e-be-tc — Book Exchange API

• Source on GitHub

Problem

Design and implement a book borrowing and return system for library members — with proper domain logic, clear API contracts, and a codebase that is maintainable and testable from day one.

Role

Solo author. Responsible for system design, implementation, testing, deployment, and documentation.

Architecture

The project applies Domain-Driven Design (DDD) and Command Query Responsibility Segregation (CQRS):

• Commands (borrow, return, create member/book) are separated from queries (list available books, member history)
• Domain logic is encapsulated in clear aggregates and value objects, not scattered across controllers
• Infrastructure (PostgreSQL, Express) is kept at the boundary — swappable without touching business logic

Stack

• Runtime: TypeScript, Node.js, Express
• Database: PostgreSQL
• Testing: Jest (unit tests)
• API Docs: Swagger UI + Bruno collection
• CI/CD: GitHub Actions
• Dev environment: Nix + Direnv (reproducible, no “works on my machine”)

Key decisions

• CQRS keeps read paths cheap and write paths explicit — making business rules easy to audit
• Nix flake ensures every developer (and CI) works with the same tool versions
• Swagger + Bruno gives two ways to explore the API: browser UI and importable request collection