Middleware { #middleware }

You can add middleware to FastAPI applications.

A "middleware" is a function that works with every request before it is processed by any specific path operation. And also with every response before returning it.

• It takes each request that comes to your application.
• It can then do something to that request or run any needed code.
• Then it passes the request to be processed by the rest of the application (by some path operation).
• It then takes the response generated by the application (by some path operation).
• It can do something to that response or run any needed code.
• Then it returns the response.

/// note | Technical Details

If you have dependencies with yield, the exit code will run after the middleware.

If there were any background tasks (covered in the Background Tasks section, you will see it later), they will run after all the middleware.

///

Create a middleware { #create-a-middleware }

To create a middleware you use the decorator @app.middleware("http") on top of a function.

The middleware function receives:

• The request.
• A function call_next that will receive the request as a parameter.
  • This function will pass the request to the corresponding path operation.
  • Then it returns the response generated by the corresponding path operation.
• You can then further modify the response before returning it.

{* ../../docs_src/middleware/tutorial001_py310.py hl[8:9,11,14] *}

/// tip

Keep in mind that custom proprietary headers can be added using the X- prefix.

But if you have custom headers that you want a client in a browser to be able to see, you need to add them to your CORS configurations (CORS (Cross-Origin Resource Sharing)) using the parameter expose_headers documented in Starlette's CORS docs.

///

/// note | Technical Details

You could also use from starlette.requests import Request.

FastAPI provides it as a convenience for you, the developer. But it comes directly from Starlette.

///

Before and after the response { #before-and-after-the-response }

You can add code to be run with the request, before any path operation receives it.

And also after the response is generated, before returning it.

For example, you could add a custom header X-Process-Time containing the time in seconds that it took to process the request and generate a response:

{* ../../docs_src/middleware/tutorial001_py310.py hl[10,12:13] *}

/// tip

Here we use time.perf_counter() instead of time.time() because it can be more precise for these use cases. 🤓

///

Multiple middleware execution order { #multiple-middleware-execution-order }

When you add multiple middlewares using either @app.middleware() decorator or app.add_middleware() method, each new middleware wraps the application, forming a stack. The last middleware added is the outermost, and the first is the innermost.

On the request path, the outermost middleware runs first.

On the response path, it runs last.

For example:

app.add_middleware(MiddlewareA)
app.add_middleware(MiddlewareB)

This results in the following execution order:

• Request: MiddlewareB → MiddlewareA → route

• Response: route → MiddlewareA → MiddlewareB

This stacking behavior ensures that middlewares are executed in a predictable and controllable order.

Other middlewares { #other-middlewares }

You can later read more about other middlewares in the Advanced User Guide: Advanced Middleware.

You will read about how to handle <abbr title="Cross-Origin Resource Sharing">CORS</abbr> with a middleware in the next section.