# Trait hyper::service::Service [−] [src]

    pub trait Service<Request> {
        type Response;
        type Error;
        type Future: Future;
        pub fn poll_ready(
            &mut self,
            cx: &mut Context<'_>
        ) -> Poll<Result<(), Self::Error>>;
        pub 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 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 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.

## Associated Types

### `type Response` [src]

Responses given by the service.

### `type Error` [src]

Errors produced by the service.

### `type Future: Future` [src]

The future response value.

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## Required methods

### `pub fn poll_ready(
&mut self,
cx: &mut Context<'_>
) -> Poll<Result<(), Self::Error>>` [src]

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(_))`.

### `pub fn call(&mut self, req: Request) -> Self::Future` [src]

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`.

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## Implementations on Foreign Types

### `impl<S, Request> Service<Request> for Box<S, Global> where
S: Service<Request> + ?Sized,` [src]

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

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

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

#### `pub fn poll_ready(
&mut self,
cx: &mut Context<'_>
) -> Poll<Result<(), <S as Service<Request>>::Error>>` [src]

#### `pub fn call(&mut self, request: Request) -> <S as Service<Request>>::Future` [src]

### `impl<'a, S, Request> Service<Request> for &'a mut S where
S: Service<Request> + 'a,` [src]

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

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

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

#### `pub fn poll_ready(
&mut self,
cx: &mut Context<'_>
) -> Poll<Result<(), <S as Service<Request>>::Error>>` [src]

#### `pub fn call(&mut self, request: Request) -> <S as Service<Request>>::Future` [src]

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## Implementors

### `impl Service<Name> for GaiResolver` [src]

#### `type Response = GaiAddrs`

#### `type Error = Error`

#### `type Future = GaiFuture`

#### `fn poll_ready(&mut self, _cx: &mut Context<'_>) -> Poll<Result<(), Error>>` [src]

#### `fn call(&mut self, name: Name) -> Self::Future` [src]

### `impl<B> Service<Request<B>> for SendRequest<B> where
B: HttpBody + 'static,` [src]

#### `type Response = Response<Body>`

#### `type Error = Error`

#### `type Future = ResponseFuture`

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

#### `fn call(&mut self, req: Request<B>) -> Self::Future` [src]

### `impl<C, B> Service<Request<B>> for &Client<C, B> where
C: Connect + Clone + Send + Sync + 'static,
B: HttpBody + Send + 'static,
B::Data: Send,
B::Error: Into<Box<dyn StdError + Send + Sync>>,` [src]

#### `type Response = Response<Body>`

#### `type Error = Error`

#### `type Future = ResponseFuture`

#### `fn poll_ready(&mut self, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>>` [src]

#### `fn call(&mut self, req: Request<B>) -> Self::Future` [src]

### `impl<C, B> Service<Request<B>> for Client<C, B> where
C: Connect + Clone + Send + Sync + 'static,
B: HttpBody + Send + 'static,
B::Data: Send,
B::Error: Into<Box<dyn StdError + Send + Sync>>,` [src]

#### `type Response = Response<Body>`

#### `type Error = Error`

#### `type Future = ResponseFuture`

#### `fn poll_ready(&mut self, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>>` [src]

#### `fn call(&mut self, req: Request<B>) -> Self::Future` [src]

### `impl<C, B, T> Service<T> for Connect<C, B, T> where
C: MakeConnection<T>,
C::Connection: Unpin + Send + 'static,
C::Future: Send + 'static,
C::Error: Into<Box<dyn StdError + Send + Sync>> + Send,
B: HttpBody + Unpin + Send + 'static,
B::Data: Send + Unpin,
B::Error: Into<Box<dyn StdError + Send + Sync>>,` [src]

#### `type Response = SendRequest<B>`

#### `type Error = Error`

#### `type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>> + Send + 'static>>`

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

#### `fn call(&mut self, req: T) -> Self::Future` [src]

### `impl<R> Service<Uri> for HttpConnector<R> where
R: Resolve + Clone + Send + Sync + 'static,
R::Future: Send,` [src]

#### `type Response = TcpStream`

#### `type Error = ConnectError`

#### `type Future = HttpConnecting<R>`

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

#### `fn call(&mut self, dst: Uri) -> Self::Future` [src]

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