Event Loop in NodeJS: How It Works!

We must have come across the terms ‘Callbacks’, ‘Non-blocking I/O’ and ‘Event Looping’ while learning or understanding Node-JS.

Here is an attempt to explain!

What is Event Looping? 

At start, we will go through concepts of Blocking and Non-Blocking executions, then jumping on to the ‘Event Loops by Reactor Pattern’!

Consider examples like reading a file into the application, or an http request to a third party api’s. There are two ways we can perform these I/O operations:

  1. Blocking I/O: Where, application will make a function call and pause its execution at this point until the data is received.
  2. Non-Blocking I/O: Where, application will make a function call, and, without waiting for the results, continue its execution.

Former is called as ‘Synchronous’, while later is called as Asynchronous’ nature of execution, and NodeJS as you know, is Asynchronous in nature.

So, despite being single-threaded framework, How NodeJS works asynchronously?Answer is, Reactor Pattern – which is at the heart of NodeJS.

But, what is Event Looping?

“It is a magical place filled with unicorns and rainbows.“

To explain this, we will compare two patterns of Event-Driven programming:

  1. Busy Waiting:

In which, continuous checks on resources is done to see if data is ready for further processing, – called as Active Polling


Though, it is the simple way to implement asynchronous, event-driven behaviour, it’s not efficient. As this process will keep running continuously, it will use the CPU extensively.

     2. Reactor Pattern:

Reactor Pattern is made of following:

  • Resources:

That are shared by multiple applications for I/O operations, generally slower in executions.

  • Synchronous Event De-multiplexer / Event Notifier:

This uses Event Loop for blocking on all resources. When a set of I/O operations completes, the Event De-multiplexer pushes the new events into the Event Queue.

  • Event Loop and Event Queue:

Event Queue queues up the new events occurred along with its event-handler, <event, event-handler> pair.

  • Request Handler / Application:

This is, generally, the application that provides the handler to be executed for registered events on resources.

See below diagram for understanding of Reactor Pattern and the place for Event Loop:


This is what happens in an application using the reactor pattern:

  1. The application generates a new I/O operation by submitting a request to
    the Event De-multiplexer. The application also specifies a handler, which will
    be invoked when the operation completes. Submitting a new request to the
    Event De-multiplexer is a non-blocking call and it immediately returns the
    control back to the application.
  2. When a set of I/O operations completes, the Event De-multiplexer pushes the
    new events into the Event Queue.
  3. At this point, the Event Loop iterates over the items of the Event Queue.
  4. For each event, the associated handler is invoked.
  5. The handler, which is part of the application code, will give back the
    control to the Event Loop when its execution completes (5a). However,
    new asynchronous operations might be requested during the execution
    of the handler (5b), causing new operations to be inserted in the Event
    De-multiplexer (1), before the control is given back to the Event Loop.
  6. When all the items in the Event Queue are processed, the loop will block
    again on the Event De-multiplexer which will then trigger another cycle.

The asynchronous behavior is now clear: the application expresses the interest to
access a resource at one point in time (without blocking) and provides a handler,
which will then be invoked at another point in time when the operation completes.

This is by far the basics behind asynchronous behavior of Node-JS.


Source: the-reactor-pattern

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