Node.js是原生的Promise.all并行或顺序处理吗？ [英] Is Node.js native Promise.all processing in parallel or sequentially?

问题描述

我想澄清这一点，如文档对此不太清楚;

I would like to clarify this point, as the documentation is not too clear about it;

Q1： Promise.all（可迭代）顺序或并行处理所有承诺？或者，更具体地说，它是否相当于运行链式承诺，如

Q1: Is Promise.all(iterable) processing all promises sequentially or in parallel? Or, more specifically, is it the equivalent of running chained promises like

p1.then(p2).then(p3).then(p4).then(p5)....

还是其他一种算法所有 p1 ， p2 ， p3 ，<$ c同时调用$ c> p4 ， p5 等（并行），并在所有解析后立即返回结果（或者一个拒绝）？

or is it some other kind of algorithm where all p1, p2, p3, p4, p5, etc. are being called at the same time (in parallel) and results are returned as soon as all resolve (or one rejects)?

Q2：如果 Promise.all 并行运行，是否有一种方便的方式来运行可迭代的顺序？

Q2: If Promise.all runs in parallel, is there a convenient way to run an iterable sequencially?

注意：我不想使用Q或Bluebird，但是所有原生的ES6规范。

Note: I don't want to use Q, or Bluebird, but all native ES6 specs.

推荐答案

Promise.all（可迭代）执行所有承诺？

不，承诺不能执行。他们在创建时开始他们的任务 - 他们只代表结果 - 并且你甚至在将它们传递给 Promise之前并行执行所有内容。全部。

No, promises cannot "be executed". They start their task when they are being created - they represent the results only - and you are executing everything in parallel even before passing them to Promise.all.

Promise.all 只做 await 多重承诺。它不关心它们解决的顺序，或者计算是否并行运行。

Promise.all does only await multiple promises. It doesn't care in what order they resolve, or whether the computations are running in parallel.

有一种方便的方法来运行可顺序迭代？

is there a convenient way to run an iterable sequencially?

如果你已经有了你的承诺，你就做不了多少但 Promise.all（ [p1，p2，p3，...]）（没有序列概念）。但是如果你有一个可迭代的异步函数，你确实可以按顺序运行它们。基本上你需要从

If you already have your promises, you can't do much but Promise.all([p1, p2, p3, …]) (which does not have a notion of sequence). But if you do have an iterable of asynchronous functions, you can indeed run them sequentially. Basically you need to get from

[fn1, fn2, fn3, …]

到

fn1().then(fn2).then(fn3).then(…)

并且做到这一点的解决方案是使用 Array :: reduce ：

and the solution to do that is using Array::reduce:

iterable.reduce((p, fn) => p.then(fn), Promise.resolve())

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