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Rust编程-核心篇-异步编程_rust异步原理

tomkeji 0 81 次浏览 编程文章

异步编程的必要性

在现代软件开发中，I/O密集型任务（如网络请求、文件操作、数据库查询）占据了很大比重。传统的同步编程模型在处理这些任务时，线程会被阻塞，导致资源利用率低下。异步编程通过非阻塞I/O和协作式多任务，能够显著提高程序的并发性能。

Rust的异步编程模型基于async/await语法，结合强大的类型系统和零成本抽象，提供了既高效又安全的异步编程体验。与传统的回调地狱和Promise链相比，Rust的异步代码看起来像同步代码，但具有异步的性能优势。

异步基础

基本语法

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

// 简单的异步函数
async fn hello_world() {
    println!("Hello, world!");
}

// 异步函数返回Future
async fn get_data() -> String {
    // 模拟异步操作
    tokio::time::sleep(tokio::time::Duration::from_secs(1)).await;
    "Data retrieved".to_string()
}

#[tokio::main]
async fn main() {
    let data = get_data().await;
    println!("{}", data);
}

异步函数组合

async fn fetch_user(id: u32) -> Result<String, String> {
    // 模拟网络请求
    tokio::time::sleep(tokio::time::Duration::from_millis(100)).await;
    Ok(format!("User {}", id))
}

async fn fetch_posts(user_id: u32) -> Result<Vec<String>, String> {
    // 模拟数据库查询
    tokio::time::sleep(tokio::time::Duration::from_millis(200)).await;
    Ok(vec![
        format!("Post 1 by user {}", user_id),
        format!("Post 2 by user {}", user_id),
    ])
}

async fn get_user_with_posts(id: u32) -> Result<(String, Vec<String>), String> {
    let user = fetch_user(id).await?;
    let posts = fetch_posts(id).await?;
    Ok((user, posts))
}

#[tokio::main]
async fn main() {
    match get_user_with_posts(1).await {
        Ok((user, posts)) => {
            println!("User: {}", user);
            for post in posts {
                println!("  - {}", post);
            }
        }
        Err(e) => println!("Error: {}", e),
    }
}

Future trait

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::{Duration, Instant};

struct Delay {
    when: Instant,
}

impl Future for Delay {
    type Output = &'static str;
    
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        if Instant::now() >= self.when {
            println!("Hello world");
            Poll::Ready("done")
        } else {
            // 获取当前任务的Waker
            let waker = cx.waker().clone();
            let when = self.when;
            
            // 在另一个线程中设置定时器
            std::thread::spawn(move || {
                let now = Instant::now();
                if now < when {
                    std::thread::sleep(when - now);
                }
                waker.wake();
            });
            
            Poll::Pending
        }
    }
}

#[tokio::main]
async fn main() {
    let when = Instant::now() + Duration::from_millis(10);
    let future = Delay { when };
    
    let out = future.await;
    assert_eq!(out, "done");
}

并发执行

使用join!并发执行

use tokio::time::{sleep, Duration};

async fn fetch_data_from_api() -> String {
    sleep(Duration::from_millis(100)).await;
    "API data".to_string()
}

async fn fetch_data_from_db() -> String {
    sleep(Duration::from_millis(150)).await;
    "DB data".to_string()
}

async fn fetch_data_from_cache() -> String {
    sleep(Duration::from_millis(50)).await;
    "Cache data".to_string()
}

#[tokio::main]
async fn main() {
    let start = std::time::Instant::now();
    
    // 并发执行多个异步操作
    let (api_data, db_data, cache_data) = tokio::join!(
        fetch_data_from_api(),
        fetch_data_from_db(),
        fetch_data_from_cache()
    );
    
    let duration = start.elapsed();
    println!("All data fetched in {:?}", duration);
    println!("API: {}, DB: {}, Cache: {}", api_data, db_data, cache_data);
}

使用select!选择最快的结果

use tokio::time::{sleep, Duration, timeout};

async fn slow_operation() -> String {
    sleep(Duration::from_secs(2)).await;
    "Slow result".to_string()
}

async fn fast_operation() -> String {
    sleep(Duration::from_millis(100)).await;
    "Fast result".to_string()
}

#[tokio::main]
async fn main() {
    tokio::select! {
        result = slow_operation() => {
            println!("Slow operation completed: {}", result);
        }
        result = fast_operation() => {
            println!("Fast operation completed: {}", result);
        }
        _ = sleep(Duration::from_secs(1)) => {
            println!("Timeout reached");
        }
    }
}

异步流处理

use tokio_stream::{self as stream, StreamExt};
use std::time::Duration;

async fn process_stream() {
    let mut stream = stream::iter(1..=10)
        .then(|i| async move {
            tokio::time::sleep(Duration::from_millis(100)).await;
            i * 2
        })
        .filter(|&x| x % 4 == 0)
        .map(|x| format!("Processed: {}", x));
    
    while let Some(value) = stream.next().await {
        println!("{}", value);
    }
}

#[tokio::main]
async fn main() {
    process_stream().await;
}

实际应用

use reqwest;
use serde_json;
use std::collections::HashMap;

#[derive(Debug, serde::Deserialize)]
struct User {
    id: u32,
    name: String,
    email: String,
}

#[derive(Debug, serde::Deserialize)]
struct Post {
    id: u32,
    title: String,
    body: String,
    userId: u32,
}

async fn fetch_user(id: u32) -> Result<User, reqwest::Error> {
    let url = format!("https://jsonplaceholder.typicode.com/users/{}", id);
    let response = reqwest::get(&url).await?;
    let user: User = response.json().await?;
    Ok(user)
}

async fn fetch_user_posts(user_id: u32) -> Result<Vec<Post>, reqwest::Error> {
    let url = format!("https://jsonplaceholder.typicode.com/posts?userId={}", user_id);
    let response = reqwest::get(&url).await?;
    let posts: Vec<Post> = response.json().await?;
    Ok(posts)
}

async fn get_user_with_posts(id: u32) -> Result<(User, Vec<Post>), reqwest::Error> {
    let user = fetch_user(id).await?;
    let posts = fetch_user_posts(id).await?;
    Ok((user, posts))
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let start = std::time::Instant::now();
    
    // 并发获取多个用户的数据
    let futures = (1..=5).map(|id| get_user_with_posts(id));
    let results = futures::future::join_all(futures).await;
    
    let duration = start.elapsed();
    println!("Fetched {} users in {:?}", results.len(), duration);
    
    for result in results {
        match result {
            Ok((user, posts)) => {
                println!("User: {} ({} posts)", user.name, posts.len());
            }
            Err(e) => {
                println!("Error fetching user: {}", e);
            }
        }
    }
    
    Ok(())
}

异步数据库操作

use sqlx::{PgPool, Row};
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)]
struct User {
    id: i32,
    name: String,
    email: String,
}

#[derive(Debug, Serialize, Deserialize)]
struct Post {
    id: i32,
    title: String,
    content: String,
    user_id: i32,
}

struct Database {
    pool: PgPool,
}

impl Database {
    async fn new(database_url: &str) -> Result<Self, sqlx::Error> {
        let pool = PgPool::connect(database_url).await?;
        Ok(Self { pool })
    }
    
    async fn create_user(&self, name: &str, email: &str) -> Result<User, sqlx::Error> {
        let row = sqlx::query!(
            "INSERT INTO users (name, email) VALUES ($1, $2) RETURNING id, name, email",
            name, email
        )
        .fetch_one(&self.pool)
        .await?;
        
        Ok(User {
            id: row.id,
            name: row.name,
            email: row.email,
        })
    }
    
    async fn get_user(&self, id: i32) -> Result<Option<User>, sqlx::Error> {
        let row = sqlx::query!(
            "SELECT id, name, email FROM users WHERE id = $1",
            id
        )
        .fetch_optional(&self.pool)
        .await?;
        
        Ok(row.map(|r| User {
            id: r.id,
            name: r.name,
            email: r.email,
        }))
    }
    
    async fn get_user_posts(&self, user_id: i32) -> Result<Vec<Post>, sqlx::Error> {
        let rows = sqlx::query!(
            "SELECT id, title, content, user_id FROM posts WHERE user_id = $1",
            user_id
        )
        .fetch_all(&self.pool)
        .await?;
        
        Ok(rows.into_iter().map(|row| Post {
            id: row.id,
            title: row.title,
            content: row.content,
            user_id: row.user_id,
        }).collect())
    }
    
    async fn create_post(&self, title: &str, content: &str, user_id: i32) -> Result<Post, sqlx::Error> {
        let row = sqlx::query!(
            "INSERT INTO posts (title, content, user_id) VALUES ($1, $2, $3) RETURNING id, title, content, user_id",
            title, content, user_id
        )
        .fetch_one(&self.pool)
        .await?;
        
        Ok(Post {
            id: row.id,
            title: row.title,
            content: row.content,
            user_id: row.user_id,
        })
    }
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let db = Database::new("postgresql://user:password@localhost/mydb").await?;
    
    // 创建用户
    let user = db.create_user("Alice", "alice@example.com").await?;
    println!("Created user: {:?}", user);
    
    // 并发创建多个帖子
    let post_futures = (1..=5).map(|i| {
        db.create_post(
            &format!("Post {}", i),
            &format!("Content of post {}", i),
            user.id
        )
    });
    
    let posts = futures::future::join_all(post_futures).await;
    for post in posts {
        match post {
            Ok(post) => println!("Created post: {:?}", post),
            Err(e) => println!("Error creating post: {}", e),
        }
    }
    
    // 获取用户及其帖子
    if let Some(user) = db.get_user(user.id).await? {
        let posts = db.get_user_posts(user.id).await?;
        println!("User {} has {} posts", user.name, posts.len());
    }
    
    Ok(())
}

异步Web服务器

use axum::{
    extract::{Path, Query},
    http::StatusCode,
    response::Json,
    routing::{get, post},
    Router,
};
use serde::{Deserialize, Serialize};
use std::collections::HashMap;
use tokio::time::{sleep, Duration};

#[derive(Serialize, Deserialize)]
struct User {
    id: u32,
    name: String,
    email: String,
}

#[derive(Deserialize)]
struct CreateUser {
    name: String,
    email: String,
}

#[derive(Deserialize)]
struct UserQuery {
    limit: Option<u32>,
    offset: Option<u32>,
}

// 模拟数据库
type UserDb = std::sync::Arc<tokio::sync::RwLock<HashMap<u32, User>>>;

async fn get_users(
    Query(params): Query<UserQuery>,
    Extension(db): Extension<UserDb>,
) -> Result<Json<Vec<User>>, StatusCode> {
    let db = db.read().await;
    let users: Vec<User> = db.values().cloned().collect();
    
    let limit = params.limit.unwrap_or(10);
    let offset = params.offset.unwrap_or(0);
    
    let users: Vec<User> = users
        .into_iter()
        .skip(offset as usize)
        .take(limit as usize)
        .collect();
    
    Ok(Json(users))
}

async fn get_user(
    Path(id): Path<u32>,
    Extension(db): Extension<UserDb>,
) -> Result<Json<User>, StatusCode> {
    let db = db.read().await;
    match db.get(&id) {
        Some(user) => Ok(Json(user.clone())),
        None => Err(StatusCode::NOT_FOUND),
    }
}

async fn create_user(
    Json(payload): Json<CreateUser>,
    Extension(db): Extension<UserDb>,
) -> Result<Json<User>, StatusCode> {
    let mut db = db.write().await;
    let id = (db.len() as u32) + 1;
    
    let user = User {
        id,
        name: payload.name,
        email: payload.email,
    };
    
    db.insert(id, user.clone());
    Ok(Json(user))
}

async fn slow_operation() -> Json<serde_json::Value> {
    // 模拟慢操作
    sleep(Duration::from_secs(2)).await;
    Json(serde_json::json!({"message": "Slow operation completed"}))
}

#[tokio::main]
async fn main() {
    let db: UserDb = std::sync::Arc::new(tokio::sync::RwLock::new(HashMap::new()));
    
    let app = Router::new()
        .route("/users", get(get_users).post(create_user))
        .route("/users/:id", get(get_user))
        .route("/slow", get(slow_operation))
        .layer(axum::Extension(db));
    
    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
    println!("Server running on http://0.0.0.0:3000");
    
    axum::serve(listener, app).await.unwrap();
}

异步错误处理

use thiserror::Error;

#[derive(Error, Debug)]
enum AppError {
    #[error("Network error: {0}")]
    Network(#[from] reqwest::Error),
    
    #[error("Database error: {0}")]
    Database(#[from] sqlx::Error),
    
    #[error("Validation error: {message}")]
    Validation { message: String },
    
    #[error("Timeout error")]
    Timeout,
}

async fn risky_operation() -> Result<String, AppError> {
    // 模拟可能失败的操作
    tokio::time::sleep(tokio::time::Duration::from_millis(100)).await;
    
    if rand::random::<f32>() < 0.3 {
        Err(AppError::Validation {
            message: "Random validation failure".to_string(),
        })
    } else {
        Ok("Success".to_string())
    }
}

async fn handle_errors() -> Result<(), AppError> {
    let result = risky_operation().await?;
    println!("Operation result: {}", result);
    Ok(())
}

#[tokio::main]
async fn main() {
    match handle_errors().await {
        Ok(_) => println!("All operations completed successfully"),
        Err(e) => println!("Error occurred: {}", e),
    }
}

性能优化技巧

1. 合理使用spawn

use tokio::task;

async fn cpu_intensive_task() -> u32 {
    // CPU密集型任务
    let mut sum = 0;
    for i in 0..1_000_000 {
        sum += i;
    }
    sum
}

async fn io_intensive_task() -> String {
    // I/O密集型任务
    tokio::time::sleep(tokio::time::Duration::from_millis(100)).await;
    "I/O completed".to_string()
}

#[tokio::main]
async fn main() {
    // CPU密集型任务应该在阻塞线程池中运行
    let cpu_handle = task::spawn_blocking(|| {
        // 这里会在线程池中运行
        cpu_intensive_task()
    });
    
    // I/O密集型任务可以在异步运行时中运行
    let io_handle = task::spawn(io_intensive_task());
    
    let (cpu_result, io_result) = tokio::join!(cpu_handle, io_handle);
    println!("CPU result: {:?}", cpu_result);
    println!("I/O result: {:?}", io_result);
}

2. 连接池管理

use sqlx::PgPool;

struct AppState {
    db_pool: PgPool,
    http_client: reqwest::Client,
}

impl AppState {
    async fn new() -> Result<Self, Box<dyn std::error::Error>> {
        let db_pool = PgPool::connect("postgresql://user:password@localhost/mydb").await?;
        let http_client = reqwest::Client::new();
        
        Ok(Self {
            db_pool,
            http_client,
        })
    }
}

常见陷阱与最佳实践

1. 避免阻塞异步运行时

// 不好的做法：在异步函数中使用阻塞操作
async fn bad_function() {
    std::thread::sleep(std::time::Duration::from_secs(1)); // 阻塞整个运行时
}

// 好的做法：使用异步睡眠
async fn good_function() {
    tokio::time::sleep(tokio::time::Duration::from_secs(1)).await;
}

2. 正确处理取消

use tokio::time::{sleep, Duration, timeout};

async fn cancellable_operation() -> Result<String, &'static str> {
    sleep(Duration::from_secs(5)).await;
    Ok("Operation completed".to_string())
}

#[tokio::main]
async fn main() {
    match timeout(Duration::from_secs(2), cancellable_operation()).await {
        Ok(result) => println!("Result: {:?}", result),
        Err(_) => println!("Operation timed out"),
    }
}

写在最后

Rust的异步编程模型提供了：

  1. 高性能：非阻塞I/O和协作式多任务
  2. 类型安全：编译时检查异步代码的正确性
  3. 零成本抽象：异步代码的性能接近手写的回调代码
  4. 易于使用：async/await语法让异步代码看起来像同步代码

异步编程是现代高性能应用程序的必备技能。通过Rust的异步编程模型，我们可以构建出既高效又安全的并发应用程序。

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汤科资源网 › 编程文章 › Rust编程-核心篇-异步编程_rust异步原理