Kotlin API Development: From Basics to Production

This book is a practical guide to creating modern web services and REST APIs in Kotlin. Unlike most textbooks, it doesn't overload you with theory but immediately immerses the reader in developing a real project: from the first line of code to deploying the finished service in production.

Who is this book for? The book is intended for developers who already have experience with Java, Python, or C# and want to learn Kotlin for backend development. If you are familiar with OOP, the HTTP protocol, and the basic principles of REST, you will easily get through all the chapters. Beginner programmers are recommended to first learn the basics of Kotlin through one of the introductory courses.

What you will learn and be able to do:

• Set up the development environment and create a project with Ktor — a lightweight framework for web services
• Design RESTful APIs considering HATEOAS principles and versioning
• Work with databases via Exposed — a type-safe ORM from JetBrains
• Implement authentication (JWT, OAuth2) and authorization (RBAC)
• Handle errors, validate requests, and write tests for endpoints
• Document APIs using OpenAPI/Swagger
• Containerize the application in Docker and configure CI/CD
• Optimize performance: coroutines, connection pools, caching

Book structure: The material is divided into 12 chapters, each containing ready-to-use code, comments, and practical tasks. At the end of the book, you will find a chapter on common mistakes and how to avoid them, as well as a code review checklist.

Why Kotlin for APIs? Kotlin combines the conciseness of Python with the performance of Java. Thanks to coroutines and null-safety, you write less code with fewer bugs. Ktor, created by the same team behind IntelliJ IDEA, provides asynchronicity "out of the box" and a lightweight footprint (the final JAR is often less than 20 MB).

What this book definitely will not teach you: It does not replace a textbook on Kotlin basics or database theory. If you have never written in JVM languages, start with an introductory course. Also, the book does not delve into DevOps — separate materials will be needed for Kubernetes and monitoring.

The book is suitable both for self-study and for teams transitioning from Java/Spring to Kotlin/Ktor. All code examples are available in the GitHub repository and have been tested under Kotlin 1.9+.