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< h1 id = "共享状态" > < a class = "header" href = "#共享状态" > 共享状态< / a > < / h1 >
< p > 上一章节中，咱们搭建了一个异步的 redis 服务器，并成功的提供了服务，但是其隐藏了一个巨大的问题：状态(数据)无法在多个连接之间共享，下面一起来看看该如何解决。< / p >
< h2 id = "解决方法" > < a class = "header" href = "#解决方法" > 解决方法< / a > < / h2 >
< p > 好在 Tokio 十分强大，上面问题对应的解决方法也不止一种：< / p >
< ul >
< li > 使用 < code > Mutex< / code > 来保护数据的共享访问< / li >
< li > 生成一个异步任务去管理状态，然后各个连接使用消息传递的方式与其进行交互< / li >
< / ul >
< p > 其中，第一种方法适合比较简单的数据，而第二种方法适用于需要异步工作的，例如 I/O 原语。由于我们使用的数据存储类型是 < code > HashMap< / code > ，使用到的相关操作是 < code > insert< / code > 和 < code > get< / code > ，又因为这两个操作都不是异步的，因此只要使用 < code > Mutex< / code > 即可解决问题。< / p >
< p > 在上面的描述中，说实话第二种方法及其适用的场景并不是很好理解，但没关系，在后面章节会进行详细介绍。< / p >
< h2 id = "添加-bytes-依赖包" > < a class = "header" href = "#添加-bytes-依赖包" > 添加 < code > bytes< / code > 依赖包< / a > < / h2 >
< p > 在上一节中，我们使用 < code > Vec< u8> < / code > 来保存目标数据，但是它有一个问题，对它进行克隆时会将底层数据也整个复制一份，效率很低，但是克隆操作对于我们在多连接间共享数据又是必不可少的。< / p >
< p > 因此这里咱们新引入一个 < code > bytes< / code > 包，它包含一个 < code > Bytes< / code > 类型，当对该类型的值进行克隆时，就不再会克隆底层数据。事实上，< code > Bytes< / code > 是一个引用计数类型，跟 < code > Arc< / code > 非常类似，或者准确的说，< code > Bytes< / code > 就是基于 < code > Arc< / code > 实现的，但相比后者< code > Bytes< / code > 提供了一些额外的能力。< / p >
< p > 在 < code > Cargo.toml< / code > 的 < code > [dependencies]< / code > 中引入 < code > bytes< / code > ： < / p >
< pre > < code class = "language-console" > bytes = "1"
< / code > < / pre >
< h2 id = "初始化-hashmap" > < a class = "header" href = "#初始化-hashmap" > 初始化 HashMap< / a > < / h2 >
< p > 由于 < code > HashMap< / code > 会在多个任务甚至多个线程间共享，再结合之前的选择，最终我们决定使用 < code > Arc< Mutex< T> > < / code > 的方式对其进行包裹。< / p >
< p > 但是，大家先来畅想一下使用它进行包裹后的类型长什么样？ 大概，可能，长这样：< code > Arc< Mutex< HashMap< String, Bytes> > > < / code > ，天哪噜，一不小心，你就遇到了 Rust 的阴暗面：类型大串烧。可以想象，如果要在代码中到处使用这样的类型，可读性会极速下降，因此我们需要一个< a href = "https://course.rs/advance/custom-type.html#类型别名type-alias" > 类型别名< / a > ( type alias )来简化下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use bytes::Bytes;
use std::collections::HashMap;
use std::sync::{Arc, Mutex};
type Db = Arc< Mutex< HashMap< String, Bytes> > > ;
< span class = "boring" > }< / span > < / code > < / pre >
< p > 此时，< code > Db< / code > 就是一个类型别名，使用它就可以替代那一大串的东东，等下你就能看到功效。< / p >
< p > 接着，我们需要在 < code > main< / code > 函数中对 < code > HashMap< / code > 进行初始化，然后使用 < code > Arc< / code > 克隆一份它的所有权并将其传入到生成的异步任务中。事实上在 Tokio 中，这里的 < code > Arc< / code > 被称为 < strong > handle< / strong > ，或者更宽泛的说，< code > handle< / code > 在 Tokio 中可以用来访问某个共享状态。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::net::TcpListener;
use std::collections::HashMap;
use std::sync::{Arc, Mutex};
#[tokio::main]
async fn main() {
let listener = TcpListener::bind("127.0.0.1:6379").await.unwrap();
println!("Listening");
let db = Arc::new(Mutex::new(HashMap::new()));
loop {
let (socket, _) = listener.accept().await.unwrap();
// 将 handle 克隆一份
let db = db.clone();
println!("Accepted");
tokio::spawn(async move {
process(socket, db).await;
});
}
}< / code > < / pre >
< h4 id = "为何使用-stdsyncmutex" > < a class = "header" href = "#为何使用-stdsyncmutex" > 为何使用 < code > std::sync::Mutex< / code > < / a > < / h4 >
< p > 上面代码还有一点非常重要，那就是我们使用了 < code > std::sync::Mutex< / code > 来保护 < code > HashMap< / code > ，而不是使用 < code > tokio::sync::Mutex< / code > 。< / p >
< p > 在使用 Tokio 编写异步代码时，一个常见的错误无条件地使用 < code > tokio::sync::Mutex< / code > ， ： < code > .await< / code > 调用时使用，而且 Tokio 的 < code > Mutex< / code > 实际上内部使用的也是 < code > std::sync::Mutex< / code > 。< / p >
< p > 多补充几句，在异步代码中，关于锁的使用有以下经验之谈：< / p >
< ul >
< li > 锁如果在多个 < code > .await< / code > 过程中持有，应该使用 Tokio 提供的锁，原因是 < code > .await< / code > 的过程中锁可能在线程间转移，若使用标准库的同步锁存在死锁的可能性，例如某个任务刚获取完锁，还没使用完就因为 < code > .await< / code > 让出了当前线程的所有权，结果下个任务又去获取了锁，造成死锁< / li >
< li > 锁竞争不多的情况下，使用 < code > std::sync::Mutex< / code > < / li >
< li > 锁竞争多，可以考虑使用三方库提供的性能更高的锁，例如 < a href = "https://docs.rs/parking_lot/0.10.2/parking_lot/type.Mutex.html" > < code > parking_lot::Mutex< / code > < / a > < / li >
< / ul >
< h2 id = "更新-process" > < a class = "header" href = "#更新-process" > 更新 < code > process()< / code > < / a > < / h2 >
< p > < code > process()< / code > 函数不再初始化 < code > HashMap< / code > ，取而代之的是它使用了 < code > HashMap< / code > 的一个 < code > handle< / code > 作为参数:< / 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, db: Db) {
use mini_redis::Command::{self, Get, Set};
let mut connection = Connection::new(socket);
while let Some(frame) = connection.read_frame().await.unwrap() {
let response = match Command::from_frame(frame).unwrap() {
Set(cmd) => {
let mut db = db.lock().unwrap();
db.insert(cmd.key().to_string(), cmd.value().clone());
Frame::Simple("OK".to_string())
}
Get(cmd) => {
let db = db.lock().unwrap();
if let Some(value) = db.get(cmd.key()) {
Frame::Bulk(value.clone())
} else {
Frame::Null
}
}
cmd => panic!("unimplemented {:?}", cmd),
};
connection.write_frame(& response).await.unwrap();
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "任务线程和锁竞争" > < a class = "header" href = "#任务线程和锁竞争" > 任务、线程和锁竞争< / a > < / h2 >
< p > 当竞争不多的时候，使用阻塞性的锁去保护共享数据是一个正确的选择。当一个锁竞争触发后，当前正在执行任务(请求锁)的线程会被阻塞，并等待锁被前一个使用者释放。这里的关键就是：< strong > 锁竞争不仅仅会导致当前的任务被阻塞，还会导致执行任务的线程被阻塞，因此该线程准备执行的其它任务也会因此被阻塞！< / strong > < / p >
< p > 默认情况下， < a href = "https://docs.rs/tokio/1.15.0/tokio/runtime/index.html#current-thread-scheduler" > < strong > current_thread< / strong > < / a > 运行时设置，在该设置下会使用一个单线程的调度器(执行器)，所有的任务都会创建并执行在当前线程上，因此不再会有锁竞争。< / p >
< blockquote >
< p > current_thread 是一个轻量级、单线程的运行时，当任务数不多或连接数不多时是一个很好的选择。例如你想在一个异步客户端库的基础上提供给用户同步的 API 访问时，该模式就很适用< / p >
< / blockquote >
< p > 当同步锁的竞争变成一个问题时，使用 Tokio 提供的异步锁几乎并不能帮你解决问题，此时可以考虑如下选项：< / p >
< ul >
< li > 创建专门的任务并使用消息传递的方式来管理状态< / li >
< li > 将锁进行分片< / li >
< li > 重构代码以避免锁< / li >
< / ul >
< p > 在我们的例子中，由于每一个 < code > key< / code > 都是独立的，因此对锁进行分片将成为一个不错的选择:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > type ShardedDb = Arc< Vec< Mutex< HashMap< String, Vec< u8> > > > > ;
fn new_sharded_db(num_shards: usize) -> ShardedDb {
let mut db = Vec::with_capacity(num_shards);
for _ in 0..num_shards {
db.push(Mutex::new(HashMap::new()));
}
Arc::new(db)
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 在这里，我们创建了 N 个不同的存储实例，每个实例都会存储不同的分片数据，例如我们有< code > a-i< / code > 共 9 个不同的 < code > key< / code > , 可以将存储分成 3 个实例，那么第一个实例可以存储 < code > a-c< / code > ，第二个< code > d-f< / code > ，以此类推。在这种情况下，访问 < code > b< / code > 时，只需要锁住第一个实例，此时二、三实例依然可以正常访问，因此锁被成功的分片了。< / p >
< p > 在分片后，使用给定的 key 找到对应的值就变成了两个步骤：首先，使用 < code > key< / code > 通过特定的算法寻找到对应的分片，然后再使用该 < code > key< / code > 从分片中查询到值:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let shard = db[hash(key) % db.len()].lock().unwrap();
shard.insert(key, value);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这里我们使用 < code > hash< / code > 算法来进行分片，但是该算法有个缺陷：分片的数量不能变，一旦变了后，那之前落入分片 1 的< code > key< / code > 很可能将落入到其它分片中，最终全部乱掉。此时你可以考虑< a href = "https://docs.rs/dashmap" > dashmap< / a > ，它提供了更复杂、更精妙的支持分片的< code > hash map< / code > 。< / p >
< h2 id = "在-await-期间持有锁" > < a class = "header" href = "#在-await-期间持有锁" > 在 < code > .await< / code > 期间持有锁< / a > < / h2 >
< p > 在某些时候，你可能会不经意写下这种代码:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::sync::{Mutex, MutexGuard};
async fn increment_and_do_stuff(mutex: & Mutex< i32> ) {
let mut lock: MutexGuard< i32> = mutex.lock().unwrap();
*lock += 1;
do_something_async().await;
} // 锁在这里超出作用域
< span class = "boring" > }< / span > < / code > < / pre >
< p > 如果你要 < code > spawn< / code > 一个任务来执行上面的函数的话，会报错:< / p >
< pre > < code class = "language-console" > error: future cannot be sent between threads safely
--> src/lib.rs:13:5
|
13 | tokio::spawn(async move {
| ^^^^^^^^^^^^ future created by async block is not `Send`
|
::: /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.21/src/task/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::sync::MutexGuard< '_, i32> `
note: future is not `Send` as this value is used across an await
--> src/lib.rs:7:5
|
4 | let mut lock: MutexGuard< i32> = mutex.lock().unwrap();
| -------- has type `std::sync::MutexGuard< '_, i32> ` which is not `Send`
...
7 | do_something_async().await;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ await occurs here, with `mut lock` maybe used later
8 | }
| - `mut lock` is later dropped here
< / code > < / pre >
< p > 错误的原因在于 < code > std::sync::MutexGuard< / code > 类型并没有实现 < code > Send< / code > 特征，这意味着你不能将一个 < code > Mutex< / code > 锁发送到另一个线程，因为 < code > .await< / code > 可能会让任务转移到另一个线程上执行，这个之前也介绍过。< / p >
< h4 id = "提前释放锁" > < a class = "header" href = "#提前释放锁" > 提前释放锁< / a > < / h4 >
< p > 要解决这个问题，就必须重构代码，让 < code > Mutex< / code > 锁在 < code > .await< / code > 被调用前就被释放掉。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // 下面的代码可以工作！
async fn increment_and_do_stuff(mutex: & Mutex< i32> ) {
{
let mut lock: MutexGuard< i32> = mutex.lock().unwrap();
*lock += 1;
} // lock在这里超出作用域 (被释放)
do_something_async().await;
}
< span class = "boring" > }< / span > < / code > < / pre >
< blockquote >
< p > 大家可能已经发现，很多错误都是因为 < code > .await< / code > 引起的，其实你只要记住，在 < code > .await< / code > 执行期间，任务可能会在线程间转移，那么这些错误将变得很好理解，不必去死记硬背< / p >
< / blockquote >
< p > 但是下面的代码不工作：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::sync::{Mutex, MutexGuard};
async fn increment_and_do_stuff(mutex: & Mutex< i32> ) {
let mut lock: MutexGuard< i32> = mutex.lock().unwrap();
*lock += 1;
drop(lock);
do_something_async().await;
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 原因我们之前解释过，编译器在这里不够聪明，目前它只能根据作用域的范围来判断，< code > drop< / code > 虽然释放了锁，但是锁的作用域依然会持续到函数的结束，未来也许编译器会改进，但是现在至少还是不行的。< / p >
< p > 聪明的读者此时的小脑袋已经飞速运转起来，既然锁没有实现 < code > Send< / code > ， < code > .await< / code > 期间被挂起，接着开始执行另一个任务，这个任务又去获取锁，就会导致死锁。< / p >
< p > 再来看看其它解决方法：< / p >
< h4 id = "重构代码在-await-期间不持有锁" > < a class = "header" href = "#重构代码在-await-期间不持有锁" > 重构代码：在 < code > .await< / code > 期间不持有锁< / a > < / h4 >
< p > 之前的代码其实也是为了在 < code > .await< / code > 期间不持有锁，但是我们还有更好的实现方式，例如，你可以把 < code > Mutex< / code > 放入一个结构体中，并且只在该结构体的非异步方法中使用该锁:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::sync::Mutex;
struct CanIncrement {
mutex: Mutex< i32> ,
}
impl CanIncrement {
// 该方法不是 `async`
fn increment(& self) {
let mut lock = self.mutex.lock().unwrap();
*lock += 1;
}
}
async fn increment_and_do_stuff(can_incr: & CanIncrement) {
can_incr.increment();
do_something_async().await;
}
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "使用异步任务和通过消息传递来管理状态" > < a class = "header" href = "#使用异步任务和通过消息传递来管理状态" > 使用异步任务和通过消息传递来管理状态< / a > < / h4 >
< p > 该方法常常用于共享的资源是 I/O 类型的资源时，我们在下一章节将详细介绍。< / p >
< h4 id = "使用-tokio-提供的异步锁" > < a class = "header" href = "#使用-tokio-提供的异步锁" > 使用 Tokio 提供的异步锁< / a > < / h4 >
< p > Tokio 提供的锁最大的优点就是：它可以在 < code > .await< / code > 执行期间被持有，而且不会有任何问题。但是代价就是，这种异步锁的性能开销会更高，因此如果可以，使用之前的两种方法来解决会更好。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use tokio::sync::Mutex; // 注意，这里使用的是 Tokio 提供的锁
// 下面的代码会编译
// 但是就这个例子而言，之前的方式会更好
async fn increment_and_do_stuff(mutex: & Mutex< i32> ) {
let mut lock = mutex.lock().await;
*lock += 1;
do_something_async().await;
} // 锁在这里被释放
< span class = "boring" > }< / span > < / code > < / pre >
< / main >
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