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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 >
< 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 >
< 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 > examples< / 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 >
< pre > < code class = "language-shell" > $ mkdir -p examples
$ mv src/main.rs examples/hello-redis.rs
< / code > < / pre >
< 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 >
< pre > < code class = "language-toml" > [[example]]
name = "hello-redis"
path = "examples/hello-redis.rs"
< / code > < / pre >
< 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 >
< h2 id = "接收-sockets" > < a class = "header" href = "#接收-sockets" > 接收 sockets< / a > < / h2 >
< 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 >
< blockquote >
< 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， ， ， < 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，
// `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， < 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 > ， < 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， < / 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：
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`，
// 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， ， ， < code > HashMap< / code > 中的值: < code > 从服务器端获取到结果=Some(b"world")< / code > 。< / p >
< 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 >
< / main >
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