• 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.

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.

import time

from fastapi import FastAPI, Request

app = FastAPI()


@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    start_time = time.perf_counter()
    response = await call_next(request)
    process_time = time.perf_counter() - start_time
    response.headers["X-Process-Time"] = str(process_time)
    return response