1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225
#![deny(missing_docs)]
#![doc(html_root_url = "https://docs.rs/tower/0.2.0")]
//! Definition of the core `Service` trait to Tower
//!
//! These traits provide the necessary abstractions for defining a request /
//! response clients and servers. They are simple but powerul and are the
//! used as the foundation for the rest of Tower.
//!
//! * [`Service`](trait.Service.html) is the primary trait and defines the request
//! / response exchange. See that trait for more details.
extern crate futures;
use futures::{Future, Poll};
/// 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.
///
/// # 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` 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 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:
///
/// ```rust,ignore
/// impl Service<http::Request> for HelloWorld {
/// type Response = http::Response;
/// type Error = http::Error;
/// type Future = Box<Future<Item = Self::Response, Error = Self::Error>>;
///
/// fn poll_ready(&mut self) -> Poll<(), Self::Error> {
/// Ok(Async::Ready(()))
/// }
///
/// 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
/// futures::finished(resp).boxed()
/// }
/// }
/// ```
///
/// # 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:
///
/// ```rust,ignore
/// 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"));
///
/// // Wait for the future to resolve
/// println!("Redis response: {:?}", await(resp));
/// ```
///
/// # Middleware
///
/// 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:
///
/// ```rust,ignore
/// 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, Request> Service<Request> for Timeout<T>
/// where
/// T: Service<Request>,
/// T::Error: From<Expired>,
/// {
/// 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> {
/// Ok(Async::Ready(()))
/// }
///
/// fn call(&mut self, req: Request) -> Self::Future {
/// let timeout = self.timer.sleep(self.delay)
/// .and_then(|_| Err(Self::Error::from(Expired)));
///
/// self.inner.call(req)
/// .select(timeout)
/// .map(|(v, _)| v)
/// .map_err(|(e, _)| e)
/// .boxed()
/// }
/// }
///
/// ```
///
/// 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 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.
pub trait Service<Request> {
/// Responses given by the service.
type Response;
/// Errors produced by the service.
type Error;
/// The future response value.
type Future: Future<Item = Self::Response, Error = Self::Error>;
/// 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.
fn poll_ready(&mut self) -> Poll<(), Self::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.
fn call(&mut self, req: Request) -> Self::Future;
}
impl<'a, S, Request> Service<Request> for &'a mut S
where
S: Service<Request> + 'a
{
type Response = S::Response;
type Error = S::Error;
type Future = S::Future;
fn poll_ready(&mut self) -> Poll<(), S::Error> {
(**self).poll_ready()
}
fn call(&mut self, request: Request) -> S::Future {
(**self).call(request)
}
}
impl<S, Request> Service<Request> for Box<S>
where
S: Service<Request> + ?Sized,
{
type Response = S::Response;
type Error = S::Error;
type Future = S::Future;
fn poll_ready(&mut self) -> Poll<(), S::Error> {
(**self).poll_ready()
}
fn call(&mut self, request: Request) -> S::Future {
(**self).call(request)
}
}