That's a great question. We get it a lot. It was asked on Hacker News in response to our birth announcement of Immutant yesterday.

So as someone who has worked in multiple enterprises, and now works for a company whose primary customers are enterprises, and since I routinely toss around the term like everyone knows what it means, and since it's one of those awful terms that means something different to everyone...

I'm compelled to answer the question, "WTF is an enterprise-grade application server?"

The answer requires answering another question first:

What's an Enterprise?

Here's my definition: it's an organization of mostly-independent teams building and maintaining applications used by other internal groups and external customers.

The key identifier of an enterprise is "a group of groups" in which more than one of them builds software.

There's usually a single "system operations" group. Their life is hell. They're ultimately responsibile for the security and integrity of the organization's data and the lifecycles of all the applications built for and used by all the other groups in the organization.

Did I mention their life is hell?

They can't afford to support all the myriad databases, message queues and web frameworks each team might decide to build their apps around. Not to mention supporting multiple languages! They prefer a single, "all in the tin" solution. Sadly, that usually means .Net or Java.

So enterprise-grade implies a capacity for supporting these types of environments. It usually involves, among other things, messaging so that the disparate apps may communicate, transactions to ensure the integrity of distributed data stores, and clustering, not only for scalability, but also to allow the lifecycles of the multiple versions of the apps to vary independently. And oh yeah, it also has to stay up, all the time.

What's an App Server?

An app server is a single product that provides all those services. It's a multi-threaded process that, once started, provides any app deployed to it with a web server, messaging, transactions, scheduling, security, caching, clustering, and more. JBoss AS7 is one such app server. It's open-source and it's fast.

Unfortunately, most popular commercial Java app servers provide those robust enterprise services at a very high price. Not only in dollars, but in the form of complex Java API's, overly-configured slow implementations, vendor-negotiated standards and general "acronym soup".

But it doesn't have to be that way. The Rails "convention over configuration" mantra inspired the creation of TorqueBox, encapsulating the enterprise-grade services provided by JBoss AS7 behind simple Ruby API's. The immediate goal for Immutant is to do the same with Clojure.

But by integrating any JVM-based language with JBoss AS7, the ultimate goal is to keep all the groups in an enterprise -- both operations and development -- happy. Or at least significantly less soul-sucky and hopefully more productive.

But why should you care?

Maybe you shouldn't. Maybe you only need a web server and a database, maybe just a JVM! Maybe you're not in a group of groups. Heck, maybe you're not even in a group!

Maybe you're perfectly content being both developer and admin for your apps and the various external processes on which they depend, or you're quite happy delegating some of those responsibilities to the fine folks at Heroku, EngineYard, or someone else.

If that's the case, you are probably a very happy person, and I'm very happy for you!

But if you feel you could benefit from a more integrated "all in the tin" solution, and especially if you're thinking along polyglot lines, I think TorqueBox (and Immutant, once it matures) is a compelling alternative, whether you work in an enterprise or not.