一次只允许一个异步操作 [英] Allowing only one async operation at a time

问题描述

我正在构建一个站点，一个特定的操作会触发一个长的服务器端进程来运行。此操作不能同时运行两次，因此我需要实现某种保护。它也不能同步，因为服务器在运行时需要继续响应其他请求。

I'm building a site and one particular operation triggers a long, server-side process to run. This operation can't be run twice at the same time, so I need to implement some sort of protection. It also can't be made synchronous, because the server needs to continue responding to other requests while it runs.

为此，我构建了这个小概念测试，使用 sleep 5 代替我实际的长期运行流程（需要 express 和 child-process-promise ，在具有 sleep 命令的系统上运行，但替换为Windows的任何内容）：

To that end I've constructed this small concept test, using sleep 5 as a substitute for my actual long-running process (requires express and child-process-promise, runs on a system with a sleep command but substitute whatever for Windows):

var site = require("express")();
var exec = require("child-process-promise").exec;

var busy = false;

site.get("/test", function (req, res) {
    if (busy) {
        res.json({status:"busy"});
    } else {
        busy = true; // <-- set busy before we start
        exec("sleep 5").then(function () {
            res.json({status:"ok"});    
        }).catch(function (err) {
            res.json({status:err.message});
        }).then(function () {
            busy = false; // <-- finally: clear busy
        });
    }
});

site.listen(8082);

这意味着当请求/ test时它会触发一个长操作，如果 / test在运行时再次被请求，它以忙回复并且什么都不做。

The intention of this is when "/test" is requested it triggers a long operation, and if "/test" is requested again while it is running, it replies with "busy" and does nothing.

我的问题是，这个实现安全且正确吗？它似乎在我的粗略测试中起作用，但它很可疑。这是基本上实现互斥+ +try-lock操作的正确方法，还是有一些更合适的Node.js构造？从习惯于标准多线程实践的语言开始，我对单线程但异步的特性感到非常满意。

My question is, is this implementation safe and correct? It appears to work in my cursory tests but it's suspiciously simple. Is this the proper way to essentially implement a mutex + a "try-lock" operation, or is there some more appropriate Node.js construct? Coming from languages where I'm used to standard multithreading practices, I'm not quite comfortable with Node's single-threaded-but-asynchronous nature yet.

推荐答案

你很好 - Javascript代码无法与Node中的其他JS代码同时运行。什么都不会改变你下面的忙碌旗帜。无需多线程样式的监视器或关键部分。

You're fine - Javascript code can't run concurrently with other JS code in Node. Nothing will change the busy flag out from under you. No need for multithreaded-styled monitors or critical sections.

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