What does it mean by " asynchronous I/O primitives" in nodejs?

Question

I was going through Node.js documentation, and couldn't understand the line :

A Node.js app runs in a single process, without creating a new thread for every request. Node.js provides a set of asynchronous I/O primitives in its standard library that prevent JavaScript code from blocking and generally, libraries in Node.js are written using non-blocking paradigms, making blocking behavior the exception rather than the norm.

Source : Introduction to node js

I couldn't understand specifically:

[...] Node.js provides a set of asynchronous I/O primitives in its standard library that prevent JavaScript code from blocking [..]

Does it simply means, it has built in functionality that provides the provision to work as asynchronous? If not then what are these set of asynchronous I/O primitives? If anyone could provide me some link for better understanding or getting started with Node.js, that would be really great.

P.S : I have practical experience with Nodejs where I understand how it's code will work but don't understand why it will work, so I want understand its theoretical part, so I can understand what actually is going on in the background.

Answer 1

Does it simply means, it has built in functionality that provides the provision to work as asynchronous?

Yes, that's basically what it means.

In a "traditional" one-thread-per-connection^* model you accept a connection and then hand off the handling of that request to a thread (either a freshly started one or from a pool, doesn't really change much) and do all work related to that connection on that one thread, including sending the response.

This can easily done with synchronous/blocking I/O: have a read method that simply returns the read bytes and a write method that blocks until the writing is done.

This does, however mean that the thread handling that request can not do anything else and also that you need many threads to be able to handle many concurrent connections/requests. And since I/O operations take a relatively huge time (when measured in the speed of memory access and computation), that means that most of these threads will be waiting for one I/O operation or another for most of the time.

Having asynchronous I/O and an event-based core architecture means that when a I/O operation is initiated the CPU can immediately go on to processing whatever action needs to be done next, which will likely be related to an entirely different request.

Therefore one can handle many more requests on a single thread.

The "primitives" just means that basic I/O operations such as "read bytes" and "write bytes" to/from network connections or files are provided as asynchronous operations and higher-level operations need to be built on top of those (again in an asynchronous way, to keep the benefits).

As a side node: many other programming environments have either had asynchronous I/O APIs for a long time or have gotten them in recent years. The one thing that sets Node.js apart is that it's the default option: If you're reading from a socket or a file, then doing it asynchronously is what is "normal" and blocking calls are the big exception. This means that the entire ecosystem surrounding Node.js (i.e. almost all third-party libraries) works with that assumption in mind and is also written in that same manner.

So while Java, for example, has asynchronous I/O you lose that advantage as soon as you use any I/O related libraries that only support blocking I/O.

^{* I use connection/request interchangeably in this answer, under the assumption that each connection contains a single request. That assumption is usually wrong these days (most common protocols allow multiple request/response pairs in a single connnection), but handling multiple requests on a single connection doesn't fundamentally change anything about this answer.}