Node.JS Processing Model

Node.js processes user requests differently when compared to a traditional web server model. Node.js runs in a single process and the application code runs in a single thread and thereby needs less resources than other platforms. All the user requests to your web application will be handled by a single thread and all the I/O work or long running job is performed asynchronously for a particular request. So, this single thread doesn't have to wait for the request to complete and is free to handle the next request. When asynchronous I/O work completes then it processes the request further and sends the response.
An event loop is constantly watching for the events to be raised for an asynchronous job and executing callback function when the job completes. Internally, Node.js uses libev for the event loop which in turn uses internal C++ thread pool to provide asynchronous I/O.

This is how a single thread can handle multiple requests at once; receiving a request and either serving static/simple content or delegating it to an I/O thread from a thread pool are both very cheap and quick operations. When the thread pool thread that is doing the long-running I/O work signals to the single listener thread that the work is done, the listener thread picks up the response and sends it back to the user; this is another very cheap operation. The core idea is that the single listener thread never blocks: it only does fast, cheap processing or delegation of requests to other threads and the serving of responses to clients.

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