什么是JavaScript的异步上下文在日志追踪中的使用，以及它如何在异步调用链中传递请求ID？

const { AsyncLocalStorage } = require('async_hooks');
const asyncLocalStorage = new AsyncLocalStorage();

// 模拟一个日志函数，它会尝试从异步上下文中获取请求ID
function log(message) {
    const store = asyncLocalStorage.getStore();
    const requestId = store ? store.requestId : 'N/A'; // 如果没有上下文，则显示N/A
    console.log(`[Request ID: ${requestId}] ${message}`);
}

// 模拟一个异步操作，比如数据库查询或外部API调用
async function fetchData(query) {
    log(`开始获取数据：${query}`);
    await new Promise(resolve => setTimeout(resolve, 50)); // 模拟网络延迟
    log(`数据获取完成：${query}`);
    return { id: Math.random().toString(36).substring(7), result: `Data for ${query}` };
}

// 模拟处理一个传入的请求
async function handleIncomingRequest(reqId, payload) {
    // 使用asyncLocalStorage.run包裹整个请求处理流程
    asyncLocalStorage.run({ requestId: reqId }, async () => {
        log(`处理请求，ID：${reqId}，载荷：${payload}`);

        // 在这个异步流中，任何地方调用log都会带上reqId
        const result = await fetchData(payload);
        log(`处理结果：${JSON.stringify(result)}`);

        // 进一步的异步操作
        await new Promise(resolve => setTimeout(resolve, 20));
        log(`请求 ${reqId} 最终完成。`);
    });
}

// 模拟两个并发的请求
handleIncomingRequest('req-alpha-1', '用户订单数据');
handleIncomingRequest('req-beta-2', '商品库存信息');

// 稍后模拟另一个请求，确保上下文隔离
setTimeout(() => {
    handleIncomingRequest('req-gamma-3', '支付流水查询');
}, 100);

const express = require('express');
const { AsyncLocalStorage } = require('async_hooks');
const { v4: uuidv4 } = require('uuid'); // 需要 npm install uuid

const app = express();
const asyncLocalStorage = new AsyncLocalStorage();

// 一个简单的日志函数，它会从AsyncLocalStorage中获取请求ID
function logWithRequestId(message) {
    const store = asyncLocalStorage.getStore();
    const requestId = store ? store.requestId : 'N/A';
    console.log(`[Request ID: ${requestId}] ${message}`);
}

// Express中间件：为每个请求设置异步上下文
app.use((req, res, next) => {
    const requestId = uuidv4(); // 为每个请求生成一个唯一的ID
    // 在AsyncLocalStorage的run方法中包裹后续的所有中间件和路由处理
    asyncLocalStorage.run({ requestId }, () => {
        // 也可以将requestId挂载到req对象上，方便直接访问，但AsyncLocalStorage是更通用的方式
        req.requestId = requestId;
        logWithRequestId(`Incoming request: ${req.method} ${req.url}`);
        // 继续处理下一个中间件或路由
        next();
    });
});

// 示例路由
app.get('/', async (req, res) => {
    logWithRequestId('处理根路径请求。');
    await new Promise(resolve => setTimeout(resolve, 100)); // 模拟异步操作
    logWithRequestId('根路径请求处理完成。');
    res.send(`Hello from Request ID: ${req.requestId}`);
});

app.get('/users/:id', async (req, res) => {
    logWithRequestId(`处理用户查询请求，用户ID：${req.params.id}`);
    await new Promise(resolve => setTimeout(resolve, 50)); // 模拟数据库查询
    logWithRequestId('用户查询请求处理完成。');
    res.json({ userId: req.params.id, name: `User_${req.requestId}` });
});

const PORT = 3000;
app.listen(PORT, () => {
    console.log(`Server running on http://localhost:${PORT}`);
});

// ... (Koa setup and AsyncLocalStorage initialization) ...

app.use(async (ctx, next) => {
    const requestId = uuidv4();
    await asyncLocalStorage.run({ requestId }, async () => {
        ctx.requestId = requestId; // Koa的ctx对象更适合挂载请求相关数据
        logWithRequestId(`Koa request: ${ctx.method} ${ctx.url}`);
        await next(); // 确保next()也在run的上下文中
        logWithRequestId(`Koa request finished: ${ctx.method} ${ctx.url}`);
    });
});