Combine use of Layer and ServiceBuilder #3457

shovelmn12 · 2025-09-05T11:20:25Z

shovelmn12
Sep 5, 2025

Summary

Hello
I would like to know if it's possible to combine use of Layer + Servicebuilder to simplify my middleware.

The API I would like to provide to my Devs:

let router = Router::new().route(...).route(...).layer(SystemLayer::new());

And SystemLayer is basically just a composition of different layers but abstracted away from the developers eyes.

pub struct SystemLayer;

impl<S> Layer<S> for SystemLayer {
      fn layer(&self, inner: S) -> Self::Service {
        ServiceBuilder::new().layer(...).layer_fn(middleware:from_fn(..))...
      }
}

The issue is, as always with rust, what type Layer::Service should be?
I tried BoxService and to use .boxed() or BoxService::new(builder) but if gives an error (unsetisfied constraints)

I like this approach as we can delegate SystemLayer to a team ...

P.S. as you probably understand I am still learning rust. The typing system is so confusing to me as I am used to the more conventional interface type but it's impossible in rust to my understanding.... I would expect to have some generic Service type with generic Request, Response, Error and the rest is taken care for ...
Would love your help !!

axum version

0.8.4

jplatte · 2025-09-05T11:25:38Z

jplatte
Sep 5, 2025
Maintainer

Try BoxCloneSyncService, and if that doesn't work, please post the full error message.

9 replies

mladedav Sep 6, 2025
Maintainer

Both of the earlier messages show you're using BoxCloneService. Use BoxCloneSyncService instead.

shovelmn12 Sep 6, 2025
Author

Thanks :)
So BoxCloseSyncService solved one issue, the one when I use .layer(ContextLayer) but as said I still get the same error as I mentioned earlier with my middleware function:

the method `service` exists for struct `ServiceBuilder<Stack<LayerFn<FromFnLayer<..., (), _>>, ...>>`, but its trait bounds were not satisfied [E0599]
Note: the following trait bounds were not satisfied:
`FromFnLayer<fn(axum_client_ip::ClientIp, axum::http::Request<AxumBody>, axum::middleware::Next) -> impl futures_util::Future<Output = axum::http::Response<AxumBody>> {test_layer}, (), _>: Fn<(_,)>`
`FromFnLayer<fn(axum_client_ip::ClientIp, axum::http::Request<AxumBody>, axum::middleware::Next) -> impl futures_util::Future<Output = axum::http::Response<AxumBody>> {test_layer}, (), _>: Fn<(_,)>`
which is required by `LayerFn<FromFnLayer<fn(axum_client_ip::ClientIp, axum::http::Request<AxumBody>, axum::middleware::Next) -> impl futures_util::Future<Output = axum::http::Response<AxumBody>> {test_layer}, (), _>>: Layer<_>`
`FromFnLayer<fn(axum_client_ip::ClientIp, axum::http::Request<AxumBody>, axum::middleware::Next) -> impl futures_util::Future<Output = axum::http::Response<AxumBody>> {test_layer}, (), _>: FnOnce<(_,)>`
which is required by `<FromFnLayer<fn(axum_client_ip::ClientIp, axum::http::Request<AxumBody>, axum::middleware::Next) -> impl futures_util::Future<Output = axum::http::Response<AxumBody>> {test_layer}, (), _> as FnOnce<(_,)>>::Output = _`
`FromFnLayer<fn(axum_client_ip::ClientIp, axum::http::Request<AxumBody>, axum::middleware::Next) -> impl futures_util::Future<Output = axum::http::Response<AxumBody>> {test_layer}, (), _>: FnOnce<(_,)>`
which is required by `LayerFn<FromFnLayer<fn(axum_client_ip::ClientIp, axum::http::Request<AxumBody>, axum::middleware::Next) -> impl futures_util::Future<Output = axum::http::Response<AxumBody>> {test_layer}, (), _>>: Layer<_>`

Code:

impl<S> Layer<S> for ContextLayer
where
    S: Service<Request<Body>, Response = Response<Body>, Error = Infallible>
        + Clone
        + Sync
        + Send
        + 'static,
    S::Future: Send + 'static,
{
    type Service = BoxCloneSyncService<
        Request<Body>,
        Response<
            tower_http::trace::ResponseBody<
                Body,
                tower_http::classify::NeverClassifyEos<ServerErrorsFailureClass>,
            >,
        >,
        Infallible,
    >;

    fn layer(&self, inner: S) -> Self::Service {
        let a = ServiceBuilder::new()
            .layer(TraceLayer::new_for_http())
            .layer_fn(axum::middleware::from_fn(test_layer))
            .service(inner); // error here

        BoxCloneSyncService::new(a)
    }
}

async fn test_layer(
    ClientIp(origin): ClientIp,
    request: Request<Body>,
    next: Next,
) -> Response<Body> {
    next.run(request).await
}

@mladedav thanks for your help 👍

shovelmn12 Sep 7, 2025
Author

@mladedav I think I managed to resolve another error, but still getting error. So I changed my code to:

    fn layer(&self, inner: S) -> Self::Service {
        let a = ServiceBuilder::new()
            .layer(TraceLayer::new_for_http())
            .layer(middleware::from_fn(build_context))
            .service(inner);

        BoxCloneSyncService::new(a) // error here
    }
    ```
    
    Error:
    
    ```
    the trait bound `FromFn<..., (), ..., _>: Service<...>` is not satisfied [E0277]
the trait `Service<http::Request<_>>` is not implemented for `FromFn<fn(ClientIp, ..., ..., ...) -> ... {build_context}, (), ..., _>`
Help: the following other types implement trait `Service<Request>`:
axum::middleware::FromFn<F, S, I, (T1, T2)>
axum::middleware::FromFn<F, S, I, (T1, T2, T3)>
axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4)>
axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5)>
axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5, T6)>
axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5, T6, T7)>
axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5, T6, T7, T8)>
axum::middleware::FromFn<F, S, I, (T1, T2, T3, T4, T5, T6, T7, T8, T9)>
and 8 others
Note: required for `Trace<FromFn<..., (), ..., _>, ...>` to implement `Service<http::Request<_>>`
Note: the full name for the type has been written to '/Users/shovelmn12/Projects/example-server/target/debug/deps/example_server-f57b5c841b4126bc.long-type-12438220873085427120.txt'
Note: consider using `--verbose` to print the full type name to the console

It seems like it's something with my middleware function because when I comment .layer(middleware::from_fn(build_context)) it works with no errors

mladedav Sep 7, 2025
Maintainer

Try using debug_middleware on the middleware method

shovelmn12 Sep 8, 2025
Author

Thanks ! now it works.

The only issue I have is that when I tried using .rate_limit() it gave an error, I can't remember which. SO I used governor instead. Ill keep posting if I add other layers and it gives an error!

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Combine use of Layer and ServiceBuilder #3457

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shovelmn12 Sep 5, 2025

Summary

axum version

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jplatte Sep 5, 2025 Maintainer

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shovelmn12
Sep 5, 2025

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jplatte
Sep 5, 2025
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mladedav Sep 6, 2025
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shovelmn12 Sep 6, 2025
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shovelmn12 Sep 7, 2025
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mladedav Sep 7, 2025
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shovelmn12 Sep 8, 2025
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