I am a big fan of Ruby on Rails: it does a lot of things, and it does most of them pretty well. When starting a new web project, it's the first thing I would reach for: most of the time, your problem is going to be figuring out a good product/market fit, or whipping up some internal tool without wasting a lot of programmer time. Once you've got a firm grasp of the problem, then maybe you can consider optimizing. Who cares if you do something that no one wants really really fast?
However, Ruby on Rails is not beautiful in terms of being particularly fast or lightweight. No complaints from me: most of the time, I'm happy to have something that does so much for me, leaving me to work on the actual problem at hand. Once in a while, though, you do need fast and relatively lightweight, and that space has been getting more interesting over the past few years, at least in terms of the web.
First of all, technologies like "Comet", utilizing web sockets or some other always-on connection are becoming more common, where a socket with the server remains open in order to quickly exchange data from the server to the client – and back. That seems to be a poor fit for something like Rails, where it can tie up a lot of resources if one isn't careful. And while computing costs continue to decline, no one minds getting more for less in terms of what their server can do. Furthermore, with frameworks like Backbone.js, pushing more and more code to the client, the server can afford to be a bit simpler and do less, so it may as well be snappy to boot.
Java has long been fairly popular for "heavy lifting" types of applications, partially because it does end up being reasonably fast. But it's not something I've ever had much fun using and is usually kind of wordy, and makes you feel like you need a crew of people in Palo Alto, one in Bangalore, and one in Stockholm just to churn out all the code. And it certainly is no lightweight in terms of memory either. So… while it can certainly do pretty much anything you need, I don't see it as being the strongest player when someone needs "real time web" code, and needs it to be reasonably light weight.
Ruby, outside of standard Rails stuff like Passenger, seems to offer some interesting possibilities for this kind of work, like Reel but they don't seem to have the traction other solutions do. Python is in a similar situation: the Twisted framework has been around for a while, and while it has some success stories, never seems to have really 'caught fire'. Neither of these languages was built for "concurrency" from the ground up, and that seems to have, to date, inhibited people from using them extensively for this kind of job.
My gut feeling is that the need for speed and "concurrency" (or at least handling a lot of concurrent users) will drive adoption of languages heretofore not so popular on the server. Let's have a look at them.
Erlang: This is by no means a new language, having been developed in Ericsson in the late 1980ies; and it implements several interesting concepts. First and foremost, concurrency is handled in the form of many small "processes", which are not actually Unix processes at all, but processes internal to the Erlang VM. The Erlang system contains a scheduler that allocates resources to all of these processes, so even if one of them dies or behaves badly, it's not a problem: the system as a whole can continue to function well. The way Erlang is built, the scheduler is preemptive: the internal "processes" don't need to yield to let other processes run. Beyond the scheduler and simple processes, Erlang gives you the tools to create elaborate trees of supervisors and workers that are quite robust to failures in any one portion of the system, as well as giving you the tools to set up a system to run on multiple, distributed computers. This kind of thinking is necessary when you write applications, such as phone switches, where downtime is really, really not ok. Erlang processes comunicate almost exclusively via message passing, meaning that state is not shared. Altogether, this makes for a fast, rock solid system that can easily handle thousands of concurrent connections without breaking a sweat.
The computing world being what it is though, Erlang is likely to be more of a Lisp or Smalltalk: it's a trailblazer that did many things years before their time in other languages, but I don't see it as ever quite catching on amongst 'the masses'. It has a wonky syntax, it's a functional programming language, and because it is used in environments where too much experimentation is not good, it does not have a lot of room to break with its own past and innovate in terms of the language itself: it's slow to change and improve, even where the need to is clearly perceived.
Go language: this is an interesting one, written by some luminaries who work at Google. It can best be described as something akin to C with some updated features, such as garbage collection, that make it more suitable for working on large projects, where things like memory leaks are going to make life very frustrating. It has the feel of a "real language, for real programmers" in that it doesn't stray far too far from what people are used to – it's not going to cause the "what the hell is this?!" reactions that Erlang might in more close-minded circles. Between the big company backing it, and the approachable syntax and concepts, Go looks like it has a good shot at the mainstream. Where it gets interesting is their concurrency model, which is apparently based on something called "communicating sequential processes" which, superficially at least, looks like it has some things in common with Erlang's "actor model" of concurrency.
Under the hood, Go apparently hives off its "goroutines" to different OS level threads, but does not have a preemptive scheduler like Erlang: http://code.google.com/p/go/issues/detail?id=543 – although according to this, they may change that in the future.
I'm not enough of a computer science guy to comment much on the details of CSP vs Actors, but both seem like valid models with strengths compared to trying to keep threads straight, which always seems to be a source of problems for programmers.
So, what's actually going to happen? I see Node.js as the clear front runner. It takes a worse-is-better approach that seems to work well enough as people get started. If they encounter difficulties later, they can always rewrite in something else, if needs be, but by "luring people in", Node.js has gathered a large group of users who continue, in turn, to churn out more code for use with the system, making it more attractive to new users.
Programming languages are not winner-take-all markets though, so perhaps there is room for a few more languages to have decent followings in this space. Hopefully the competition will lead to ever better tools for those of us utilizing them!
What do you think?