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<h1 id="tokio-初印象"><a class="header" href="#tokio-初印象">tokio 初印象</a></h1>
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<p>又到了喜闻乐见的初印象环节，这个环节决定了你心中的那 24 盏灯最终是全亮还是全灭。</p>
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<p>在本文中，我们将看看本专题的学习目标、<code>tokio</code>该怎么引入以及如何实现一个 <code>Hello Tokio</code> 项目，最终亮灯还是灭灯的决定权留给各位看官。但我提前说好，如果你全灭了，但却找不到更好的，未来还是得回来真香 :P</p>
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<h2 id="专题目标"><a class="header" href="#专题目标">专题目标</a></h2>
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<p>通过 API 学项目无疑是无聊的，因此我们采用一个与众不同的方式：边学边练，在本专题的最后你将拥有一个 <code>redis</code> 客户端和服务端，当然不会实现一个完整版本的 <code>redis</code> ，只会提供基本的功能和部分常用的命令。</p>
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<h4 id="mini-redis"><a class="header" href="#mini-redis">mini-redis</a></h4>
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<p><code>redis</code> 的项目源码可以在<a href="https://github.com/sunface/rust-by-practice/tree/master/zh-CN/assets/mini-redis">这里访问</a>，本项目是从<a href="https://github.com/tokio-rs/mini-redis">官方地址</a> <code>fork</code> 而来，在未来会提供注释和文档汉化。</p>
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<p>再次声明：该项目仅仅用于学习目的，因此它的文档注释非常全，但是它完全无法作为 <code>redis</code> 的替代品。</p>
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<h2 id="环境配置"><a class="header" href="#环境配置">环境配置</a></h2>
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<p>首先，我们假定你已经安装了 Rust 和相关的工具链，例如 <code>cargo</code>。其中 Rust 版本的最低要求是 <code>1.45.0</code>，建议使用最新版 <code>1.58</code>:</p>
|
|
|
<pre><code class="language-shell">sunfei@sunface $ rustc --version
|
|
|
rustc 1.58.0 (02072b482 2022-01-11)
|
|
|
</code></pre>
|
|
|
<p>接下来，安装 <code>mini-redis</code> 的服务器端，它可以用来测试我们后面将要实现的 <code>redis</code> 客户端：</p>
|
|
|
<pre><code class="language-shell">$ cargo install mini-redis
|
|
|
</code></pre>
|
|
|
<blockquote>
|
|
|
<p>如果下载失败，也可以通过<a href="https://github.com/sunface/rust-by-practice/tree/master/zh-CN/assets/mini-redis">这个地址</a>下载源码，然后在本地通过 <code>cargo run</code>运行。</p>
|
|
|
</blockquote>
|
|
|
<p>下载成功后，启动服务端:</p>
|
|
|
<pre><code class="language-shell">$ mini-redis-server
|
|
|
</code></pre>
|
|
|
<p>然后，再使用客户端测试下刚启动的服务端:</p>
|
|
|
<pre><code class="language-shell">$ mini-redis-cli set foo 1
|
|
|
OK
|
|
|
$ mini-redis-cli get foo
|
|
|
"1"
|
|
|
</code></pre>
|
|
|
<p>不得不说，还挺好用的，先自我陶醉下 :) 此时，万事俱备，只欠东风，接下来是时候亮“箭“了：实现我们的 <code>Hello Tokio</code> 项目。</p>
|
|
|
<h2 id="hello-tokio"><a class="header" href="#hello-tokio">Hello Tokio</a></h2>
|
|
|
<p>与简单无比的 <code>Hello World</code> 有所不同(简单？还记得本书开头时，湖畔边的那个多国语言版本的<code>你好，世界</code>嘛~~)，<code>Hello Tokio</code> 它承载着“非常艰巨“的任务，那就是向刚启动的 <code>redis</code> 服务器写入一个 <code>key=hello, value=world</code> ，然后再读取出来，嗯，使用 <code>mini-redis</code> 客户端 :)</p>
|
|
|
<h4 id="分析未到代码先行"><a class="header" href="#分析未到代码先行">分析未到，代码先行</a></h4>
|
|
|
<p>在详细讲解之前，我们先来看看完整的代码，让大家有一个直观的印象。首先，创建一个新的 <code>Rust</code> 项目:</p>
|
|
|
<pre><code class="language-shell">$ cargo new my-redis
|
|
|
$ cd my-redis
|
|
|
</code></pre>
|
|
|
<p>然后在 <code>Cargo.toml</code> 中添加相关的依赖:</p>
|
|
|
<pre><code class="language-toml">[dependencies]
|
|
|
tokio = { version = "1", features = ["full"] }
|
|
|
mini-redis = "0.4"
|
|
|
</code></pre>
|
|
|
<p>接下来，使用以下代码替换 <code>main.rs</code> 中的内容：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use mini_redis::{client, Result};
|
|
|
|
|
|
#[tokio::main]
|
|
|
async fn main() -> Result<()> {
|
|
|
// 建立与mini-redis服务器的连接
|
|
|
let mut client = client::connect("127.0.0.1:6379").await?;
|
|
|
|
|
|
// 设置 key: "hello" 和 值: "world"
|
|
|
client.set("hello", "world".into()).await?;
|
|
|
|
|
|
// 获取"key=hello"的值
|
|
|
let result = client.get("hello").await?;
|
|
|
|
|
|
println!("从服务器端获取到结果={:?}", result);
|
|
|
|
|
|
Ok(())
|
|
|
}</code></pre>
|
|
|
<p>不知道你之前启动的 <code>mini-redis-server</code> 关闭没有，如果关了，记得重新启动下，否则我们的代码就是意大利空气炮。</p>
|
|
|
<p>最后，运行这个项目:</p>
|
|
|
<pre><code class="language-shell">$ cargo run
|
|
|
从服务器端获取到结果=Some("world")
|
|
|
</code></pre>
|
|
|
<p>Perfect, 代码成功运行，是时候来解释下其中蕴藏的至高奥秘了。</p>
|
|
|
<h2 id="原理解释"><a class="header" href="#原理解释">原理解释</a></h2>
|
|
|
<p>代码篇幅虽然不长，但是还是有不少值得关注的地方，接下来我们一起来看看。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let mut client = client::connect("127.0.0.1:6379").await?;
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p><a href="https://docs.rs/mini-redis/0.4.1/mini_redis/client/fn.connect.html"><code>client::connect</code></a> 函数由<code>mini-redis</code> 包提供，它使用异步的方式跟指定的远程 <code>IP</code> 地址建立 TCP 长连接，一旦连接建立成功，那 <code>client</code> 的赋值初始化也将完成。</p>
|
|
|
<p>特别值得注意的是：虽然该连接是异步建立的，但是从代码本身来看，完全是<strong>同步的代码编写方式</strong>，唯一能说明异步的点就是 <code>.await</code>。</p>
|
|
|
<h4 id="什么是异步编程"><a class="header" href="#什么是异步编程">什么是异步编程</a></h4>
|
|
|
<p>大部分计算机程序都是按照代码编写的顺序来执行的：先执行第一行，然后第二行，以此类推(当然，还要考虑流程控制，例如循环)。当进行同步编程时，一旦程序遇到一个操作无法被立即完成，它就会进入阻塞状态，直到该操作完成为止。</p>
|
|
|
<p>因此同步编程非常符合我们人类的思维习惯，是一个顺其自然的过程，被几乎每一个程序员所喜欢(本来想说所有，但我不敢打包票，毕竟总有特立独行之士)。例如，当建立 TCP 连接时，当前线程会被阻塞，直到等待该连接建立完成，然后才往下继续进行。</p>
|
|
|
<p>而使用异步编程，无法立即完成的操作会被切到后台去等待，因此当前线程不会被阻塞，它会接着执行其它的操作。一旦之前的操作准备好可以继续执行后，它会通知执行器，然后执行器会调度它并从上次离开的点继续执行。但是大家想象下，如果没有使用 <code>await</code>，而是按照这个异步的流程使用通知 -> 回调的方式实现，代码该多么的难写和难读！</p>
|
|
|
<p>好在 Rust 为我们提供了 <code>async/await</code> 的异步编程特性，让我们可以像写同步代码那样去写异步的代码，也让这个世界美好依旧。</p>
|
|
|
<h4 id="编译时绿色线程"><a class="header" href="#编译时绿色线程">编译时绿色线程</a></h4>
|
|
|
<p>一个函数可以通过<code>async fn</code>的方式被标记为异步函数:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use mini_redis::Result;
|
|
|
use mini_redis::client::Client;
|
|
|
use tokio::net::ToSocketAddrs;
|
|
|
|
|
|
pub async fn connect<T: ToSocketAddrs>(addr: T) -> Result<Client> {
|
|
|
// ...
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>在上例中，<code>redis</code> 的连接函数 <code>connect</code> 实现如上，它看上去很像是一个同步函数，但是 <code>async fn</code> 出卖了它。
|
|
|
<code>async fn</code> 异步函数并不会直接返回值，而是返回一个 <code>Future</code>，顾名思义，该 <code>Future</code> 会在未来某个时间点被执行，然后最终获取到真实的返回值 <code>Result<Client></code>。</p>
|
|
|
<blockquote>
|
|
|
<p>async/await 的原理就算大家不理解，也不妨碍使用 <code>tokio</code> 写出能用的服务，但是如果想要更深入的用好，强烈建议认真读下本书的 <a href="https://course.rs/advance/async/intro.html"><code>async/await</code> 异步编程章节</a>，你会对 Rust 的异步编程有一个全新且深刻的认识。</p>
|
|
|
</blockquote>
|
|
|
<p>由于 <code>async</code> 会返回一个 <code>Future</code>，因此我们还需要配合使用 <code>.await</code> 来让该 <code>Future</code> 运行起来，最终获得返回值:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">async fn say_to_world() -> String {
|
|
|
String::from("world")
|
|
|
}
|
|
|
|
|
|
#[tokio::main]
|
|
|
async fn main() {
|
|
|
// 此处的函数调用是惰性的，并不会执行 `say_to_world()` 函数体中的代码
|
|
|
let op = say_to_world();
|
|
|
|
|
|
// 首先打印出 "hello"
|
|
|
println!("hello");
|
|
|
|
|
|
// 使用 `.await` 让 `say_to_world` 开始运行起来
|
|
|
println!("{}", op.await);
|
|
|
}</code></pre>
|
|
|
<p>上面代码输出如下:</p>
|
|
|
<pre><code class="language-shell">hello
|
|
|
world
|
|
|
</code></pre>
|
|
|
<p>而大家可能很好奇 <code>async fn</code> 到底返回什么吧？它实际上返回的是一个实现了 <code>Future</code> 特征的匿名类型: <code>impl Future<Output = String></code>。</p>
|
|
|
<h4 id="async-main"><a class="header" href="#async-main">async main</a></h4>
|
|
|
<p>在代码中，使用了一个与众不同的 <code>main</code> 函数 : <code>async fn main</code> ，而且是用 <code>#[tokio::main]</code> 属性进行了标记。异步 <code>main</code> 函数有以下意义：</p>
|
|
|
<ul>
|
|
|
<li><code>.await</code> 只能在 <code>async</code> 函数中使用，如果是以前的 <code>fn main</code>，那它内部是无法直接使用 <code>async</code> 函数的！这个会极大的限制了我们的使用场景</li>
|
|
|
<li>异步运行时本身需要初始化</li>
|
|
|
</ul>
|
|
|
<p>因此 <code>#[tokio::main]</code> 宏在将 <code>async fn main</code> 隐式的转换为 <code>fn main</code> 的同时还对整个异步运行时进行了初始化。例如以下代码:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">#[tokio::main]
|
|
|
async fn main() {
|
|
|
println!("hello");
|
|
|
}</code></pre>
|
|
|
<p>将被转换成:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let mut rt = tokio::runtime::Runtime::new().unwrap();
|
|
|
rt.block_on(async {
|
|
|
println!("hello");
|
|
|
})
|
|
|
}</code></pre>
|
|
|
<p>最终，Rust 编译器就愉快地执行这段代码了。</p>
|
|
|
<h2 id="cargo-feature"><a class="header" href="#cargo-feature">cargo feature</a></h2>
|
|
|
<p>在引入 <code>tokio</code> 包时，我们在 <code>Cargo.toml</code> 文件中添加了这么一行:</p>
|
|
|
<pre><code class="language-toml">tokio = { version = "1", features = ["full"] }
|
|
|
</code></pre>
|
|
|
<p>里面有个 <code>features = ["full"]</code> 可能大家会比较迷惑，当然，关于它的具体解释在本书的 <a href="https://course.rs/cargo/intro.html">Cargo 详解专题</a> 有介绍，这里就简单进行说明。</p>
|
|
|
<p><code>Tokio</code> 有很多功能和特性，例如 <code>TCP</code>，<code>UDP</code>，<code>Unix sockets</code>，同步工具，多调度类型等等，不是每个应用都需要所有的这些特性。为了优化编译时间和最终生成可执行文件大小、内存占用大小，应用可以对这些特性进行可选引入。</p>
|
|
|
<p>而这里为了演示的方便，我们使用 <code>full</code> ，表示直接引入所有的特性。</p>
|
|
|
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
|
|
|
<p>大家对 <code>tokio</code> 的初印象如何？可否 24 灯全亮通过？</p>
|
|
|
<p>总之，<code>tokio</code> 做的事情其实是细雨润无声的，在大多数时候，我们并不能感觉到它的存在，但是它确实是异步编程中最重要的一环(或者之一)，深入了解它对我们的未来之路会有莫大的帮助。</p>
|
|
|
<p>接下来，正式开始 <code>tokio</code> 的学习之旅。</p>
|
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