If you follow the world of Javascript and/or high-performance networking, you have probably heard of node.js. If you already grok Node, then this post is not for you; move along. If, however, you are a bit confused as to exactly what Node.js is and how it works, then you should read on.

The node.js website doesn’t mince words in describing the software: “Evented I/O for V8 JavaScript.” While that statement is precise and captures the essence of node.js succinctly, at first glance it did not tell me much about node.js. I did what anyone interested in node.js should do: downloaded the source and started playing around with it.

So what exactly is node.js? Well, first and foremost it is a Javascript runtime. Think of your web browser; how does it run Javascript? It implements a Javascript runtime and supports APIs that make sense in the browser such as DOM manipulation etc. Javascript as a language itself is fairly browser agnostic. So node.js is yet another runtime for Javascript, implemented primarily in C++.

Because node.js focuses on networking, it does not support the standard APIs available in a browser. Instead, it provides a different set of APIs (with fantastic documentation). Thus, for instance, HTTP support is built into node.js — it is not an external library.

The other salient feature of node.js is that it is event driven. If you are familiar with event driven programming (ala Python Twisted, Ruby’s Event Machine, the event loop in Qt etc), you know what I’m talking about. The key difference though is that unlike all these systems, you never explicitly invoke a blocking call to start the event loop — node.js automatically enters the event loop as soon as it has finished loaded the program. A corollary is that you can only write event driven programs in node.js, no other programming models are supported. Another consequence of this design choice is that node.js is single-threaded. To exploit CPU parallelism, you need to run multiple node.js instances. Of course, there are several node.js modules and projects already available to address this very issue.

To implement a runtime for Javascript, node.js first needs to parse the input Javascript. node.js leverages Google’s V8 Javascript engine to do this. V8 takes care of interpreting the Javascript so node.js need not worry about syntactical issues; it only need to implement the appropriate hooks and callbacks for V8.

node.js claims to be extremely memory efficient and scalable. This is possible because node.js does not expose any blocking APIs. As a result, the program is completely callback driven. Of course, any kind of I/O (disk or network) will eventually block. node.js does all blocking I/O in an internal thread pool — thus even though the application executes in a single thread, internally there are multiple threads that node.js manages.

Overall, node.js is very refreshing. The community seems great and there is a lot of buzz around the project right now, with some big companies like Yahoo starting to use experiment with node.js. node.js is also driving the “server side Javascript” movement. For instance, Joyent’s Smart platform allows you to write your server code in Javascript, which they can then execute on their hosted platforms.

Finally, no blog post about node.js is complete without an example of node.js code. Here is a simple web server: