockerfile into Kubernetes descriptors
• Build image directly into k8s.io namespace to avoid using image registry for local development

Kubernetes Objects

• Use imagePullPolicy: Never for local development to avoid using image registry
• Limit CPU and memory resources
• Use Spring Boot liveness and readiness health check probes (set initialDelaySeconds as default is 0)
• Use pod lifecycle preStop pause to avoid that Kubernetes routes traffic to pods that are shutting down
• Externalize JVM options and app configuration into ConfigMap that is injected into pod's environment variables
  • Use relative heap limits: -XX:MaxRAMPercentage=50.0
  • Expose JMX port (see test-app.yaml / test-app-config / JDK_JAVA_OPTIONS)
  • Reference dependent stateful services by StatefulSets hostnames
  • Reference dependent stateless services hosts and ports from environment variables directly in env section
  • Materialize application configuration file for expanded properties to be visible in container
• Use volumes for app logs?
• Install databases as StatefulSets with PersistentVolume and VolumeClaimTemplates