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The Challenge
A hotel booking system has tricky consistency requirements: two users booking the same room at the same time must not both succeed. Building this correctly — with proper auth, availability windows, and payment flow — from scratch is a systems design exercise that surfaces real database and API design decisions.
The Solution
Designed a layered FastAPI backend: route handlers → service layer → SQLAlchemy repository. Booking conflicts are prevented using database-level row locking (SELECT FOR UPDATE) inside a transaction, ensuring only one booking can claim a room for overlapping dates. JWT tokens (access + refresh) handle auth. Auto-generated OpenAPI docs via FastAPI serve as live documentation.
Architecture
FastAPI routers → Pydantic schemas for validation → Service layer for business logic → SQLAlchemy ORM with PostgreSQL → Alembic for migrations. JWT auth middleware. Row-level locking for booking conflict prevention. Mock Stripe integration in the payment service layer.
Impact
- Live Swagger UI at airbnblite-api.onrender.com/api/v1/docs — fully interactive documentation
- Zero double-booking bugs: row-level locking prevents race conditions under concurrent load
- Full booking lifecycle: search → reserve → pay → cancel with status transitions
- JWT refresh token rotation implemented correctly — no silent token reuse attacks
- Alembic migrations make schema changes safe and reversible across environments
What I Learned
- SELECT FOR UPDATE inside a transaction is the correct primitive for booking conflict prevention — optimistic locking would work too but adds retry complexity
- FastAPI's dependency injection made adding auth middleware to specific routes clean without wrapping every handler
- Pydantic v2's model_validator enabled cross-field validation (check-out must be after check-in) at the schema layer, keeping the service layer clean