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<main>
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<h1 id="创建异步任务"><a class="header" href="#创建异步任务">创建异步任务</a></h1>
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<p>同志们，抓稳了，我们即将换挡提速，通向 <code>mini-redis</code> 服务端的高速之路已经开启。</p>
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<p>不过在开始之前，先来做点收尾工作：上一章节中，我们实现了一个简易的 <code>mini-redis</code> 客户端并支持了 <code>SET</code>/<code>GET</code> 操作, 现在将该<a href="https://course.rs/advance-practice/getting-startted.html#分析未到代码先行">代码</a>移动到 <code>examples</code> 文件夹下，因为我们这个章节要实现的是服务器，后面可以通过运行 <code>example</code> 的方式，用之前客户端示例对我们的服务器端进行测试:</p>
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<pre><code class="language-shell">$ mkdir -p examples
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$ mv src/main.rs examples/hello-redis.rs
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</code></pre>
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<p>并在 <code>Cargo.toml</code> 里添加 <code>[[example]]</code> 说明。关于 <code>example</code> 的详细说明，可以在<a href="https://course.rs/cargo/reference/cargo-target.html#示例对象examples">Cargo使用指南</a>里进一步了解。</p>
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<pre><code class="language-toml">[[example]]
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name = "hello-redis"
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path = "examples/hello-redis.rs"
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</code></pre>
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<p>然后再重新创建一个空的 <code>src/main.rs</code> 文件，至此替换文档已经完成，提速正式开始。</p>
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<h2 id="接收-sockets"><a class="header" href="#接收-sockets">接收 sockets</a></h2>
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<p>作为服务器端，最基础的工作无疑是接收外部进来的 TCP 连接，可以通过 <code>tokio::net::TcpListener</code> 来完成。</p>
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<blockquote>
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|
|
<p>Tokio 中大多数类型的名称都和标准库中对应的同步类型名称相同，而且，如果没有特殊原因，Tokio 的 API 名称也和标准库保持一致，只不过用 <code>async fn</code> 取代 <code>fn</code> 来声明函数。</p>
|
|
|
</blockquote>
|
|
|
<p><code>TcpListener</code> 监听 <strong>6379</strong> 端口，然后通过循环来接收外部进来的连接，每个连接在处理完后会被关闭。对于目前来说，我们的任务很简单：读取命令、打印到标准输出 <code>stdout</code>，最后回复给客户端一个错误。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use tokio::net::{TcpListener, TcpStream};
|
|
|
use mini_redis::{Connection, Frame};
|
|
|
|
|
|
#[tokio::main]
|
|
|
async fn main() {
|
|
|
// Bind the listener to the address
|
|
|
// 监听指定地址，等待 TCP 连接进来
|
|
|
let listener = TcpListener::bind("127.0.0.1:6379").await.unwrap();
|
|
|
|
|
|
loop {
|
|
|
// 第二个被忽略的项中包含有新连接的 `IP` 和端口信息
|
|
|
let (socket, _) = listener.accept().await.unwrap();
|
|
|
process(socket).await;
|
|
|
}
|
|
|
}
|
|
|
|
|
|
async fn process(socket: TcpStream) {
|
|
|
// `Connection` 对于 redis 的读写进行了抽象封装，因此我们读到的是一个一个数据帧frame(数据帧 = redis命令 + 数据)，而不是字节流
|
|
|
// `Connection` 是在 mini-redis 中定义
|
|
|
let mut connection = Connection::new(socket);
|
|
|
|
|
|
if let Some(frame) = connection.read_frame().await.unwrap() {
|
|
|
println!("GOT: {:?}", frame);
|
|
|
|
|
|
// 回复一个错误
|
|
|
let response = Frame::Error("unimplemented".to_string());
|
|
|
connection.write_frame(&response).await.unwrap();
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p>现在运行我们的简单服务器 :</p>
|
|
|
<pre><code class="language-shell">cargo run
|
|
|
</code></pre>
|
|
|
<p>此时服务器会处于循环等待以接收连接的状态，接下来在一个新的终端窗口中启动上一章节中的 <code>redis</code> 客户端，由于相关代码已经放入 <code>examples</code> 文件夹下，因此我们可以使用 <code>--example</code> 来指定运行该客户端示例:</p>
|
|
|
<pre><code class="language-shell">$ cargo run --example hello-redis
|
|
|
</code></pre>
|
|
|
<p>此时，客户端的输出是: <code>Error: "unimplemented"</code>, 同时服务器端打印出了客户端发来的由 <strong>redis 命令和数据</strong> 组成的数据帧: <code>GOT: Array([Bulk(b"set"), Bulk(b"hello"), Bulk(b"world")])</code>。</p>
|
|
|
<h2 id="生成任务"><a class="header" href="#生成任务">生成任务</a></h2>
|
|
|
<p>上面的服务器，如果你仔细看，它其实一次只能接受和处理一条 TCP 连接，只有等当前的处理完并结束后，才能开始接收下一条连接。原因在于 <code>loop</code> 循环中的 <code>await</code> 会导致当前任务进入阻塞等待，也就是 <code>loop</code> 循环会被阻塞。</p>
|
|
|
<p>而这显然不是我们想要的，服务器能并发地处理多条连接的请求，才是正确的打开姿势，下面来看看如何实现真正的并发。</p>
|
|
|
<blockquote>
|
|
|
<p>关于并发和并行，在<a href="https://course.rs/advance/concurrency-with-threads/concurrency-parallelism.html">多线程章节中</a>有详细的解释</p>
|
|
|
</blockquote>
|
|
|
<p>为了并发的处理连接，需要为每一条进来的连接都生成( <code>spawn</code> )一个新的任务, 然后在该任务中处理连接:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use tokio::net::TcpListener;
|
|
|
|
|
|
#[tokio::main]
|
|
|
async fn main() {
|
|
|
let listener = TcpListener::bind("127.0.0.1:6379").await.unwrap();
|
|
|
|
|
|
loop {
|
|
|
let (socket, _) = listener.accept().await.unwrap();
|
|
|
// 为每一条连接都生成一个新的任务，
|
|
|
// `socket` 的所有权将被移动到新的任务中，并在那里进行处理
|
|
|
tokio::spawn(async move {
|
|
|
process(socket).await;
|
|
|
});
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<h4 id="任务"><a class="header" href="#任务">任务</a></h4>
|
|
|
<p>一个 Tokio 任务是一个异步的绿色线程，它们通过 <code>tokio::spawn</code> 进行创建，该函数会返回一个 <code>JoinHandle</code> 类型的句柄，调用者可以使用该句柄跟创建的任务进行交互。</p>
|
|
|
<p><code>spawn</code> 函数的参数是一个 <code>async</code> 语句块，该语句块甚至可以返回一个值，然后调用者可以通过 <code>JoinHandle</code> 句柄获取该值:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">#[tokio::main]
|
|
|
async fn main() {
|
|
|
let handle = tokio::spawn(async {
|
|
|
10086
|
|
|
});
|
|
|
|
|
|
let out = handle.await.unwrap();
|
|
|
println!("GOT {}", out);
|
|
|
}</code></pre>
|
|
|
<p>以上代码会打印出<code>GOT 10086</code>。实际上，上面代码中<code>.await</code> 会返回一个 <code>Result</code> ，若 <code>spawn</code> 创建的任务正常运行结束，则返回一个 <code>Ok(T)</code>的值，否则会返回一个错误 <code>Err</code>：例如任务内部发生了 <code>panic</code> 或任务因为运行时关闭被强制取消时。</p>
|
|
|
<p>任务是调度器管理的执行单元。<code>spawn</code>生成的任务会首先提交给调度器，然后由它负责调度执行。需要注意的是，执行任务的线程未必是创建任务的线程，任务完全有可能运行在另一个不同的线程上，而且任务在生成后，它还可能会在线程间被移动。</p>
|
|
|
<p>任务在 Tokio 中远比看上去要更轻量，例如创建一个任务仅仅需要一次 64 字节大小的内存分配。因此应用程序在生成任务上，完全不应该有任何心理负担，除非你在一台没那么好的机器上疯狂生成了几百万个任务。。。</p>
|
|
|
<h4 id="static-约束"><a class="header" href="#static-约束"><code>'static</code> 约束</a></h4>
|
|
|
<p>当使用 Tokio 创建一个任务时，该任务类型的生命周期必须是 <code>'static</code>。意味着，在任务中不能使用外部数据的引用:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use tokio::task;
|
|
|
|
|
|
#[tokio::main]
|
|
|
async fn main() {
|
|
|
let v = vec![1, 2, 3];
|
|
|
|
|
|
task::spawn(async {
|
|
|
println!("Here's a vec: {:?}", v);
|
|
|
});
|
|
|
}</code></pre>
|
|
|
<p>上面代码中，<code>spawn</code> 出的任务引用了外部环境中的变量 <code>v</code> ，导致以下报错:</p>
|
|
|
<pre><code class="language-console">error[E0373]: async block may outlive the current function, but
|
|
|
it borrows `v`, which is owned by the current function
|
|
|
--> src/main.rs:7:23
|
|
|
|
|
|
|
7 | task::spawn(async {
|
|
|
| _______________________^
|
|
|
8 | | println!("Here's a vec: {:?}", v);
|
|
|
| | - `v` is borrowed here
|
|
|
9 | | });
|
|
|
| |_____^ may outlive borrowed value `v`
|
|
|
|
|
|
|
note: function requires argument type to outlive `'static`
|
|
|
--> src/main.rs:7:17
|
|
|
|
|
|
|
7 | task::spawn(async {
|
|
|
| _________________^
|
|
|
8 | | println!("Here's a vector: {:?}", v);
|
|
|
9 | | });
|
|
|
| |_____^
|
|
|
help: to force the async block to take ownership of `v` (and any other
|
|
|
referenced variables), use the `move` keyword
|
|
|
|
|
|
|
7 | task::spawn(async move {
|
|
|
8 | println!("Here's a vec: {:?}", v);
|
|
|
9 | });
|
|
|
|
|
|
|
</code></pre>
|
|
|
<p>原因在于：默认情况下，变量并不是通过 <code>move</code> 的方式转移进 <code>async</code> 语句块的， <code>v</code> 变量的所有权依然属于 <code>main</code> 函数，因为任务内部的 <code>println!</code> 是通过借用的方式使用了 <code>v</code>，但是这种借用并不能满足 <code>'static</code> 生命周期的要求。</p>
|
|
|
<p>在报错的同时，Rust 编译器还给出了相当有帮助的提示：为 <code>async</code> 语句块使用 <code>move</code> 关键字，这样就能将 <code>v</code> 的所有权从 <code>main</code> 函数转移到新创建的任务中。</p>
|
|
|
<p>但是 <code>move</code> 有一个问题，一个数据只能被一个任务使用，如果想要多个任务使用一个数据，就有些强人所难。不知道还有多少同学记得 <a href="https://course.rs/advance/smart-pointer/rc-arc.html"><code>Arc</code></a>，它可以轻松解决该问题，还是线程安全的。</p>
|
|
|
<p>在上面的报错中，还有一句很奇怪的信息<code>function requires argument type to outlive `'static` </code>， 函数要求参数类型的生命周期必须比 <code>'static</code> 长，问题是 <code>'static</code> 已经活得跟整个程序一样久了，难道函数的参数还能活得更久？大家可能会觉得编译器秀逗了，毕竟其它语言编译器也有秀逗的时候:)</p>
|
|
|
<p>先别急着给它扣帽子，虽然我有时候也想这么做。。原因是它说的是类型必须活得比 <code>'static</code> 长，而不是值。当我们说一个值是 <code>'static</code> 时，意味着它将永远存活。这个很重要，因为编译器无法知道新创建的任务将存活多久，所以唯一的办法就是让任务永远存活。</p>
|
|
|
<p>如果大家对于 <code>&'static</code> 和 <code>T: 'static</code> 较为模糊，强烈建议回顾下<a href="https://course.rs/advance/lifetime/static.html">该章节</a>。</p>
|
|
|
<h4 id="send-约束"><a class="header" href="#send-约束">Send 约束</a></h4>
|
|
|
<p><code>tokio::spawn</code> 生成的任务必须实现 <code>Send</code> 特征，因为当这些任务在 <code>.await</code> 执行过程中发生阻塞时，Tokio 调度器会将任务在线程间移动。</p>
|
|
|
<p><strong>一个任务要实现 <code>Send</code> 特征，那它在 <code>.await</code> 调用的过程中所持有的全部数据都必须实现 <code>Send</code> 特征</strong>。当 <code>.await</code> 调用发生阻塞时，任务会让出当前线程所有权给调度器，然后当任务准备好后，调度器会从上一次暂停的位置继续执行该任务。该流程能正确的工作，任务必须将<code>.await</code>之后使用的所有状态保存起来，这样才能在中断后恢复现场并继续执行。若这些状态实现了 <code>Send</code> 特征(可以在线程间安全地移动)，那任务自然也就可以在线程间安全地移动。</p>
|
|
|
<p>例如以下代码可以工作:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use tokio::task::yield_now;
|
|
|
use std::rc::Rc;
|
|
|
|
|
|
#[tokio::main]
|
|
|
async fn main() {
|
|
|
tokio::spawn(async {
|
|
|
// 语句块的使用强制了 `rc` 会在 `.await` 被调用前就被释放，
|
|
|
// 因此 `rc` 并不会影响 `.await`的安全性
|
|
|
{
|
|
|
let rc = Rc::new("hello");
|
|
|
println!("{}", rc);
|
|
|
}
|
|
|
|
|
|
// `rc` 的作用范围已经失效，因此当任务让出所有权给当前线程时，它无需作为状态被保存起来
|
|
|
yield_now().await;
|
|
|
});
|
|
|
}</code></pre>
|
|
|
<p>但是下面代码就不行：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use tokio::task::yield_now;
|
|
|
use std::rc::Rc;
|
|
|
|
|
|
#[tokio::main]
|
|
|
async fn main() {
|
|
|
tokio::spawn(async {
|
|
|
let rc = Rc::new("hello");
|
|
|
|
|
|
|
|
|
// `rc` 在 `.await` 后还被继续使用，因此它必须被作为任务的状态保存起来
|
|
|
yield_now().await;
|
|
|
|
|
|
|
|
|
// 事实上，注释掉下面一行代码，依然会报错
|
|
|
// 原因是：是否保存，不取决于 `rc` 是否被使用，而是取决于 `.await`在调用时是否仍然处于 `rc` 的作用域中
|
|
|
println!("{}", rc);
|
|
|
|
|
|
// rc 作用域在这里结束
|
|
|
});
|
|
|
}</code></pre>
|
|
|
<p>这里有一个很重要的点，代码注释里有讲到，但是我们再重复一次： <code>rc</code> 是否会保存到任务状态中，取决于 <code>.await</code> 的调用是否处于它的作用域中，上面代码中，就算你注释掉 <code>println!</code> 函数，该报错依然会报错，因为 <code>rc</code> 的作用域直到 <code>async</code> 的末尾才结束！</p>
|
|
|
<p>下面是相应的报错，在下一章节，我们还会继续深入讨论该错误:</p>
|
|
|
<pre><code class="language-shell">error: future cannot be sent between threads safely
|
|
|
--> src/main.rs:6:5
|
|
|
|
|
|
|
6 | tokio::spawn(async {
|
|
|
| ^^^^^^^^^^^^ future created by async block is not `Send`
|
|
|
|
|
|
|
::: [..]spawn.rs:127:21
|
|
|
|
|
|
|
127 | T: Future + Send + 'static,
|
|
|
| ---- required by this bound in
|
|
|
| `tokio::task::spawn::spawn`
|
|
|
|
|
|
|
= help: within `impl std::future::Future`, the trait
|
|
|
| `std::marker::Send` is not implemented for
|
|
|
| `std::rc::Rc<&str>`
|
|
|
note: future is not `Send` as this value is used across an await
|
|
|
--> src/main.rs:10:9
|
|
|
|
|
|
|
7 | let rc = Rc::new("hello");
|
|
|
| -- has type `std::rc::Rc<&str>` which is not `Send`
|
|
|
...
|
|
|
10 | yield_now().await;
|
|
|
| ^^^^^^^^^^^^^^^^^ await occurs here, with `rc` maybe
|
|
|
| used later
|
|
|
11 | println!("{}", rc);
|
|
|
12 | });
|
|
|
| - `rc` is later dropped here
|
|
|
</code></pre>
|
|
|
<h2 id="使用-hashmap-存储数据"><a class="header" href="#使用-hashmap-存储数据">使用 HashMap 存储数据</a></h2>
|
|
|
<p>现在，我们可以继续前进了，下面来实现 <code>process</code> 函数，它用于处理进入的命令。相应的值将被存储在 <code>HashMap</code> 中: 通过 <code>SET</code> 命令存值，通过 <code>GET</code> 命令来取值。</p>
|
|
|
<p>同时，我们将使用循环的方式在同一个客户端连接中处理多次连续的请求:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use tokio::net::TcpStream;
|
|
|
use mini_redis::{Connection, Frame};
|
|
|
|
|
|
async fn process(socket: TcpStream) {
|
|
|
use mini_redis::Command::{self, Get, Set};
|
|
|
use std::collections::HashMap;
|
|
|
|
|
|
// 使用 hashmap 来存储 redis 的数据
|
|
|
let mut db = HashMap::new();
|
|
|
|
|
|
// `mini-redis` 提供的便利函数，使用返回的 `connection` 可以用于从 socket 中读取数据并解析为数据帧
|
|
|
let mut connection = Connection::new(socket);
|
|
|
|
|
|
// 使用 `read_frame` 方法从连接获取一个数据帧：一条redis命令 + 相应的数据
|
|
|
while let Some(frame) = connection.read_frame().await.unwrap() {
|
|
|
let response = match Command::from_frame(frame).unwrap() {
|
|
|
Set(cmd) => {
|
|
|
// 值被存储为 `Vec<u8>` 的形式
|
|
|
db.insert(cmd.key().to_string(), cmd.value().to_vec());
|
|
|
Frame::Simple("OK".to_string())
|
|
|
}
|
|
|
Get(cmd) => {
|
|
|
if let Some(value) = db.get(cmd.key()) {
|
|
|
// `Frame::Bulk` 期待数据的类型是 `Bytes`， 该类型会在后面章节讲解，
|
|
|
// 此时，你只要知道 `&Vec<u8>` 可以使用 `into()` 方法转换成 `Bytes` 类型
|
|
|
Frame::Bulk(value.clone().into())
|
|
|
} else {
|
|
|
Frame::Null
|
|
|
}
|
|
|
}
|
|
|
cmd => panic!("unimplemented {:?}", cmd),
|
|
|
};
|
|
|
|
|
|
// 将请求响应返回给客户端
|
|
|
connection.write_frame(&response).await.unwrap();
|
|
|
}
|
|
|
}
|
|
|
|
|
|
// main 函数在之前已实现
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>使用 <code>cargo run</code> 运行服务器，然后再打开另一个终端窗口，运行 <code>hello-redis</code> 客户端示例: <code>cargo run --example hello-redis</code>。</p>
|
|
|
<p>Bingo，在看了这么多原理后，我们终于迈出了小小的第一步，并获取到了存在 <code>HashMap</code> 中的值: <code>从服务器端获取到结果=Some(b"world")</code>。</p>
|
|
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<p>但是问题又来了：这些值无法在 TCP 连接中共享，如果另外一个用户连接上来并试图同时获取 <code>hello</code> 这个 <code>key</code>，他将一无所获。</p>
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