Work toward cross-framework middleware interface

[aturon]

Originally posted by @rolftimmermans in #162 (comment)

Having a universal middleware concept in the Rust web-ecosystem is something I look forward to. In the Ruby community, for example, every framework builds on Rack middleware. I see this as the sole factor that allows for (some) code reuse across frameworks.

My questions would be:

• If Actix middleware is different, why is it different? Does Tide middleware offer the same functionality, and could they be mapped to each other? Is it technically possible to move to a common middleware in the future?
• Can we come up with a definition of middleware that is not dependent on any implementation details of the framework? For example I see in the current Tide middleware definition that it depends on the DynEndpoint type. Ideally the endpoint itself should also be expressed as a middleware so that no knowledge about endpoints is needed in order to define middleware.
• Maybe the Middleware type and related code needs to be the "core", kind of in the same way that Rust stdlib provides futures, but you need other projects to actually work with them.

[aturon]

If folks want to help on this front, one great step would be to start documenting the various middleware interfaces defined by frameworks today -- Actix, Gotham, Rocket, Tower-Web, Tide. For now, maybe just drop copies of the relevant traits and traits as comments here on this thread.

[DoumanAsh]

Actix's middleware

P.s. personally I think there is no value in trying to work out Middleware since it almost impossible without all other components being shared

[secretfader]

Gotham middleware relies on two traits: Middleware defines the request handler specification, while NewMiddleware creates per-request clones of the middleware implementation.

It isn't terribly complex, but I have questions surrounding how to best wrap RefUnwindSafe.

[taqtiqa-mark]

Sylvain Kerkour has an interesting examination of what the interfaces look like for different exercises, Tide isn't considered but how the exercises look in tide would be interesting (note to self):

https://kerkour.com/rust-web-framework-2022

The "Inventing the Service trait" post shows how Tower's fundamental Service trait could be designed from scratch. @aturon as requested, they end up with something like this (full detail is more involved:

pub trait Service<Request> {
    type Response;
    type Error;
    type Future: Future<Output = Result<Self::Response, Self::Error>>;

    fn poll_ready(
        &mut self,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), Self::Error>>;

    fn call(&mut self, req: Request) -> Self::Future;
}