在原子操作中测试和减少？ [英] Testing and decrementing in an atomic operation?

问题描述

我刚刚找到了一个令人讨厌的bug，这基本上就是竞争条件。为了论证，我们假设一个非常简单的文档结构，如 {_id：'XXX'，金额：100} 。

I have just traced down an annoying bug which is basicly a race condition. For the sake of the argument lets assume a very simple document structure like { _id : 'XXX', amount : 100 }.

这些文档中有数百个这样的文档存在于一个集合中，并且由多个编写者访问，这些编写者有效地尝试将任何值减少但从不低于0.目前这是通过两个查询完成的：

Hundreds of these these documents exist inside a collection and are accessed by multiple writers which effectively try to lower the amount by any value but never below 0. Currently this is done with two queries:

• 第一个查询确定当前金额。


• 只有当金额大于要减去的金额时，才会触发第二个查询。递减当前金额。

当多个线程同时执行此操作时，这当然容易导致混乱。但有没有办法在原子操作中做到这一点？我在文档中找不到类似 test-and-set 的操作。

This is of course prone to result in a mess when multiple threads do this simultaneously. But is there a way to do this in an atomic operation? I couldn't find anything like a test-and-set operation in the docs.

如果相关：运行这些操作的代码是运行多个Node.js实例的服务器端JavaScript和 mongodb npm包。

If its relevant: The code running these operations is serverside JavaScript running with multiple Node.js instances and the mongodb npm package.

推荐答案

如果金额大于0，则将更新限定为仅更新文档：

Qualify the update to only update the doc if amount is greater than 0:

db.coll.update({_id: 'XXX', amount: {$gt: 0}}, {$inc: {amount: -1}})

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