node.js - express.router()的路由是同步的吗？

Question

这一直让我困惑...下面的测试显示请求是异步接收的，异步响应的，但所有请求都是首先接收的，然后响应...给定以下 3 个文件:

package.json

{
    "name": "express-router-sync-test",
    "version": "1.0.0",
    "description": "Testing if express router has sync aspect",
    "scripts": {
        "start": "node server.js"
    },
    "dependencies": {
        "body-parser": "1.16.0",
        "express": "4.14.1",
        "request": "2.79.0"
    }
}

服务器.js

const express = require('express');
const bodyParser = require('body-parser');
const router = express.Router();
const request = require('request');

// Create our Express application
let app = express();

let port = 1111;

// body parser
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));

app.use(router);

// Start the server
app.listen(port);
console.log('Server listening on port ' + port + '.');


router.get('/', _get);
router.post('/test', _test);

function _test(req, res) {
    console.log('starting to process request # ', req.body.requestNumber);
    request({
        method: 'GET',
        url: 'http://localhost:1111'
    }, (err, response, body) => {
        console.log('process # ' +  req.body.requestNumber + ' has ended');
    });
}

function _get(req, res) {
    res.json({ success: true });
}

测试.js

const request = require('request');

let i;
let len = 500;
let options = {
    method: 'POST',
    url: 'http://localhost:1111/test'
}

for (i = 0; i < len; i++) {
    options.form = { requestNumber: i + 1 };
    request(options, (err, response, body) => {
        if (err) console.log(err);
        if (response) console.log(body)
    }); 
}

给定目录结构:

app/
--package.json
--server.js
--test.js

采取以下步骤重现测试:

• 在终端中，导航到应用程序目录并运行 npm install
• 运行node server.js
• 打开另一个终端，导航到应用程序目录并运行 node test.js

如果您查看 server.js 终端的输出，您会注意到所有“启动进程”日志记录都分组在一起，但以异步顺序排列。您还会注意到，所有“进程结束”日志记录都分组在一起，但也以异步顺序排列。

我现在问两个问题:

• 为什么响应会延迟到收到所有请求之后？
• 发生这种情况是因为我收到了大量请求吗？

任何信息将不胜感激。

编辑:这是此测试运行的示例日志，其中有 50 个请求而不是 500 个。您会注意到，在收到所有请求之前，不会响应任何请求。您发送的请求越多，等待第一个响应的时间就越长。

starting to process request #  1
starting to process request #  2
starting to process request #  3
starting to process request #  4
starting to process request #  5
starting to process request #  6
starting to process request #  9
starting to process request #  8
starting to process request #  7
starting to process request #  10
starting to process request #  12
starting to process request #  11
starting to process request #  13
starting to process request #  17
starting to process request #  16
starting to process request #  15
starting to process request #  14
starting to process request #  21
starting to process request #  19
starting to process request #  20
starting to process request #  18
starting to process request #  22
starting to process request #  23
starting to process request #  25
starting to process request #  24
starting to process request #  27
starting to process request #  28
starting to process request #  26
starting to process request #  32
starting to process request #  31
starting to process request #  30
starting to process request #  29
starting to process request #  36
starting to process request #  35
starting to process request #  33
starting to process request #  34
starting to process request #  40
starting to process request #  38
starting to process request #  39
starting to process request #  37
starting to process request #  44
starting to process request #  42
starting to process request #  43
starting to process request #  41
starting to process request #  45
starting to process request #  46
starting to process request #  47
starting to process request #  49
starting to process request #  48
starting to process request #  50
process # 1 has ended
process # 2 has ended
process # 4 has ended
process # 3 has ended
process # 5 has ended
process # 6 has ended
process # 9 has ended
process # 8 has ended
process # 11 has ended
process # 12 has ended
process # 10 has ended
process # 7 has ended
process # 13 has ended
process # 17 has ended
process # 15 has ended
process # 16 has ended
process # 14 has ended
process # 21 has ended
process # 22 has ended
process # 18 has ended
process # 20 has ended
process # 19 has ended
process # 27 has ended
process # 24 has ended
process # 25 has ended
process # 23 has ended
process # 31 has ended
process # 32 has ended
process # 26 has ended
process # 28 has ended
process # 30 has ended
process # 29 has ended
process # 34 has ended
process # 35 has ended
process # 33 has ended
process # 36 has ended
process # 40 has ended
process # 38 has ended
process # 39 has ended
process # 37 has ended
process # 44 has ended
process # 42 has ended
process # 46 has ended
process # 45 has ended
process # 41 has ended
process # 43 has ended
process # 47 has ended
process # 50 has ended
process # 48 has ended
process # 49 has ended

Answer 1

This following test shows that requests are being received asynchronously, responded to asynchronously, yet all requests are first received, then responded to...

尚不清楚这里有什么惊喜。首先接收请求然后响应？在收到请求之前对其进行响应没有多大意义。

由于这是一个非常普遍的问题，那么也许我会添加一些一般信息。首先，异步并不一定意味着无序，如果这是您的假设的话。操作可以按顺序或无序异步处理，这与所使用的并发模型无关。

现在，一般来说，每个返回值的函数(除非该值是一个 promise )都必须同步工作，只是因为它不能返回任何它没有的东西。从这个意义上说，promise 本身实际上是同步创建和返回的。异步完成的是 promise 的解决。

每个接受回调的函数通常都是异步工作的，但并非必须如此。回调可以同步调用，有时也可以同步调用，就像数组和字符串的 .map() 方法的回调一样。每个返回 Promise 的函数都应该异步工作，但也可以在返回 Promise 之前同步解析 Promise。

重点是，同步和异步与执行顺序有很大不同，它还取决于您是否使用 promise 或连续传递风格。