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
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
#![doc(html_root_url = "https://docs.rs/tower-service/0.3.1")]
#![warn(
    missing_debug_implementations,
    missing_docs,
    rust_2018_idioms,
    unreachable_pub
)]
#![deny(broken_intra_doc_links)]

//! Definition of the core `Service` trait to Tower
//!
//! The [`Service`] trait provides the necessary abstractions for defining
//! request / response clients and servers. It is simple but powerful and is
//! used as the foundation for the rest of Tower.

use std::future::Future;
use std::task::{Context, Poll};

/// 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:
///
/// ```rust
/// # use std::pin::Pin;
/// # use std::task::{Poll, Context};
/// # use std::future::Future;
/// # use tower_service::Service;
/// 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:
///
/// ```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")).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:
///
/// ```rust
/// 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.
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<Output = Result<Self::Response, 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(_))`.
    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), 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`.
    ///
    /// 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`.
    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, cx: &mut Context<'_>) -> Poll<Result<(), S::Error>> {
        (**self).poll_ready(cx)
    }

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

    fn call(&mut self, request: Request) -> S::Future {
        (**self).call(request)
    }
}