Middleware¶
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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.
"Technical Details"
If you have dependencies with yield
, the exit code will run after the middleware.
If there were any background tasks (documented later), they will run after all the 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 therequest
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.
- This function will pass the
- 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
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.
"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
¶
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:
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
Tip
Here we use time.perf_counter()
instead of time.time()
because it can be more precise for these use cases. 🤓
Other middlewares¶
You can later read more about other middlewares in the Advanced User Guide: Advanced Middleware.
You will read about how to handle CORS with a middleware in the next section.