Trait Service

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

    // Required methods
    fn poll_ready(
        &mut self,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), Self::Error>>;
    fn call(&mut self, req: Request) -> Self::Future;
}

An asynchronous function from a Request to a Response.

The Service trait is a simplified interface making it easy to write network applications in a modular and reusable way, decoupled from the underlying protocol. It is one of Tower’s fundamental abstractions.

Functional

A Service is a function of a Request. It immediately returns a Future representing the eventual completion of processing the request. The actual request processing may happen at any time in the future, on any thread or executor. The processing may depend on calling other services. At some point in the future, the processing will complete, and the Future will resolve to a response or error.

At a high level, the Service::call function represents an RPC request. The Service value can be a server or a client.

Server

An RPC server implements the Service trait. Requests received by the server over the network are deserialized and then passed as an argument to the server value. The returned response is sent back over the network.

As an example, here is how an HTTP request is processed by a server:

use http::{Request, Response, StatusCode};

struct HelloWorld;

impl Service<Request<Vec<u8>>> for HelloWorld {
    type Response = Response<Vec<u8>>;
    type Error = http::Error;
    type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;

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

    fn call(&mut self, req: Request<Vec<u8>>) -> Self::Future {
        // create the body
        let body: Vec<u8> = "hello, world!\n"
            .as_bytes()
            .to_owned();
        // Create the HTTP response
        let resp = Response::builder()
            .status(StatusCode::OK)
            .body(body)
            .expect("Unable to create `http::Response`");

        // create a response in a future.
        let fut = async {
            Ok(resp)
        };

        // Return the response as an immediate future
        Box::pin(fut)
    }
}

Client

A client consumes a service by using a Service value. The client may issue requests by invoking call and passing the request as an argument. It then receives the response by waiting for the returned future.

As an example, here is how a Redis request would be issued:

let client = redis::Client::new()
    .connect("127.0.0.1:6379".parse().unwrap())
    .unwrap();

let resp = client.call(Cmd::set("foo", "this is the value of foo")).await?;

// Wait for the future to resolve
println!("Redis response: {:?}", resp);

Middleware / Layer

More often than not, all the pieces needed for writing robust, scalable network applications are the same no matter the underlying protocol. By unifying the API for both clients and servers in a protocol agnostic way, it is possible to write middleware that provide these pieces in a reusable way.

Take timeouts as an example:

use tower_service::Service;
use tower_layer::Layer;
use futures::FutureExt;
use std::future::Future;
use std::task::{Context, Poll};
use std::time::Duration;
use std::pin::Pin;
use std::fmt;
use std::error::Error;

// Our timeout service, which wraps another service and
// adds a timeout to its response future.
pub struct Timeout<T> {
    inner: T,
    timeout: Duration,
}

impl<T> Timeout<T> {
    pub const fn new(inner: T, timeout: Duration) -> Timeout<T> {
        Timeout {
            inner,
            timeout
        }
    }
}

// The error returned if processing a request timed out
#[derive(Debug)]
pub struct Expired;

impl fmt::Display for Expired {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "expired")
    }
}

impl Error for Expired {}

// We can implement `Service` for `Timeout<T>` if `T` is a `Service`
impl<T, Request> Service<Request> for Timeout<T>
where
    T: Service<Request>,
    T::Future: 'static,
    T::Error: Into<Box<dyn Error + Send + Sync>> + 'static,
    T::Response: 'static,
{
    // `Timeout` doesn't modify the response type, so we use `T`'s response type
    type Response = T::Response;
    // Errors may be either `Expired` if the timeout expired, or the inner service's
    // `Error` type. Therefore, we return a boxed `dyn Error + Send + Sync` trait object to erase
    // the error's type.
    type Error = Box<dyn Error + Send + Sync>;
    type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;

    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        // Our timeout service is ready if the inner service is ready.
        // This is how backpressure can be propagated through a tree of nested services.
       self.inner.poll_ready(cx).map_err(Into::into)
    }

    fn call(&mut self, req: Request) -> Self::Future {
        // Create a future that completes after `self.timeout`
        let timeout = tokio::time::sleep(self.timeout);

        // Call the inner service and get a future that resolves to the response
        let fut = self.inner.call(req);

        // Wrap those two futures in another future that completes when either one completes
        //
        // If the inner service is too slow the `sleep` future will complete first
        // And an error will be returned and `fut` will be dropped and not polled again
        //
        // We have to box the errors so the types match
        let f = async move {
            tokio::select! {
                res = fut => {
                    res.map_err(|err| err.into())
                },
                _ = timeout => {
                    Err(Box::new(Expired) as Box<dyn Error + Send + Sync>)
                },
            }
        };

        Box::pin(f)
    }
}

// A layer for wrapping services in `Timeout`
pub struct TimeoutLayer(Duration);

impl TimeoutLayer {
    pub const fn new(delay: Duration) -> Self {
        TimeoutLayer(delay)
    }
}

impl<S> Layer<S> for TimeoutLayer {
    type Service = Timeout<S>;

    fn layer(&self, service: S) -> Timeout<S> {
        Timeout::new(service, self.0)
    }
}

The above timeout implementation is decoupled from the underlying protocol and is also decoupled from client or server concerns. In other words, the same timeout middleware could be used in either a client or a server.

Backpressure

Calling a Service which is at capacity (i.e., it is temporarily unable to process a request) should result in an error. The caller is responsible for ensuring that the service is ready to receive the request before calling it.

Service provides a mechanism by which the caller is able to coordinate readiness. Service::poll_ready returns Ready if the service expects that it is able to process a request.

Be careful when cloning inner services

Services are permitted to panic if call is invoked without obtaining Poll::Ready(Ok(())) from poll_ready. You should therefore be careful when cloning services for example to move them into boxed futures. Even though the original service is ready, the clone might not be.

Therefore this kind of code is wrong and might panic:

struct Wrapper<S> {
    inner: S,
}

impl<R, S> Service<R> for Wrapper<S>
where
    S: Service<R> + Clone + 'static,
    R: 'static,
{
    type Response = S::Response;
    type Error = S::Error;
    type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;

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

    fn call(&mut self, req: R) -> Self::Future {
        let mut inner = self.inner.clone();
        Box::pin(async move {
            // `inner` might not be ready since its a clone
            inner.call(req).await
        })
    }
}

You should instead use std::mem::replace to take the service that was ready:

struct Wrapper<S> {
    inner: S,
}

impl<R, S> Service<R> for Wrapper<S>
where
    S: Service<R> + Clone + 'static,
    R: 'static,
{
    type Response = S::Response;
    type Error = S::Error;
    type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;

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

    fn call(&mut self, req: R) -> Self::Future {
        let clone = self.inner.clone();
        // take the service that was ready
        let mut inner = std::mem::replace(&mut self.inner, clone);
        Box::pin(async move {
            inner.call(req).await
        })
    }
}

Required Associated Types

type Response

Responses given by the service.

type Error

Errors produced by the service.

type Future: Future<Output = Result<Self::Response, Self::Error>>

The future response value.

Required Methods

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

Returns Poll::Ready(Ok(())) when the service is able to process requests.

If the service is at capacity, then Poll::Pending is returned and the task is notified when the service becomes ready again. This function is expected to be called while on a task. Generally, this can be done with a simple futures::future::poll_fn call.

If Poll::Ready(Err(_)) is returned, the service is no longer able to service requests and the caller should discard the service instance.

Once poll_ready returns Poll::Ready(Ok(())), a request may be dispatched to the service using call. Until a request is dispatched, repeated calls to poll_ready must return either Poll::Ready(Ok(())) or Poll::Ready(Err(_)).

Note that poll_ready may reserve shared resources that are consumed in a subsequent invocation of call. Thus, it is critical for implementations to not assume that call will always be invoked and to ensure that such resources are released if the service is dropped before call is invoked or the future returned by call is dropped before it is polled.

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

Process the request and return the response asynchronously.

This function is expected to be callable off task. As such, implementations should take care to not call poll_ready.

Before dispatching a request, poll_ready must be called and return Poll::Ready(Ok(())).

Panics

Implementations are permitted to panic if call is invoked without obtaining Poll::Ready(Ok(())) from poll_ready.

Implementations on Foreign Types

impl<'a, S, Request> Service<Request> for &'a mut S

where S: Service<Request> + 'a,

type Response = <S as Service<Request>>::Response

type Error = <S as Service<Request>>::Error

type Future = <S as Service<Request>>::Future

fn poll_ready( &mut self, cx: &mut Context<'_>, ) -> Poll<Result<(), <S as Service<Request>>::Error>>

fn call(&mut self, request: Request) -> <S as Service<Request>>::Future

impl<S, Request> Service<Request> for Box<S>

where S: Service<Request> + ?Sized,

type Response = <S as Service<Request>>::Response

type Error = <S as Service<Request>>::Error

type Future = <S as Service<Request>>::Future

fn poll_ready( &mut self, cx: &mut Context<'_>, ) -> Poll<Result<(), <S as Service<Request>>::Error>>

fn call(&mut self, request: Request) -> <S as Service<Request>>::Future

Implementors

impl<A, B, Request> Service<Request> for Either<A, B>

where A: Service<Request>, B: Service<Request, Response = A::Response, Error = A::Error>,

Available on crate feature util only.

type Response = <A as Service<Request>>::Response

type Error = <A as Service<Request>>::Error

type Future = EitherResponseFuture<<A as Service<Request>>::Future, <B as Service<Request>>::Future>

impl<F, S, R, E> Service<R> for FutureService<F, S>

where F: Future<Output = Result<S, E>> + Unpin, S: Service<R, Error = E>,

Available on crate feature util only.

type Response = <S as Service<R>>::Response

type Error = E

type Future = <S as Service<R>>::Future

impl<R, S, F, T, E, Fut> Service<R> for MapFuture<S, F>

where S: Service<R>, F: FnMut(S::Future) -> Fut, E: From<S::Error>, Fut: Future<Output = Result<T, E>>,

Available on crate feature util only.

type Response = T

type Error = E

type Future = Fut

impl<S, F, R1, R2> Service<R1> for MapRequest<S, F>

where S: Service<R2>, F: FnMut(R1) -> R2,

Available on crate feature util only.

type Response = <S as Service<R2>>::Response

type Error = <S as Service<R2>>::Error

type Future = <S as Service<R2>>::Future

impl<S, F, Request, Error> Service<Request> for MapErr<S, F>

where S: Service<Request>, F: FnOnce(S::Error) -> Error + Clone,

Available on crate feature util only.

type Response = <S as Service<Request>>::Response

type Error = Error

type Future = MapErrFuture<<S as Service<Request>>::Future, F>

impl<S, F, Request, Fut> Service<Request> for AndThen<S, F>

where S: Service<Request>, S::Error: Into<Fut::Error>, F: FnOnce(S::Response) -> Fut + Clone, Fut: TryFuture,

Available on crate feature util only.

type Response = <Fut as TryFuture>::Ok

type Error = <Fut as TryFuture>::Error

type Future = AndThenFuture<<S as Service<Request>>::Future, Fut, F>

impl<S, F, Request, Response> Service<Request> for MapResponse<S, F>

where S: Service<Request>, F: FnOnce(S::Response) -> Response + Clone,

Available on crate feature util only.

type Response = Response

type Error = <S as Service<Request>>::Error

type Future = MapResponseFuture<<S as Service<Request>>::Future, F>

impl<S, F, Request, Response, Error> Service<Request> for MapResult<S, F>

where S: Service<Request>, Error: From<S::Error>, F: FnOnce(Result<S::Response, S::Error>) -> Result<Response, Error> + Clone,

Available on crate feature util only.

type Response = Response

type Error = Error

type Future = MapResultFuture<<S as Service<Request>>::Future, F>

impl<S, F, Request, Response, Error, Fut> Service<Request> for Then<S, F>

where S: Service<Request>, S::Error: Into<Error>, F: FnOnce(Result<S::Response, S::Error>) -> Fut + Clone, Fut: Future<Output = Result<Response, Error>>,

Available on crate feature util only.

type Response = Response

type Error = Error

type Future = ThenFuture<<S as Service<Request>>::Future, Fut, F>

impl<T, F, Request, R, E> Service<Request> for ServiceFn<T>

where T: FnMut(Request) -> F, F: Future<Output = Result<R, E>>,

Available on crate feature util only.

type Response = R

type Error = E

type Future = F

impl<T, Request> Service<Request> for Optional<T>

where T: Service<Request>, T::Error: Into<BoxError>,

Available on crate feature util only.

type Response = <T as Service<Request>>::Response

type Error = Box<dyn Error + Send + Sync>

type Future = ResponseFuture<<T as Service<Request>>::Future>

impl<T, U, E> Service<T> for BoxCloneService<T, U, E>

Available on crate feature util only.

type Response = U

type Error = E

type Future = Pin<Box<dyn Future<Output = Result<U, E>> + Send>>

impl<T, U, E> Service<T> for BoxService<T, U, E>

Available on crate feature util only.

type Response = U

type Error = E

type Future = Pin<Box<dyn Future<Output = Result<U, E>> + Send>>

impl<T, U, E> Service<T> for UnsyncBoxService<T, U, E>

Available on crate feature util only.

type Response = U

type Error = E

type Future = Pin<Box<dyn Future<Output = Result<U, E>>>>