Kotlin Server Template

This project is a server code template using Kotlin, Gradle, and Ktor. It aims to help you build a service by providing reusable code examples that is likely essential for a server.

Tech stack

• Kotlin as a main language.
  • kotlinx.coroutines for asynchronous, non-blocking programming.
  • kotlinx.serialization for serialization.
• Gradle as a build tool.
  • Kotlin DSL as well.
• Ktor as a server framework.
• Logback for logging.
• Typesafe Config for configuration using HOCON files.
• Arrow Core for type-safe error handling.
  • Uses only basic features such as Either.
• Spring Data R2DBC and PostgreSQL for persistence.
  • Uses Spring Data R2DBC as a client of R2DBC, but does not depend on Spring IoC or Spring Boot.
  • Uses r2dbc-pool to manage database connections, r2dbc-postgresql as a R2DBC driver.
• Testcontainers for integration testing.

Architecture

This project consists of several Gradle subprojects separated based on Domain-driven design (DDD) as below.

graph TD
    Infrastructure --> Domain
    Presentation --> Domain
    Boot --> Domain
    Boot --> Infrastructure
    Boot --> Presentation

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domain

Domain contains pure conceptual business logic implemented in Kotlin code which uses limited and minimal external dependencies.

infrastructure

Infrastructure contains actual implementations of domain interfaces which use external dependencies such as database, network, and libraries that possibly be replaced.

presentation

Presentation provides domain functions as actual network APIs. It contains logic to support network protocols such as HTTP, gRPC and also API serialization / deserialization for domain.

boot

Boot depends on all other subprojects and connects implementations of them to run a server application. In other words, it is responsible for dependency injection (DI). It also contains several resource files to run a server.

Other subprojects

• logging provides logback library dependency and configuration of it for global use in the project.

Gradle Setting

Root build.gradle.kts contains settings commonly shared to all subprojects, and also several task settings for the entire application.

• Common external library dependencies
• Main class definition
• Tasks that build a fat jar and containerize it.

Each subproject's build.gradle.kts contains settings only for its own subproject.

• Dependencies for the subproject.
• Several plugin settings for the subproject.

Testing

Examples of unit test, integration test, and end-to-end test are all included in this project.

• Unit tests are usually written for entities, services in domain layer.
  • UserSpec
  • UserServiceSpec
• Integration tests are usually written for repositories in infrastructure layer by using Testcontainers.
  • RepositorySpec
• End-to-end tests are written in boot layer usually for testing the server APIs.
  • EndToEndSpec

Server API

The server provides simple API examples as follows.

• GET /greeting/{name}

  > curl -X GET http://localhost:8080/greeting/alice
  HTTP/1.1 200 OK
  Content-Type: text/plain; charset=UTF-8
  Hello, alice.
  

• GET /users/{id}

  > curl -X GET http://localhost:8080/users/existing-user
  HTTP/1.1 200 OK
  Content-Type: application/json; charset=UTF-8
  {"id":"existing-user","name":"..."}
  
  > curl -X GET http://localhost:8080/users/non-existing-user
  HTTP/1.1 404 Not Found
  

• POST /users/{id}: Creates a user.

  > curl -i -X POST http://localhost:8080/users -H "Content-Type: application/json" -d '{ "name": "bob" }'
  HTTP/1.1 200 OK
  Content-Type: application/json; charset=UTF-8
  {"id":"some-random-id","name":"bob"}
  
  > curl -i -X POST http://localhost:8080/users -H "Content-Type: application/json" -d '{ "name": "!@" }'
  HTTP/1.1 400 Bad Request
  Content-Type: text/plain; charset=UTF-8
  User name must be alphanumeric.
  

• PATCH /users/{id}: Updates a user.

  > curl -i -X PATCH http://localhost:8080/users/existing-user -H "Content-Type: application/json" -d '{ "name": "charlie" }'
  HTTP/1.1 200 OK
  Content-Type: application/json; charset=UTF-8
  {"id":"existing-user","name":"charlie"}
  
  > curl -i -X POST http://localhost:8080/users/existing-user -H "Content-Type: application/json" -d '{ "name": "!@" }'
  HTTP/1.1 400 Bad Request
  Content-Type: text/plain; charset=UTF-8
  User name must be alphanumeric.
  
  > curl -i -X POST http://localhost:8080/users/non-existing-user -H "Content-Type: application/json" -d '{ "name": "damien" }'
  HTTP/1.1 404 Not Found
  

• DELETE /users/{id}

  > curl -i -X DELETE http://localhost:8080/users/existing-user
  HTTP/1.1 200 OK
  
  > curl -i -X POST http://localhost:8080/users/non-existing-user
  HTTP/1.1 404 Not Found