How node.js works?

Node is completely event-driven. Basically the server consists of one thread processing one event after another.

A new request coming in is one kind of event. The server starts processing it and when there is a blocking IO operation, it does not wait until it completes and instead registers a callback function. The server then immediately starts to process another event (maybe another request). When the IO operation is finished, that is another kind of event, and the server will process it (i.e. continue working on the request) by executing the callback as soon as it has time.

So the server never needs to create additional threads or switch between threads, which means it has very little overhead. If you want to make full use of multiple hardware cores, you just start multiple instances of node.js

Update At the lowest level (C++ code, not Javascript), there actually are multiple threads in node.js: there is a pool of IO workers whose job it is to receive the IO interrupts and put the corresponding events into the queue to be processed by the main thread. This prevents the main thread from being interrupted.

Although Question is already explained before a long time, I'm putting my thoughts on the same.

Node.js is single threaded JavaScript runtime environment. Basically it's creator Ryan Dahl concern was that parallel processing using multiple threads is not the right way or too complicated.

if Node.js doesn't use threads how does it handle concurrent requests in parallel

Ans: It's completely wrong sentence when you say it doesn't use threads, Node.js use threads but in a smart way. It uses single thread to serve all the HTTP requests & multiple threads in thread pool(in libuv) for handling any blocking operation

Libuv: A library to handle asynchronous I/O.

What does event I/O model means?

Ans: The right term is non-blocking I/O. It almost never blocks as Node.js official site says. When any request goes to node server it never queues the request. It take request and start executing if it's blocking operation then it's been sent to working threads area and registered a callback for the same as soon as code execution get finished, it trigger the same callback and goes to event queue and processed by event loop again after that create response and send to the respective client.

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