tower_service/lib.rs
1#![warn(
2 missing_debug_implementations,
3 missing_docs,
4 rust_2018_idioms,
5 unreachable_pub
6)]
7#![forbid(unsafe_code)]
8// `rustdoc::broken_intra_doc_links` is checked on CI
9
10//! Definition of the core `Service` trait to Tower
11//!
12//! The [`Service`] trait provides the necessary abstractions for defining
13//! request / response clients and servers. It is simple but powerful and is
14//! used as the foundation for the rest of Tower.
15
16#![no_std]
17
18extern crate alloc;
19
20use alloc::boxed::Box;
21
22use core::future::Future;
23use core::marker::Sized;
24use core::result::Result;
25use core::task::{Context, Poll};
26
27/// An asynchronous function from a `Request` to a `Response`.
28///
29/// The `Service` trait is a simplified interface making it easy to write
30/// network applications in a modular and reusable way, decoupled from the
31/// underlying protocol. It is one of Tower's fundamental abstractions.
32///
33/// # Functional
34///
35/// A `Service` is a function of a `Request`. It immediately returns a
36/// `Future` representing the eventual completion of processing the
37/// request. The actual request processing may happen at any time in the
38/// future, on any thread or executor. The processing may depend on calling
39/// other services. At some point in the future, the processing will complete,
40/// and the `Future` will resolve to a response or error.
41///
42/// At a high level, the `Service::call` function represents an RPC request. The
43/// `Service` value can be a server or a client.
44///
45/// # Server
46///
47/// An RPC server *implements* the `Service` trait. Requests received by the
48/// server over the network are deserialized and then passed as an argument to the
49/// server value. The returned response is sent back over the network.
50///
51/// As an example, here is how an HTTP request is processed by a server:
52///
53/// ```rust
54/// # use std::pin::Pin;
55/// # use std::task::{Poll, Context};
56/// # use std::future::Future;
57/// # use tower_service::Service;
58/// use http::{Request, Response, StatusCode};
59///
60/// struct HelloWorld;
61///
62/// impl Service<Request<Vec<u8>>> for HelloWorld {
63/// type Response = Response<Vec<u8>>;
64/// type Error = http::Error;
65/// type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;
66///
67/// fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
68/// Poll::Ready(Ok(()))
69/// }
70///
71/// fn call(&mut self, req: Request<Vec<u8>>) -> Self::Future {
72/// // create the body
73/// let body: Vec<u8> = "hello, world!\n"
74/// .as_bytes()
75/// .to_owned();
76/// // Create the HTTP response
77/// let resp = Response::builder()
78/// .status(StatusCode::OK)
79/// .body(body)
80/// .expect("Unable to create `http::Response`");
81///
82/// // create a response in a future.
83/// let fut = async {
84/// Ok(resp)
85/// };
86///
87/// // Return the response as an immediate future
88/// Box::pin(fut)
89/// }
90/// }
91/// ```
92///
93/// # Client
94///
95/// A client consumes a service by using a `Service` value. The client may
96/// issue requests by invoking `call` and passing the request as an argument.
97/// It then receives the response by waiting for the returned future.
98///
99/// As an example, here is how a Redis request would be issued:
100///
101/// ```rust,ignore
102/// let mut client = redis::Client::new()
103/// .connect("127.0.0.1:6379".parse().unwrap())
104/// .unwrap();
105///
106/// ServiceExt::<Cmd>::ready(&mut client).await?;
107///
108/// let resp = client.call(Cmd::set("foo", "this is the value of foo")).await?;
109///
110/// println!("Redis response: {:?}", resp);
111/// ```
112///
113/// # Middleware / Layer
114///
115/// More often than not, all the pieces needed for writing robust, scalable
116/// network applications are the same no matter the underlying protocol. By
117/// unifying the API for both clients and servers in a protocol agnostic way,
118/// it is possible to write middleware that provide these pieces in a
119/// reusable way.
120///
121/// Take timeouts as an example:
122///
123/// ```rust
124/// use tower_service::Service;
125/// use tower_layer::Layer;
126/// use futures::FutureExt;
127/// use std::future::Future;
128/// use std::task::{Context, Poll};
129/// use std::time::Duration;
130/// use std::pin::Pin;
131/// use std::fmt;
132/// use std::error::Error;
133///
134/// // Our timeout service, which wraps another service and
135/// // adds a timeout to its response future.
136/// pub struct Timeout<T> {
137/// inner: T,
138/// timeout: Duration,
139/// }
140///
141/// impl<T> Timeout<T> {
142/// pub const fn new(inner: T, timeout: Duration) -> Timeout<T> {
143/// Timeout {
144/// inner,
145/// timeout
146/// }
147/// }
148/// }
149///
150/// // The error returned if processing a request timed out
151/// #[derive(Debug)]
152/// pub struct Expired;
153///
154/// impl fmt::Display for Expired {
155/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
156/// write!(f, "expired")
157/// }
158/// }
159///
160/// impl Error for Expired {}
161///
162/// // We can implement `Service` for `Timeout<T>` if `T` is a `Service`
163/// impl<T, Request> Service<Request> for Timeout<T>
164/// where
165/// T: Service<Request>,
166/// T::Future: 'static,
167/// T::Error: Into<Box<dyn Error + Send + Sync>> + 'static,
168/// T::Response: 'static,
169/// {
170/// // `Timeout` doesn't modify the response type, so we use `T`'s response type
171/// type Response = T::Response;
172/// // Errors may be either `Expired` if the timeout expired, or the inner service's
173/// // `Error` type. Therefore, we return a boxed `dyn Error + Send + Sync` trait object to erase
174/// // the error's type.
175/// type Error = Box<dyn Error + Send + Sync>;
176/// type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;
177///
178/// fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
179/// // Our timeout service is ready if the inner service is ready.
180/// // This is how backpressure can be propagated through a tree of nested services.
181/// self.inner.poll_ready(cx).map_err(Into::into)
182/// }
183///
184/// fn call(&mut self, req: Request) -> Self::Future {
185/// // Create a future that completes after `self.timeout`
186/// let timeout = tokio::time::sleep(self.timeout);
187///
188/// // Call the inner service and get a future that resolves to the response
189/// let fut = self.inner.call(req);
190///
191/// // Wrap those two futures in another future that completes when either one completes
192/// //
193/// // If the inner service is too slow the `sleep` future will complete first
194/// // And an error will be returned and `fut` will be dropped and not polled again
195/// //
196/// // We have to box the errors so the types match
197/// let f = async move {
198/// tokio::select! {
199/// res = fut => {
200/// res.map_err(|err| err.into())
201/// },
202/// _ = timeout => {
203/// Err(Box::new(Expired) as Box<dyn Error + Send + Sync>)
204/// },
205/// }
206/// };
207///
208/// Box::pin(f)
209/// }
210/// }
211///
212/// // A layer for wrapping services in `Timeout`
213/// pub struct TimeoutLayer(Duration);
214///
215/// impl TimeoutLayer {
216/// pub const fn new(delay: Duration) -> Self {
217/// TimeoutLayer(delay)
218/// }
219/// }
220///
221/// impl<S> Layer<S> for TimeoutLayer {
222/// type Service = Timeout<S>;
223///
224/// fn layer(&self, service: S) -> Timeout<S> {
225/// Timeout::new(service, self.0)
226/// }
227/// }
228/// ```
229///
230/// The above timeout implementation is decoupled from the underlying protocol
231/// and is also decoupled from client or server concerns. In other words, the
232/// same timeout middleware could be used in either a client or a server.
233///
234/// # Backpressure
235///
236/// Calling a `Service` which is at capacity (i.e., it is temporarily unable to process a
237/// request) should result in an error. The caller is responsible for ensuring
238/// that the service is ready to receive the request before calling it.
239///
240/// `Service` provides a mechanism by which the caller is able to coordinate
241/// readiness. `Service::poll_ready` returns `Ready` if the service expects that
242/// it is able to process a request.
243///
244/// # Be careful when cloning inner services
245///
246/// Services are permitted to panic if `call` is invoked without obtaining `Poll::Ready(Ok(()))`
247/// from `poll_ready`. You should therefore be careful when cloning services for example to move
248/// them into boxed futures. Even though the original service is ready, the clone might not be.
249///
250/// Therefore this kind of code is wrong and might panic:
251///
252/// ```rust
253/// # use std::pin::Pin;
254/// # use std::task::{Poll, Context};
255/// # use std::future::Future;
256/// # use tower_service::Service;
257/// #
258/// struct Wrapper<S> {
259/// inner: S,
260/// }
261///
262/// impl<R, S> Service<R> for Wrapper<S>
263/// where
264/// S: Service<R> + Clone + 'static,
265/// R: 'static,
266/// {
267/// type Response = S::Response;
268/// type Error = S::Error;
269/// type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;
270///
271/// fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
272/// self.inner.poll_ready(cx)
273/// }
274///
275/// fn call(&mut self, req: R) -> Self::Future {
276/// let mut inner = self.inner.clone();
277/// Box::pin(async move {
278/// // `inner` might not be ready since its a clone
279/// inner.call(req).await
280/// })
281/// }
282/// }
283/// ```
284///
285/// You should instead use [`core::mem::replace`] to take the service that was ready:
286///
287/// ```rust
288/// # use std::pin::Pin;
289/// # use std::task::{Poll, Context};
290/// # use std::future::Future;
291/// # use tower_service::Service;
292/// #
293/// struct Wrapper<S> {
294/// inner: S,
295/// }
296///
297/// impl<R, S> Service<R> for Wrapper<S>
298/// where
299/// S: Service<R> + Clone + 'static,
300/// R: 'static,
301/// {
302/// type Response = S::Response;
303/// type Error = S::Error;
304/// type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;
305///
306/// fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
307/// self.inner.poll_ready(cx)
308/// }
309///
310/// fn call(&mut self, req: R) -> Self::Future {
311/// let clone = self.inner.clone();
312/// // take the service that was ready
313/// let mut inner = std::mem::replace(&mut self.inner, clone);
314/// Box::pin(async move {
315/// inner.call(req).await
316/// })
317/// }
318/// }
319/// ```
320pub trait Service<Request> {
321 /// Responses given by the service.
322 type Response;
323
324 /// Errors produced by the service.
325 type Error;
326
327 /// The future response value.
328 type Future: Future<Output = Result<Self::Response, Self::Error>>;
329
330 /// Returns `Poll::Ready(Ok(()))` when the service is able to process requests.
331 ///
332 /// If the service is at capacity, then `Poll::Pending` is returned and the task
333 /// is notified when the service becomes ready again. This function is
334 /// expected to be called while on a task. Generally, this can be done with
335 /// a simple `futures::future::poll_fn` call.
336 ///
337 /// If `Poll::Ready(Err(_))` is returned, the service is no longer able to service requests
338 /// and the caller should discard the service instance.
339 ///
340 /// Once `poll_ready` returns `Poll::Ready(Ok(()))`, a request may be dispatched to the
341 /// service using `call`. Until a request is dispatched, repeated calls to
342 /// `poll_ready` must return either `Poll::Ready(Ok(()))` or `Poll::Ready(Err(_))`.
343 ///
344 /// Note that `poll_ready` may reserve shared resources that are consumed in a subsequent
345 /// invocation of `call`. Thus, it is critical for implementations to not assume that `call`
346 /// will always be invoked and to ensure that such resources are released if the service is
347 /// dropped before `call` is invoked or the future returned by `call` is dropped before it
348 /// is polled.
349 fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
350
351 /// Process the request and return the response asynchronously.
352 ///
353 /// This function is expected to be callable off task. As such,
354 /// implementations should take care to not call `poll_ready`.
355 ///
356 /// Before dispatching a request, `poll_ready` must be called and return
357 /// `Poll::Ready(Ok(()))`.
358 ///
359 /// # Panics
360 ///
361 /// Implementations are permitted to panic if `call` is invoked without
362 /// obtaining `Poll::Ready(Ok(()))` from `poll_ready`.
363 #[must_use = "futures do nothing unless you `.await` or poll them"]
364 fn call(&mut self, req: Request) -> Self::Future;
365}
366
367impl<S, Request> Service<Request> for &mut S
368where
369 S: Service<Request> + ?Sized,
370{
371 type Response = S::Response;
372 type Error = S::Error;
373 type Future = S::Future;
374
375 fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), S::Error>> {
376 (**self).poll_ready(cx)
377 }
378
379 fn call(&mut self, request: Request) -> S::Future {
380 (**self).call(request)
381 }
382}
383
384impl<S, Request> Service<Request> for Box<S>
385where
386 S: Service<Request> + ?Sized,
387{
388 type Response = S::Response;
389 type Error = S::Error;
390 type Future = S::Future;
391
392 fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), S::Error>> {
393 (**self).poll_ready(cx)
394 }
395
396 fn call(&mut self, request: Request) -> S::Future {
397 (**self).call(request)
398 }
399}