Trait tower_service::Service[][src]

pub trait Service {
    type Request;
    type Response;
    type Error;
    type Future: Future<Item = Self::Response, Error = Self::Error>;
    fn poll_ready(&mut self) -> Poll<(), Self::Error>;
fn call(&mut self, req: Self::Request) -> Self::Future; fn ready(self) -> Ready<Self>
        Self: Sized
, { ... } }

An asynchronous function from 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.


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 represents an RPC request. The Service value can be a server or a client.


An RPC server implements the Service trait. Requests received by the server over the network are deserialized 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:

This example is not tested
impl Service for HelloWorld {
    type Request = http::Request;
    type Response = http::Response;
    type Error = http::Error;
    type Future = Box<Future<Item = Self::Response, Error = http::Error>>;

    fn poll_ready(&mut self) -> Poll<(), Self::Error> {

    fn call(&mut self, req: http::Request) -> Self::Future {
        // Create the HTTP response
        let resp = http::Response::ok()
            .with_body(b"hello world\n");

        // Return the response as an immediate future


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:

This example is not tested
let client = redis::Client::new()

let resp ="foo", "this is the value of foo"));

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


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:

This example is not tested
use tower_service::Service;
use futures::Future;
use std::time::Duration;

use tokio_timer::Timer;

pub struct Timeout<T> {
    inner: T,
    delay: Duration,
    timer: Timer,

pub struct Expired;

impl<T> Timeout<T> {
    pub fn new(inner: T, delay: Duration) -> Timeout<T> {
        Timeout {
            inner: inner,
            delay: delay,
            timer: Timer::default(),

impl<T> Service for Timeout<T>
    where T: Service,
          T::Error: From<Expired>,
    type Request = T::Request;
    type Response = T::Response;
    type Error = T::Error;
    type Future = Box<Future<Item = Self::Response, Error = Self::Error>>;

    fn poll_ready(&mut self) -> Poll<(), Self::Error> {

    fn call(&mut self, req: Self::Req) -> Self::Future {
        let timeout = self.timer.sleep(self.delay)
            .and_then(|_| Err(Self::Error::from(Expired)));
            .map(|(v, _)| v)
            .map_err(|(e, _)| e)

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.


Calling an at capacity Service (i.e., it 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

Requests handled by the service.

Responses given by the service.

Errors produced by the service.

The future response value.

Required Methods

Returns Ready when the service is able to process requests.

If the service is at capacity, then NotReady is returned and the task is notified when the service becomes ready again. This function is expected to be called while on a task.

This is a best effort implementation. False positives are permitted. It is permitted for the service to return Ready from a poll_ready call and the next invocation of call results in an error.

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. If the service is at capacity and the request is unable to be handled, the returned Future should resolve to an error.

Calling call without calling poll_ready is permitted. The implementation must be resilient to this fact.

Provided Methods

A future yielding the service when it is ready to accept a request.

Implementations on Foreign Types

impl<'a, S: Service + 'a> Service for &'a mut S

impl<S: Service + ?Sized> Service for Box<S>