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Advanced Middleware

In the main tutorial you read how to add Custom Middleware to your application.

And then you also read how to handle CORS with the CORSMiddleware.

In this section we'll see how to use other middlewares.

Adding ASGI middlewares

As FastAPI is based on Starlette and implements the ASGI specification, you can use any ASGI middleware.

A middleware doesn't have to be made for FastAPI or Starlette to work, as long as it follows the ASGI spec.

In general, ASGI middlewares are classes that expect to receive an ASGI app as the first argument.

So, in the documentation for third-party ASGI middlewares they will probably tell you to do something like:

from unicorn import UnicornMiddleware

app = SomeASGIApp()

new_app = UnicornMiddleware(app, some_config="rainbow")

But FastAPI (actually Starlette) provides a simpler way to do it that makes sure that the internal middlewares to handle server errors and custom exception handlers work properly.

For that, you use app.add_middleware() (as in the example for CORS).

from fastapi import FastAPI
from unicorn import UnicornMiddleware

app = FastAPI()

app.add_middleware(UnicornMiddleware, some_config="rainbow")

app.add_middleware() receives a middleware class as the first argument and any additional arguments to be passed to the middleware.

Integrated middlewares

FastAPI includes several middlewares for common use cases, we'll see next how to use them.

Technical Details

For the next examples, you could also use from starlette.middleware.something import SomethingMiddleware.

FastAPI provides several middlewares in fastapi.middleware just as a convenience for you, the developer. But most of the available middlewares come directly from Starlette.

HTTPSRedirectMiddleware

Enforces that all incoming requests must either be https or wss.

Any incoming requests to http or ws will be redirected to the secure scheme instead.

from fastapi import FastAPI
from fastapi.middleware.httpsredirect import HTTPSRedirectMiddleware

app = FastAPI()

app.add_middleware(HTTPSRedirectMiddleware)


@app.get("/")
async def main():
    return {"message": "Hello World"}

TrustedHostMiddleware

Enforces that all incoming requests have a correctly set Host header, in order to guard against HTTP Host Header attacks.

from fastapi import FastAPI
from fastapi.middleware.trustedhost import TrustedHostMiddleware

app = FastAPI()

app.add_middleware(
    TrustedHostMiddleware, allowed_hosts=["example.com", "*.example.com"]
)


@app.get("/")
async def main():
    return {"message": "Hello World"}

The following arguments are supported:

  • allowed_hosts - A list of domain names that should be allowed as hostnames. Wildcard domains such as *.example.com are supported for matching subdomains to allow any hostname either use allowed_hosts=["*"] or omit the middleware.

If an incoming request does not validate correctly then a 400 response will be sent.

GZipMiddleware

Handles GZip responses for any request that includes "gzip" in the Accept-Encoding header.

The middleware will handle both standard and streaming responses.

from fastapi import FastAPI
from fastapi.middleware.gzip import GZipMiddleware

app = FastAPI()

app.add_middleware(GZipMiddleware, minimum_size=1000)


@app.get("/")
async def main():
    return "somebigcontent"

The following arguments are supported:

  • minimum_size - Do not GZip responses that are smaller than this minimum size in bytes. Defaults to 500.

Other middlewares

There are many other ASGI middlewares.

For example:

To see other available middlewares check Starlette's Middleware docs and the ASGI Awesome List.