这篇文章给大家分享的是有关JS如何实现请求调度器的内容。小编觉得挺实用的，因此分享给大家做个参考，一起跟随小编过来看看吧。
为了获取一批互不依赖的资源，通常从性能考虑可以用 Promise.all(arrayOfPromises)来并发执行。比如我们已有 100 个应用的 id，需求是聚合所有应用的 PV，我们通常会这么写：
const ids = [1001, 1002, 1003, 1004, 1005];const urlPrefix = 'http://opensearch.example.com/api/apps';// fetch 函数发送 HTTP 请求，返回 Promiseconst appPromises = ids.map(id => `${urlPrefix}/${id}`).map(fetch);Promise.all(appPromises) // 通过 reduce 做累加 .then(apps => apps.reduce((initial, current) => initial + current.pv, 0)) .catch((error) => console.log(error));上面的代码在应用个数不多的情况下，可以运行正常。当应用个数达到成千上万时，对支持并发数不是很好的系统，你的「压测」会把第三放服务器搞挂，暂时无法响应请求：
<html><head><title>502 Bad Gateway</title></head><body bgcolor="white"><center><h2>502 Bad Gateway</h2></center><hr><center>nginx/1.10.1</center></body></html>如何解决呢？
一个很自然的想法是，既然不支持这么多的并发请求，那就分割成几大块，每块为一个 chunk，chunk 内部的请求依然并发，但块的大小（chunkSize）限制在系统支持的最大并发数以内。前一个 chunk 结束后一个 chunk 才能继续执行，也就是说 chunk 内部的请求是并发的，但 chunk 之间是串行的。思路其实很简单，写起来却有一定难度。总结起来三个操作：分块、串行、聚合
难点在如何串行执行 Promise，Promise 仅提供了并行（Promise.all）功能，并没有提供串行功能。我们从简单的三个请求开始，看如何实现，启发式解决问题（heuristic）。
// task1, task2, task3 是三个返回 Promise 的工厂函数，模拟我们的异步请求const task1 = () => new Promise((resolve) => { setTimeout(() => { resolve(1); console.log('task1 executed'); }, 1000);});const task2 = () => new Promise((resolve) => { setTimeout(() => { resolve(2); console.log('task2 executed'); }, 1000);});const task3 = () => new Promise((resolve) => { setTimeout(() => { resolve(3); console.log('task3 executed'); }, 1000);});// 聚合结果let result = 0;const resultPromise = [task1, task2, task3].reduce((current, next) => current.then((number) => { console.log('resolved with number', number); // task2, task3 的 Promise 将在这里被 resolve result += number; return next(); }), Promise.resolve(0)) // 聚合初始值 .then(function(last) { console.log('The last promise resolved with number', last); // task3 的 Promise 在这里被 resolve result += last; console.log('all executed with result', result); return Promise.resolve(result); });运行结果如图 1：
代码解析：我们想要的效果，直观展示其实是 fn1().then(() => fn2()).then(() => fn3())。上面代码能让一组 Promise 按顺序执行的关键之处就在 reduce 这个“引擎”在一步步推动 Promise 工厂函数的执行。
难点解决了，我们看看最终代码：
function fetch(url) { console.log(`Fetching ${url}`); return new Promise((resolve) => { setTimeout(() => resolve({ pv: Number(url.match(/\d+$/)) }), 2000); });}const urlPrefix = 'http://opensearch.example.com/api/apps';const aggregator = { start() { return this.fetchAppIds() .then(ids => this.fetchAppsSerially(ids, 2)) .then(apps => this.sumPv(apps)) .catch(error => console.error(error)); }, fetchAppIds() { return Promise.resolve([1001, 1002, 1003, 1004, 1005]); }, promiseFactory(ids) { return () => Promise.all(ids.map(id => `${urlPrefix}/${id}`).map(fetch)); }, fetchAppsSerially(ids, concurrency = 100) { // 分块 let chunkOfIds = ids.splice(0, concurrency); const tasks = []; while (chunkOfIds.length !== 0) { tasks.push(this.promiseFactory(chunkOfIds)); chunkOfIds = ids.splice(0, concurrency); } // 按块顺序执行 const result = []; return tasks.reduce((current, next) => current.then((chunkOfApps) => { console.info('Chunk of', chunkOfApps.length, 'concurrency requests has finished with result:', chunkOfApps, '\n\n'); result.push(...chunkOfApps); // 拍扁数组 return next(); }), Promise.resolve([])) .then((lastchunkOfApps) => { console.info('Chunk of', lastchunkOfApps.length, 'concurrency requests has finished with result:', lastchunkOfApps, '\n\n'); result.push(...lastchunkOfApps); // 再次拍扁它 console.info('All chunks has been executed with result', result); return result; }); }, sumPv(apps) { const initial = { pv: 0 }; return apps.reduce((accumulator, app) => ({ pv: accumulator.pv + app.pv }), initial); }};// 开始运行aggregator.start().then(console.log);运行结果如图 2：
抽象和复用
目的达到了，因具备通用性，下面开始抽象成一个模式以便复用。
串行
先模拟一个 http get 请求。
function httpGet(url) { const delay = Math.random() * 1000; console.info('GET', url); return new Promise((resolve) => { setTimeout(() => { resolve({ url, delay, at: Date.now() }) }, delay); })}串行执行一批请求。
const ids = [1, 2, 3, 4, 5, 6, 7];// 批量请求函数，注意是 delay 执行的『函数』对了，否则会立即将请求发送出去，达不到串行的目的const httpGetters = ids.map(id => () => httpGet(`https://jsonplaceholder.typicode.com/posts/${id}`));// 串行执行之const tasks = await httpGetters.reduce((acc, cur) => { return acc.then(cur); // 简写，等价于 // return acc.then(() => cur());}, Promise.resolve());tasks.then(() => { console.log('done');});注意观察控制台输出，应该串行输出以下内容：
GET https://jsonplaceholder.typicode.com/posts/1GET https://jsonplaceholder.typicode.com/posts/2GET https://jsonplaceholder.typicode.com/posts/3GET https://jsonplaceholder.typicode.com/posts/4GET https://jsonplaceholder.typicode.com/posts/5GET https://jsonplaceholder.typicode.com/posts/6GET https://jsonplaceholder.typicode.com/posts/7分段串行，段中并行
重点来了。本文的请求调度器实现
function schedulePromises(factories, concurrency) { const chunk = (arr, size = 1) => { return arr.reduce((acc, cur, idx) => { const modulo = idx % size; if (modulo === 0) { acc[acc.length] = [cur]; } else { acc[acc.length - 1].push(cur); } return acc; }, []) }; const chunks = chunk(factories, concurrency); let resps = []; return chunks.reduce( (acc, cur) => { return acc .then(() => { console.log('---'); return Promise.all(cur.map(f => f())); }) .then((intermediateResponses) => { resps.push(...intermediateResponses); return resps; }) }, Promise.resolve() );}测试下，执行调度器：
// 分段串行，段中并行schedulePromises(httpGetters, 3).then((resps) => { console.log('resps:', resps);});控制台输出：
---GET https://jsonplaceholder.typicode.com/posts/1GET https://jsonplaceholder.typicode.com/posts/2GET https://jsonplaceholder.typicode.com/posts/3---GET https://jsonplaceholder.typicode.com/posts/4GET https://jsonplaceholder.typicode.com/posts/5GET https://jsonplaceholder.typicode.com/posts/6---GET https://jsonplaceholder.typicode.com/posts/7resps: [ { "url": "https://jsonplaceholder.typicode.com/posts/1", "delay": 733.010980640727, "at": 1615131322163 }, { "url": "https://jsonplaceholder.typicode.com/posts/2", "delay": 594.5056229848931, "at": 1615131322024 }, { "url": "https://jsonplaceholder.typicode.com/posts/3", "delay": 738.8230109146299, "at": 1615131322168 }, { "url": "https://jsonplaceholder.typicode.com/posts/4", "delay": 525.4604386109747, "at": 1615131322698 }, { "url": "https://jsonplaceholder.typicode.com/posts/5", "delay": 29.086379722201183, "at": 1615131322201 }, { "url": "https://jsonplaceholder.typicode.com/posts/6", "delay": 592.2345027398272, "at": 1615131322765 }, { "url": "https://jsonplaceholder.typicode.com/posts/7", "delay": 513.0684467560949, "at": 1615131323284 }]感谢各位的阅读！关于“JS如何实现请求调度器”这篇文章就分享到这里了，希望以上内容可以对大家有一定的帮助，让大家可以学到更多知识，如果觉得文章不错，可以把它分享出去让更多的人看到吧！
