API Architecture: Subdomain vs. Path Prefix for FastAPI and React

When building a modern web application with a separate frontend (like React) and backend API (like FastAPI), deciding on the URL structure for the API is a critical design choice. I've been exploring the generally accepted conventions for a production deployment, which primarily boil down to two main strategies.

The Subdomain Approach (e.g., api.example.com)

This is the most common and often preferred method for larger, public, or microservice-based applications, as it provides clear separation and simplifies independent scaling.

Frontend URL Structure: The React application is served from the main domain:
```
https://example.com
```
API Backend URL Structure: The FastAPI backend is exposed via a dedicated subdomain, often including a version path:
```
https://api.example.com/v1/users
```

This separation is beneficial for several reasons:

Independent Deployment: You can deploy and scale the frontend and backend services separately.
Authentication and Cookies: Subdomains make it easier to isolate cookies or set up different authentication mechanisms for the API versus the main web app.
Clear Public Interface: Using api. clearly signals that this endpoint is for programmatic consumption by various clients (mobile apps, partners, other services), not just your React app.

The Path Prefix Approach (e.g., example.com/api)

This alternative is excellent for simpler, single-service applications where the API is primarily consumed by its own frontend.

Frontend URL Structure: The React application uses the main domain for primary content:
```
https://example.com/dashboard
```
API Backend URL Structure: The FastAPI backend endpoints are prefixed within the main domain path, often using an /api/ segment:
```
https://example.com/api/v1/items
```

The primary advantage of this method is simplification of CORS (Cross-Origin Resource Sharing).

Since both the frontend and API share the same domain (origin), browser security policies are easier to manage without complex CORS headers.

This setup is typically managed using a reverse proxy (like Nginx) to route traffic based on the path.

Site References

Subdomains vs. Subdirectories for API Design: https://www.cloudflare.com/learning/dns/subdomain-vs-subdirectory/
Best Practices for RESTful API Naming Conventions: https://restfulapi.net/resource-naming/
FastAPI and React Integration Guides Focusing on Deployment Separation: https://testdriven.io/blog/fastapi-react/

Disclaimer: This blog post was created with assistance from Grok 3, an AI developed by xAI, under my direct supervision and guidance to ensure accuracy and alignment with my vision for the content.