FastAPI Middleware: How to Handle Cross-Origin, Authentication, and Other Request Interceptions

FastAPI middleware acts as a request/response interceptor, processing data before requests enter and responses return. Its core function is to uniformly handle requests and responses, with earlier-registered middleware executing first and executing in reverse order when returning. Typical applications include: 1. Cross-origin resource sharing (CORS): Implemented via CORSMiddleware, configuring allowed origins (e.g., "*" for development, specific domains for production), credentials, methods, and headers to resolve front-end cross-origin request issues. 2. Authentication interception: Custom middleware for global Token verification (e.g., Bearer Token), returning 401 for failed verification, distinct from dependencies (which target specific routes). Considerations include execution order, avoiding excessive interception, and distinguishing between middleware (for general logic) and dependencies (for local logic).