Kotlin Application Template

Kotlin service built with the Spring WebFlux reactive stack.

Prerequisites

Kotlin 2.0 w/ Java 21, Docker for building + running the containerized application:

brew install openjdk@21
brew install --cask docker # or just `brew install docker` if you don't want the Desktop app

For the provided Git hooks you will need:

brew install lefthook node talisman

Getting started

To get started with development run:

./run.sh init

This will replace placeholders in the application template and install a couple of Git hooks.

Tests

The project has distinct unit and integration test suites.

To run just the unit tests:

./gradlew test

To run the integration tests:

./gradlew integrationTest

You can find the unit tests in src/test/kotlin and the integration tests in src/integrationTest/kotlin.

Furthermore, there is another type of test worth mentioning. We're using ArchUnit for ensuring certain architectural characteristics, for instance making sure that there are no cyclic dependencies.

Formatting & Linting

For linting and formatting Kotlin code ktlint is used.

Consistent formatting for Kotlin, as well as various other types of source code (JSON, Markdown, Yaml, ...), is being enforced via Spotless.

Check formatting:

./gradlew spotlessCheck

Autoformat sources:

./gradlew spotlessApply

IntelliJ IDEA setup

See https://github.com/pinterest/ktlint#-with-intellij-idea

brew install ktlint
ktlint applyToIDEAProject

Git hooks

The repo contains a Lefthook configuration, providing a Git hooks setup out of the box.

To install these hooks, run:

./run.sh init

The hooks are supposed to help you to:

• commit properly formatted source code only (and not break the build otherwise)
• write conventional commit messages
• not accidentally push secrets and sensitive information

Code quality analysis

Continuous code quality analysis is performed in the pipeline upon pushing to trunk; it requires a token provided as SONAR_TOKEN repository secret that needs to be obtained from https://sonarcloud.io.

To run the analysis locally:

SONAR_TOKEN=[sonar-token] ./gradlew sonar

Go to https://sonarcloud.io for the analysis results.

Container image

Container images running the application are automatically published by the pipeline to the GitHub Packages Container registry.

To run the latest published image:

docker run -p8080:8080 "ghcr.io/digitalservicebund/kotlin-application-template:$(git log -1 origin/main --format='%H')"

The service will be accessible at http://localhost:8080.

We are using Spring's built-in support for producing an optimized container image:

./gradlew bootBuildImage
docker run -p8080:8080 ghcr.io/digitalservicebund/kotlin-application-template

Container images in the registry are signed with keyless signatures.

To verify an image:

cosign verify "ghcr.io/digitalservicebund/kotlin-application-template:$(git log -1 origin/main --format='%H')" --certificate-identity="https://github.com/digitalservicebund/kotlin-application-template/.github/workflows/pipeline.yml@refs/heads/main" --certificate-oidc-issuer="https://token.actions.githubusercontent.com"

If you need to push a new container image to the registry manually there are two ways to do this:

Via built-in Gradle task:

export CONTAINER_REGISTRY=ghcr.io
export CONTAINER_IMAGE_NAME=digitalservicebund/kotlin-application-template
export CONTAINER_IMAGE_VERSION="$(git log -1 --format='%H')"
CONTAINER_REGISTRY_USER=[github-user] CONTAINER_REGISTRY_PASSWORD=[github-token] ./gradlew bootBuildImage --publishImage

Note: Make sure you're using a GitHub token with the necessary write:packages scope for this to work.

Using Docker:

echo [github-token] | docker login ghcr.io -u [github-user] --password-stdin
docker push "ghcr.io/digitalservicebund/kotlin-application-template:$(git log -1 --format='%H')"

Note: Make sure you're using a GitHub token with the necessary write:packages scope for this to work.

Vulnerability Scanning

Scanning container images for vulnerabilities is performed with Trivy as part of the pipeline's build job, as well as each night for the latest published image in the container repository.

To run a scan locally:

Install Trivy:

brew install aquasecurity/trivy/trivy

./gradlew bootBuildImage
trivy image --severity HIGH,CRITICAL ghcr.io/digitalservicebund/kotlin-application-template:latest

As part of the automated vulnerability scanning we are generating a Cosign vulnerability scan record using Trivy, and then use Cosign to attach an attestation of it to the container image, again signed with keyless signatures similar to signing the container image itself. Using a policy engine in a cluster the vulnerability scan can be verified and for instance running a container rejected if a scan is not current.

License Scanning

License scanning is performed as part of the pipeline's build job. Whenever a production dependency is being added with a yet unknown license the build is going to fail.

To run a scan locally:

./gradlew checkLicense

Architecture Decision Records

Architecture decisions are kept in the docs/adr directory. For adding new records install the adr-tools package:

brew install adr-tools

See https://github.com/npryce/adr-tools regarding usage.

Slack notifications

Opt in to CI posting notifications for failing jobs to a particular Slack channel by setting a repository secret with the name SLACK_WEBHOOK_URL, containing a URL for Incoming Webhooks.

Contributing

🇬🇧 Everyone is welcome to contribute the development of the kotlin-application-template. You can contribute by opening pull request, providing documentation or answering questions or giving feedback. Please always follow the guidelines and our Code of Conduct.

🇩🇪 Jede:r ist herzlich eingeladen, die Entwicklung der kotlin-application-template mitzugestalten. Du kannst einen Beitrag leisten, indem du Pull-Requests eröffnest, die Dokumentation erweiterst, Fragen beantwortest oder Feedback gibst. Bitte befolge immer die Richtlinien und unseren Verhaltenskodex.

Contributing code

🇬🇧 Open a pull request with your changes and it will be reviewed by someone from the team. When you submit a pull request, you declare that you have the right to license your contribution to the DigitalService and the community. By submitting the patch, you agree that your contributions are licensed under the MIT license.

Please make sure that your changes have been tested befor submitting a pull request.

🇩🇪 Nach dem Erstellen eines Pull Requests wird dieser von einer Person aus dem Team überprüft. Wenn du einen Pull Request einreichst, erklärst du dich damit einverstanden, deinen Beitrag an den DigitalService und die Community zu lizenzieren. Durch das Einreichen des Patches erklärst du dich damit einverstanden, dass deine Beiträge unter der MIT-Lizenz lizenziert sind.

Bitte stelle sicher, dass deine Änderungen getestet wurden, bevor du einen Pull Request sendest.