For the past N months, it seems like there is no new technology stack that is either hotter or more controversial than node.js. node.js is cancer! node.js cures cancer! node.js is bad ass rock star tech!. I myself have given node.js a lot of shit, often involving the phrase “explicit continuation-passing style.”

Most of the arguments I’ve seen seem to center around whether node.js is “scalable” or high-performance, and the relative merits of single-threaded event loops versus threading for scaling out, or other such noise. Or how to best write a Fibonacci server in node.js (wat?).

I am going to completely ignore all of that (and I think you should, too!), and argue that node.js is in fact on to something really cool, and is worth using and thinking about, but for a reason that has absolutely nothing to do with scalability or performance.

The Problem 🔗︎

node.js is cool because it solves a problem shared by virtually every mainstream language. That problem is the fact that, as long as “ordinary” blocking code is the default, it is difficult and unnatural to write networked code in a way that it can be combined with other network code, while allowing them to interact.

In most languages/environments – virtually every other language people use today – when you write networked code, you can either make it fully blocking itself, and implement your own main loop – which is almost always easiest – or you can pick your favorite event loop library (you probably have half a dozen choices), and write your code around that. If you do the latter, not only will your code likely be more awkward than if you chose the blocking main loop approach, but your only reward for the effort is that your code is combinable with the small fraction of other code that also chose the same event loop library.

The upshot of this situation is that if you pick a couple of random networked libraries written by different people – let’s say, an HTTP server, a Twitter client, and an IRC client, for example – and want to combine them – maybe you want a Twitter <-> IRC bridge with a web-based admin panel – you will end up at best having to write some awkward glue code, and at worst doing something truly hackish in order to make them communicate at all.

(In case you’re not convinced about the existence or scope of this problem, you can detour ahead to the optional example of how this problem manifests with typical Python libraries)

node.js solves this problem, somewhat paradoxically by reducing the number of options available to developers. By having a built-in event loop, and by making that event loop the default way to accomplish virtually anything at all (e.g. all of the builtin IO functions work asynchronously on the same event loop), node.js provides a very strong pressure to write networked code in a certain way. The upshot of this pressure is that, since essentially every node.js library works this way, you can pick and choose arbitrary node.js libraries and combine them in the same program, without even having to think about the fact that you’re doing so.

node.js is cool, then, not for any performance or scalability reasons, but because it makes composable networked components the default.

Concluding Notes 🔗︎

An interesting point here is that there is not really any fundamental differences between what you can do in Python and node.js. The Twisted project, for example, is basically an attempt to implement the node.js ideology in Python (yes, I’m being terribly anachronistic describing it that way). Twisted, however, has a fairly steep learning curve, and “feels” unnatural to developers used to writing “normal” Python, and so relatively few libraries get written for the Twisted environment, compared to the rest of the Python ecosystem. Twisted suffers from the fact that the Python language and Python community are not set up to make Twisted the default way to do things.

The key is that node.js makes it, both technically and socially, easier to write code in this composable way than not to do so. The built-in event loop and nonblocking primitives make it technically easy, and the social culture that has grown up around it discourages libraries that don’t work this way, so libraries that attempt to just block for IO are looked down on and are unlikely to thrive and gain adoption and development resources.

I’m also not ignoring the downside of the node.js style – the potentially convoluted callback-based style, the risk of bringing the whole world to a halt with an accidental blocking call, the single-threaded model that makes it hard to effectively exploit multiple cores. node.js definitely makes you do more work than you might otherwise have to, in many circumstances. But the key is, in exchange for this work, you get something really cool – and something much more valuable, in my opinion, than nebulous performance gains.

I also don’t want to claim that node.js is the only system, or the only technical approach, that makes this property possible. But node.js is the most successful such system I know of, and that’s worth at least as much as the technical possibility – what’s the use of being able to combine third-party libraries, if no one has written anything worth using in the first place?

And similarly, while the single-threaded callback model may not be the best possible model, it seems to have hit some sweet spot for finding a sweet spot in terms of what developers are willing to put up with. Certainly, people are writing node.js code like mad – check out the npm registry for a partial list.

So, node.js is not magic, and it definitely doesn’t cure cancer. But there is something here worth looking at. The next time you need to glue some unrelated networked services together, give node.js a shot – I think you’ll like it. And if you’re still not convinced, glue a quick HTTP frontend onto whatever you’ve created. I promise you’ll be shocked by how easy it is.

Postscript - A Python Example

As an optional addendum, here’s a step-by-step discussion of how just bad the situation is in most other languages.

Let’s imagine we want to write a trivial Jabber -> IRC bridge: A bot that lurks in an IRC channel, and signs on to Jabber, and sends all the messages it receives on Jabber into the IRC channel. This is the kind of simple problem that can be described in one sentence, and sounds like it should take all of 20 lines of code, but actually turns out to be rather a nuisance.

Python has, by this point, a great library ecosystem, so we happily start googling, and find the plausible looking python-irclib and xmpppy libraries. Great. So, what does code in each of those look like? Well, in python-irclib, we construct a subclass of SingleServerIRCBot , and call the start function, which runs the IRC main loop:

bot = MyBot(channel, nickname, server, port) bot.start()

And in xmpppy, we construct an xmpp.Client object, and call Client.Process in a loop, with a timeout:

conn = xmpp.Client(server) # connect to the server while True: conn.Process(1)

Ok, so, we launch a thread for each one, and a few minutes of fumbling later, we’re connected to both Jabber and IRC. So far, so good. We’re using Python’s threads, which will inevitably bring us a world of pain, but I’ll ignore that for now, since a better threading implementation could fix most of the pain.

But now, what do we do when we receive a Jabber message? We want to send a message out the python-irclib instance, but how do we do that? python-irclib isn’t thread safe, so we can’t just call .send() from the Jabber thread. Ok, so we add in a Queue.Queue , and have the Jabber thread push messages onto it.

Now we just need to make the IRC thread fetch messages from this queue. But how do we do that? The IRC thread is blocked somewhere deep inside python-irclib , waiting for network traffic. How do we wake it up to read messages from the queue? The easiest way is to switch from calling start to calling process_once in a loop with a short timeout.

This will work, and we’ll eventually get something working, but now we’re forced into polling, with all the annoying latency/CPU tradeoffs that entails, and also half of our code so far has just been spent gluing these two libraries together.

In node.js, on the other hand, we’d just instantiate client objects for both protocols, set up some event handlers, and … well, that’s about it. Because everything’s hooked into the same main loop, they’ll Just Work together, and because we’re all running single-threaded, we can mostly just communicate directly between the two libraries without having to think too hard about race conditions or anything.

The point, of course, is not that this is impossible to write this program in Python. It is, and I’ve done it, and it’s not that terrible. But any option you take will involve some annoying tradeoffs, and will involve making lots of irrelevant plumbing decisions about how to make your pieces play well together. And compared to all that, node.js feels like a breeze.