# PRD: FastAPI Framework

FastAPI is a modern, high-performance Python web framework for building APIs with Python type hints. This document outlines the product requirements for the FastAPI framework itself.

## Implementation Status

The core framework is mature and well-established. This PRD tracks major feature initiatives and improvements beyond the stable release.

## User Stories

### P0: Core Framework (Must Have) — ✅ Complete

**As a** Python developer,
**I want** to build APIs quickly using type hints,
**So that** I get automatic validation, documentation, and IDE support.

- [P0-US1] ✅ Route definition via decorators with path parameters
- [P0-US2] ✅ Query, header, and cookie parameter extraction with type hints
- [P0-US3] ✅ Request body validation using Pydantic models
- [P0-US4] ✅ Automatic OpenAPI schema generation
- [P0-US5] ✅ Interactive API documentation (Swagger UI + ReDoc)
- [P0-US6] ✅ Dependency injection system
- [P0-US7] ✅ Error handling with custom exception handlers
- [P0-US8] ✅ Response model typing with automatic serialization

**Acceptance Criteria:**
- All core HTTP methods supported (GET, POST, PUT, DELETE, PATCH, OPTIONS)
- Type hints enable validation and documentation without boilerplate
- OpenAPI 3.0+ compliant schema generation

### P1: Async & Performance (Must Have) — ✅ Complete

**As a** developer building high-throughput APIs,
**I want** first-class async support,
**So that** I can handle many concurrent connections efficiently.

- [P1-US1] ✅ Async route handlers with `async def`
- [P1-US2] ✅ Background tasks for deferred processing
- [P1-US3] ✅ WebSocket support for real-time communication
- [P1-US4] ✅ Middleware stack (request/response processing)
- [P1-US5] ✅ Sub-application mounting for modular apps

**Acceptance Criteria:**
- Async handlers perform comparably to Node.js/Go
- WebSocket connections maintained efficiently
- Middleware applies correctly to all routes

### P1: Security & Authentication (Should Have) — ✅ Complete

**As a** developer building secure APIs,
**I want** built-in authentication and authorization,
**So that** I don't have to implement common patterns from scratch.

- [P1-US6] ✅ OAuth2 password flow implementation
- [P1-US7] ✅ JWT token support
- [P1-US8] ✅ CORS middleware
- [P1-US9] ✅ HTTP Basic Auth
- [P1-US10] ✅ Dependency-based authorization scopes

**Acceptance Criteria:**
- OAuth2 with JWT tokens works out of the box
- Security schemes documented in OpenAPI spec

### P2: Developer Experience (Should Have) — ✅ Complete

**As a** FastAPI user,
**I want** excellent tooling and error messages,
**So that** I can develop quickly and debug easily.

- [P2-US1] ✅ FastAPI CLI for running and generating projects
- [P2-US2] ✅ Clear validation error messages with field locations
- [P2-US3] ✅ Static type checking support with mypy
- [P2-US4] ✅ Response model documentation in OpenAPI

**Acceptance Criteria:**
- CLI provides `fastapi dev` and `fastapi run`
- Error responses include details for each validation failure
- Type hints work with pyright/mypy

### P2: Extensibility (Should Have) — ✅ Complete

**As a** framework contributor,
**I want** to extend FastAPI without modifying core code,
**So that** I can add custom behaviors.

- [P2-US5] ✅ Custom response classes
- [P2-US6] ✅ Custom parameter types
- [P2-US7] ✅ Custom middleware interfaces
- [P2-US8] ✅ OpenAPI extension hooks

**Acceptance Criteria:**
- Extensions integrate seamlessly with FastAPI
- Custom components documented in OpenAPI

### P3: Advanced Features (Could Have) — ✅ Complete

**As an** advanced user,
**I want** sophisticated deployment options,
**So that** I can optimize for production.

- [P3-US1] ✅ Testing utilities with TestClient
- [P3-US2] ✅ Dependency overrides for testing
- [P3-US3] ✅ ASGI server integration (Uvicorn, Hypercorn)
- [P3-US4] ✅ Gunicorn worker support
- [P3-US5] ✅ Docker integration and multi-stage builds

**Acceptance Criteria:**
- Testing patterns documented and working
- Production deployment guides available

## Architecture: Starlette + Pydantic

FastAPI builds on two key libraries:

| Component | Role |
|-----------|------|
| Starlette | Routing, middleware, ASGI handler |
| Pydantic | Data validation, serialization, OpenAPI generation |

### Request/Response Flow

```
Request → ASGI Handler → Middleware → Route Handler → Dependency Injection
                                                        ↓
Response ← Serialization ← Pydantic Model ← Return Value
```

### Key Components

| Component | File | Role |
|-----------|------|------|
| FastAPI app | `fastapi/applications.py` | Main application class |
| Routing | `fastapi/routing.py` | Route resolution, OpenAPI generation |
| Dependencies | `fastapi/dependencies.py` | Dependency injection system |
| Params | `fastapi/params.py` | Parameter definitions |
| Response | `fastapi/response.py` | Response handling |
| OpenAPI | `fastapi/openapi/` | Schema generation |

## Non-Functional Requirements

### Performance
- Request/response latency comparable to Node.js and Go
- Minimal overhead for sync handlers
- Efficient async task scheduling

### Compatibility
- Python 3.8+ (subject to PEP 594)
- OpenAPI 3.0+ and JSON Schema
- ASGI specification compliance

### Security
- Input validation via Pydantic
- Secure defaults for authentication
- No sensitive data in error messages

### Reliability
- Comprehensive test coverage
- Deprecation warnings for breaking changes
- Stable API surface within major versions

## Test Scenarios

1. **Basic API**: Create endpoint, verify request/response
2. **Validation**: Send invalid data, verify 422 error with details
3. **Async**: Test async handler with concurrent requests
4. **Dependencies**: Verify dependency injection ordering
5. **Auth**: Test OAuth2 flow end-to-end
6. **Documentation**: Verify OpenAPI schema accuracy
7. **Middleware**: Test request/response modification
8. **Sub-app**: Mount app, verify routing isolation