英文:
Write blocking javascript
问题
以下是您要翻译的内容：
我有一个小型服务器，我正在尝试从中获取一堆文件；大约100个要获取。我正在使用JS通过forEach循环来获取这些文件。不幸的是，这大约100个请求会导致服务器崩溃。
links.forEach((link) => {const name = link.split("/")[5];const file = fs.createWriteStream(name);const request = http.get(link, (res) => {res.pipe(file);file.on("finish", () => {file.close();});});});
我正在尝试编写同步阻塞的JavaScript；我希望在开始下一个获取操作之前文件写入完成。我一直在尝试使用生成器、while循环和fs.writeFileSync。我还使用setTimeout来模拟慢速网络调用（这样我就不必在每次崩溃后重新设置服务器了）。
看起来我漏掉了一些概念。这是我的一个非常天真的实验，我认为应该需要3秒，但实际只需要1秒。经过思考，很明显所有的超时都是同时发生的。
function writeSlow(path) {setTimeout(function () {console.log("write now");fs.writeFileSync(path, "lorem");}, 1000);}writeSlow("junk/one");writeSlow("junk/two");writeSlow("junk/three");
我进行了一些阅读，确信使用Promises是正确的方法，但似乎也不起作用：
function sleep(ms) {return new Promise((resolve) => setTimeout(resolve, ms));}async function run() {arr.forEach(async (str) => {await sleep(1000);fs.writeFileSync("junk/" + str + ".txt", "lorem");});}
我的期望，或者我试图达到的目标，是在实验性代码中，我可以监视文件系统，每秒钟会出现一个新文件。
（编辑）
我寻求的实际最终结果是，只有在上一个http请求完成后，才会触发下一个请求。
英文:
I have a tiny server I'm trying to fetch a bunch of files from; ~100 to fetch. I'm using JS to fetch the files using a forEach loop. Unfortunately, the ~100 requests knocks over the server.
links.forEach((link) => {const name = link.split("/")[5];const file = fs.createWriteStream(name);const request = http.get(link, (res) => {res.pipe(file);file.on("finish", () => {file.close();});});});
I'm trying to write synchronous blocking JavaScript; I want the file writing to complete before beginning the next fetching action. I've been experimenting with generators, while loops, and fs.writeFileSync. I've also been using setTimeout to emulate a slow network call (just so I don't have to reset the server after knocking it over each time).
It seems like I'm missing some concept. Here's my very naive experiment that I thought should take 3 seconds, but only takes 1. After thinking about it, it's clear all the timeouts are happening at once.
function writeSlow(path) {setTimeout(function () {console.log("write now");fs.writeFileSync(path, "lorem");}, 1000);}writeSlow("junk/one");writeSlow("junk/two");writeSlow("junk/three");
I did some reading and was convinced using Promises was the way to go, but this doesn't appear to work either:
function sleep(ms) {return new Promise((resolve) => setTimeout(resolve, ms));}async function run() {arr.forEach(async (str) => {await sleep(1000);fs.writeFileSync("junk/" + str + ".txt", "lorem");});}
My expectation, or what I'm trying to get to, with the experimental code, is the point where I can watch the filesystem, and every second, a new file appears.
(edit)
The actual end result I'm looking for is for the next http request to only fire once the last one completes.
答案1
得分: 1
以下是翻译好的部分：
你可以编写一个异步循环：function loop(links, i) {if (i >= links.length) return; // all doneconst link = links[i];const name = link.split("/")[5];const file = fs.createWriteStream(name);const request = http.get(link, (res) => {res.pipe(file);file.on("finish", () => {file.close();loop(links, i+1); // Continue with next});});}// Start the asynchronous loop:loop(links, 0);或者你可以切换到基于Promise的库，或者像这样将你的基于回调的函数promisify（未经测试）：// Promisify your callback-based functionsconst asyncWrite = (name, res) => new Promise((resolve) => {const file = fs.createWriteStream(name);res.pipe(file);file.on("finish", () => {file.close();resolve();});});const asyncHttpGet = (link) => new Promise((resolve) =>http.get(link, resolve));// ... and use them:async function writeAllLinks(links) {for (const link of links) {await asyncWrite(link.split("/")[5], await asyncHttpGet(link));}}// Start the asynchronous loop:writeAllLinks(links).then( /* .... */ );你可能想要添加错误处理...
英文:
You could write an asynchronous loop:
function loop(links, i) {if (i >= links.length) return; // all doneconst link = links[i];const name = link.split("/")[5];const file = fs.createWriteStream(name);const request = http.get(link, (res) => {res.pipe(file);file.on("finish", () => {file.close();loop(links, i+1); // Continue with next});});}// Start the asynchronous loop:loop(links, 0);
Alternatively you could switch to promise-based libraries, or else, promisfy the callback-based functions you have, like so (not tested):
// Promisify your callback-based functionsconst asyncWrite = (name, res) => new Promise((resolve) => {const file = fs.createWriteStream(name);res.pipe(file);file.on("finish", () => {file.close();resolve();});});const asyncHttpGet = (link) => new Promise((resolve) =>http.get(link, resolve));// ... and use them:async function writeAllLinks(links) {for (const link of links) {await asyncWrite(link.split("/")[5], await asyncHttpGet(link));}}// Start the asynchronous loop:writeAllLinks(links).then( /* .... */ );
You would probably want to add error handling...
答案2
得分: 0
forEach 不像你可能认为的那样在使用 async/await 时表现。查看此 StackOverflow 帖子以获取更多信息：使用 async/await 与 forEach 循环
相反，在你的情况下（如你在上面看到的），你可以使用标准的 for 循环：
function sleep(ms, fileName) {return new Promise(resolve => setTimeout(() => resolve(fileName), ms))}const files = ["file1", "file2", "file3"];async function run() {for(let i = 0; i < files.length; i++) {const fileName = await sleep(1000, files[i])console.log(fileName, new Date().toLocaleTimeString())}}run();
英文:
forEach doesn't behave like you may think it will with async/await. Take a look at this post on SO for more information:Using async/await with a forEach loop
Instead, in your case (as you will see above), you can use a standard for loop:
function sleep(ms, fileName) {return new Promise(resolve => setTimeout(() => resolve(fileName), ms))}const files = ["file1", "file2", "file3"];async function run() {for(let i = 0; i < files.length; i ++) {const fileName = await sleep(1000, files[i])console.log(fileName, new Date().toLocaleTimeString())}}run();
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