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I can't imagine how can node.js in one single thread execute two scripts with different code simultaneously. For example I have two different scripts A and B. What will happen if almost simultaneously several clients request A and B. For PHP it is understandable, for example, will be created five threads to handle A and five threads to handle B, and for each request script executes again. But what happens in Node.js? Thank you!

mihai
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Molarro
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    See also: [this](http://stackoverflow.com/a/19823583/201952), [this](http://stackoverflow.com/a/21488067/201952), and [this](http://stackoverflow.com/a/22849860/201952). – josh3736 Apr 14 '14 at 15:26

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It uses the so called event loop, implemented by libuv

A very simple explanation would be: when a new event occurs, it will be put into a queue. Every now and then, the node process will interupt execution to process these events.

The main difference between PHP and node is that a node.js process is essentially a stand-alone web server (single threaded), while PHP is an interpreter that runs within a web server (i.e. Apache), which is responsible for creating new threads for each request.

mihai
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Node.js is very good for network applications (like web sites) because in these applications most of the work is I/O which node.js handles asynchronously.

Even if two requests arrive at the same time and Node.js only has one single thread of execution, each one of the requests (in sequence) will be handed off to the operating system for I/O (via libuv as mihai pointed out) and the fact that there is only one JavaScript thread of execution becomes irrelevant. As the I/O completes, the JavaScript thread picks up the result and returns a response.

Hector Correa
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