Systems

The framework is small enough that "systems" maps neatly onto sub-modules of fastapi/. This section walks them.

System	Source	Role
Application core	fastapi/applications.py	The FastAPI class — entry point, OpenAPI orchestration, exception-handler registry.
Routing	fastapi/routing.py	APIRouter, APIRoute, APIWebSocketRoute, request/response wrapping, response serialization.
Parameters	fastapi/params.py, fastapi/param_functions.py	The Param/Body/Form/File/Depends/Security markers.
Security	fastapi/security/	Auth schemes (API key, HTTP, OAuth2, OpenID Connect).
OpenAPI	fastapi/openapi/	Schema generator, OpenAPI Pydantic models, Swagger UI / ReDoc HTML.
Encoders	fastapi/encoders.py	jsonable_encoder — JSON-coerce arbitrary Python values.
Exceptions	fastapi/exceptions.py, fastapi/exception_handlers.py	Error classes and default handlers.
Middleware	fastapi/middleware/	Re-exports + the dependency cleanup middleware.
Pydantic compatibility	fastapi/_compat/	Wrapper over Pydantic v2 internals.

Cross-cutting features (under features/)

The systems above implement these end-to-end behaviours, which are documented separately:

• Dependency injection cuts across Routing, Parameters, and Security.
• OpenAPI generation cuts across Application core, Routing, and OpenAPI.
• Server-Sent Events is implemented in fastapi/sse.py but its encoding is in fastapi/routing.py.
• Request lifecycle is the end-to-end view spanning Application core, Middleware, Routing, and Dependency injection.