API Development in Kotlin: From Basics to Microservices

This book is a comprehensive guide to modern web API development in Kotlin. The author, a practicing developer with 8 years of experience, systematically guides the reader from basic concepts to creating a production-ready microservice architecture. The material is built on the "from simple to complex" principle: each chapter contains a theoretical part, practical code examples, and assignments for independent work.

What this book is about

The book covers the complete cycle of creating an API in Kotlin:

• Kotlin Basics for Backend — coroutines, Flow, serialization, working with databases via Exposed and JDBC
• Frameworks — detailed analysis of Ktor (lightweight, asynchronous) and Spring Boot (powerful, enterprise-ready)
• API Types — creating RESTful services with HATEOAS, implementing GraphQL via graphql-kotlin
• Security — JWT, OAuth 2.0, API keys, protection against SQL injections and CSRF attacks
• Testing — unit tests, integration tests with Testcontainers, load testing with k6
• DevOps — containerization with Docker, orchestration in Kubernetes, CI/CD with GitHub Actions
• Advanced Topics — data exchange via WebSocket and Server-Sent Events, gRPC, event-driven architecture with Kafka

Who this book is for

• Java/Kotlin developers who want to master modern approaches to creating APIs
• Backend engineers transitioning from other languages (Python, C#, Go) to the JVM ecosystem
• Students and beginner programmers seeking structured knowledge in web development
• Team Leads and architects evaluating Kotlin for new projects

What you will learn

• How to choose the right framework for a specific task (Ktor vs Spring Boot)
• Principles of designing RESTful APIs and working with OpenAPI/Swagger
• Performance optimization using coroutines and connection pools
• Monitoring and logging methods (Prometheus, Grafana, ELK stack)
• How to migrate an existing Java project to Kotlin
• Patterns for high-load systems: CQRS, Saga, Circuit Breaker

The book contains over 200 code listings, ready for use in real projects. All examples are accompanied by links to a GitHub repository and work with current library versions (Kotlin 2.x, Ktor 3.x, Spring Boot 3.x).

This is not just a reference guide — it is a navigator through the modern Kotlin ecosystem that will help you create reliable, scalable, and maintainable API services.