案例：HTTP 伺服器

我們用 async/.await 打造一個 echo 伺服器吧！

首先，執行 rustup update nightly 讓你去的最新最棒的 Rust，我們正使用最前沿的功能，確保最新版本非常重要。當你完成上述指令，執行 cargo +nightly new async-await-echo 來建立新專案，並開啟 async-await-echo 資料夾。

讓我們在 Cargo.toml 中加上一些相依函式庫：

[dependencies]
# The latest version of the "futures" library, which has lots of utilities
# for writing async code. Enable the "compat" feature to include the
# functions for using futures 0.3 and async/await with the Hyper library,
# which use futures 0.1.
futures-preview = { version = "=0.3.0-alpha.17", features = ["compat"] }

# Hyper is an asynchronous HTTP library. We'll use it to power our HTTP
# server and to make HTTP requests.
hyper = "0.12.9"

現在已經取得我們的相依函式庫，可以開始寫程式了。開啟 src/main.rs 並在檔案最上面開啟 async_await 功能：


# #![allow(unused_variables)]
#![feature(async_await)]
#fn main() {
#}

這將會加入 nightly 版專屬但很快就會穩定的 async/await 語法支援。

此外，有些 import 需要新增：


# #![allow(unused_variables)]
#fn main() {
use {
    hyper::{
        // Miscellaneous types from Hyper for working with HTTP.
        Body, Client, Request, Response, Server, Uri,

        // This function turns a closure which returns a future into an
        // implementation of the the Hyper `Service` trait, which is an
        // asynchronous function from a generic `Request` to a `Response`.
        service::service_fn,

        // A function which runs a future to completion using the Hyper runtime.
        rt::run,
    },
    futures::{
        // Extension trait for futures 0.1 futures, adding the `.compat()` method
        // which allows us to use `.await` on 0.1 futures.
        compat::Future01CompatExt,
        // Extension traits providing additional methods on futures.
        // `FutureExt` adds methods that work for all futures, whereas
        // `TryFutureExt` adds methods to futures that return `Result` types.
        future::{FutureExt, TryFutureExt},
    },
    std::net::SocketAddr,
};
#}

當這些 imports 加入後，將這些鍋碗瓢盆放在一起，就能開始接收請求：

async fn serve_req(_req: Request<Body>) -> Result<Response<Body>, hyper::Error> {
    // Always return successfully with a response containing a body with
    // a friendly greeting ;)
    Ok(Response::new(Body::from("hello, world!")))
}

async fn run_server(addr: SocketAddr) {
    println!("Listening on http://{}", addr);

    // Create a server bound on the provided address
    let serve_future = Server::bind(&addr)
        // Serve requests using our `async serve_req` function.
        // `serve` takes a closure which returns a type implementing the
        // `Service` trait. `service_fn` returns a value implementing the
        // `Service` trait, and accepts a closure which goes from request
        // to a future of the response. To use our `serve_req` function with
        // Hyper, we have to box it and put it in a compatability
        // wrapper to go from a futures 0.3 future (the kind returned by
        // `async fn`) to a futures 0.1 future (the kind used by Hyper).
        .serve(|| service_fn(|req| serve_req(req).boxed().compat()));

    // Wait for the server to complete serving or exit with an error.
    // If an error occurred, print it to stderr.
    if let Err(e) = serve_future.compat().await {
        eprintln!("server error: {}", e);
    }
}

fn main() {
    // Set the address to run our socket on.
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));

    // Call our `run_server` function, which returns a future.
    // As with every `async fn`, for `run_server` to do anything,
    // the returned future needs to be run. Additionally,
    // we need to convert the returned future from a futures 0.3 future into a
    // futures 0.1 future.
    let futures_03_future = run_server(addr);
    let futures_01_future = futures_03_future.unit_error().boxed().compat();

    // Finally, we can run the future to completion using the `run` function
    // provided by Hyper.
    run(futures_01_future);
}

如果現在跑 cargo run，應該會看到「Listening on http://127.0.0.1:3000」輸出在終端機上。若在你的瀏覽器開啟這個鏈結，會看到「hello, world」出現在你的瀏覽器。恭喜！你剛寫下第一個 Rust 非同步網頁伺服器。

你也可以檢視這個請求本身，會包含譬如請求的 URI、HTTP 版本、headers、以及其他詮釋資料。舉例來說，你可以打印出請求的 URI：


# #![allow(unused_variables)]
#fn main() {
println!("Got request at {:?}", req.uri());
#}

你可能注意到我們尚未做任何非同步的事情來處理這個請求，我們就只是即刻回應罷了，所以我們並無善用 async fn 給予我們的彈性。與其只返回一個靜態訊息，讓我們使用 Hyper 的 HTTP 客戶端來代理使用者的請求到其他網站。

我們從解析想要請求的 URL 開始：


# #![allow(unused_variables)]
#fn main() {
let url_str = "http://www.rust-lang.org/en-US/";
let url = url_str.parse::<Uri>().expect("failed to parse URL");
#}

接下來我們建立一個新的 hyper::Client 並用之生成一個 GET 請求，這個請求會回傳一個回應給使用者。


# #![allow(unused_variables)]
#fn main() {
let res = Client::new().get(url).compat().await;
// Return the result of the request directly to the user
println!("request finished-- returning response");
res
#}

Client::get 返回一個 hyper::client::FutureResponse，這個 future 實作了 Future<Output = Result<Response, Error>>（或 futures 0.1 的 Future<Item = Response, Error = Error>）。當我們 .await 這個 future，將會發送一個 HTTP 請求，當前的任務會暫時停止（suspend），而這個任務會進入佇列中，在收到回應後繼續執行。

現在，執行 cargo run 並在瀏覽器中打開 http://127.0.0.1:3000/foo，會看到 Rust 首頁，以及以下的終端機輸出：

Listening on http://127.0.0.1:3000
Got request at /foo
making request to http://www.rust-lang.org/en-US/
request finished-- returning response

恭喜呀！你剛剛代理了一個 HTTP 請求。

Rust 非同步程式設計

案例：HTTP 伺服器