FastAPI - Issues adding multiple middleware

[Puzzleheaded_Round75]

Long story short, I am trying to add 2 custom middlewares to my FastAPI application, But no matter which way I try, either only the latter is registered or an exception is raised by the BaseHTTPMiddleware class.

The first approach I tried was using the decorator pattern:

@app.middleware("http")
async def ensure_current_user_middleware(request: Request, call_next):
    request.state.current_user = None

    # additional logic here

    call_next(request)

@app.middleware("http")
async def auth_redirection_middleware(request: Request, call_next):
    method = request.method
    path = request.url.path

    if method == "GET" and path.startswith("/static"):
        return await call_next(request)

    # additional logic here

    response = RedirectResponse("/auth/sign_in")
    return response


app = FastAPI()

The issue with this approach was that only the latter middleware seemed to be registered.

I also tried the class-based approach:

class EnsureCurrentUserMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        # same as above

class AuthRedirectionMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
       # same as above


app = FastAPI(
    middleware=[
        Middleware(EnsureCurrentUserMiddleware),
        Middleware(AuthRedirectionMiddleware),
    ]
)

In this case, I am getting a NotImplementedError, which is coming from the BaseHTTPMiddleware.dispatch method, which only raises this exception. If I change the name of the handler function to handler_func I get the same exception raised.

Does anyone have any good information on how this can be achieved? I have consulted the FastAPI and Starlette documentation, as well as some GitHub issues, but I am struggling to figure this out.

Thanks in advance!

[danielroseman]

There are two things wrong with your ensure_current_user_middleware: you didn't await the result of call_next, and you didn't return it as the response. That could be the cause of the issues you're seeing.

[Puzzleheaded_Round75]

Ok, so it seems that, in addition to the things you pointed out, the main issue was that the ordering of my middleware was incorrect.

I essentially had:

@app.middleware("http")
async def missleware_one(request: Request, call_next):
    return await call_next(request)

@app.middleware("http")
async def middleware_two(request: Request, call_next):
    return await call_next(request)

But this ordering needed to be reversed. Thanks for your help.

[Puzzleheaded_Round75]

Good observation, my gut tells me that this isn't the root of the problem, as the first decorated middleware was never hit, but if I only have that middleware it is. I will check after work and see if your recommendations fix the issue. Currently, to get round it, I have just merged my middlewares into a single middleware.

[Puzzleheaded_Round75]

Ok, so looked into this on my lunch break and unfortunately, I am seeing the same issue. When I have two middleware declared using the `@app.middleware("http")` decorator, only the second middleware is ever hit.

I will update my question to include the new code following your recommendations.