#NODEJS EVENT LOOP UPDATE#
#NODEJS EVENT LOOP CODE#
#NODEJS EVENT LOOP WINDOWS#

Non-preemptive (run-to-completion) Īnother important thing about JavaScript concurrency is that it is non-preemptive.

#NODEJS EVENT LOOP CODE#

Note that, although the JavaScript code runs in a single thread, the engine might use several threads for performing asynchronous operations like retrieving data from a server.

The part of the engine responsible for determining what code to run when is the Event Loop, which in essence constantly checks if there is something ready to be executed and also determines what to execute first in case there are multiple candidates. But while the engine was waiting for the data, other pieces of JavaScript that are triggered by other events can already be run. When the engine has retrieved the data, it can then execute the program starting from the function it received. The task in question could be something like retrieving data from a URL. When making asynchronous calls, JavaScript code tells the JavaScript engine something like 'ok, I'm done for now, but please perform this task for me and pass the resulting data to this function I'm giving you'. JavaScript is able to accomplish this using a combination of asynchronous calls and the Event Loop.

#NODEJS EVENT LOOP UPDATE#

Example: while a part of the JavaScript code is getting some data from the backend to update the page with, another part of the code might be creating some visual effects whenever the user hovers over a button.This does not prevent JavaScript from making it seem like multiple things are going on at the same time!.This means that, at any point in time, at most one code path is executing.LibUv performs, maintains and manages all the io and events in the event pool.A high-level overview of how JavaScript handles concurrency using its Event Loop JavaScript and concurrency Single-threaded Ī JavaScript program typically executes inside a single thread (let's ignore some less common cases where code spawns child processes, Web Workers, Worker Threads or similar)

#NODEJS EVENT LOOP WINDOWS#

It handles the file descriptors, data handler, sockets etc.ģ)LibUv :- is an abstraction layer on the top of libeio, libev, c-ares ( for DNS ) and iocp (for windows asyncronous-io). Implicitly EventLoop in code level is done by:-ġ)LibEv :- is the event loop which actually runs internally in node.js to perform simple event loop operations.Ģ)LibEio :- is a library to perform input-output asynchronously. In this way, nodejs handle multiple concurrent request simultaneously. So Event loop is only responsible for shifting or pushing the task from task queue to call stack. If call stack is empty then it pushes the task from the task queue to call stack (This is how it works) Now here Event loop is checking that weather the call stack is empty or not. In the above snapshot as you see It works as follows:-įirst of all, Hi comes in the call stack and console the string.Īfter that setTimeout comes in the callStack and because its related to web-app its response takes time so it will go to Web-API and after 5 seconds its come into task-queue.

To understand this in a simple way then you can understand like there are major 4 components: So main question arises in mind is how nodejs handle multiple requests concurrently.

Like we all know most of the web application frameworks uses Multithreading models like Java, Spring Mvc.NET, Ajax etc.

But how nodejs can handle multiple requests if it uses single thread model. You have always listened that nodejs uses Single Threaded Event Loop Model to handle multiple requests.

Nodejs event loop