The DJabberd module is event-based so we can have really low per-connection memory requirements, smaller than is possible with a threaded jabber server.DJabberd was the answer to LiveJournal's Jabber (XMPP) server needs. They needed:
* good performance for tons of connected users
* ability to scale to multiple nodes
* ability to hook into LiveJournal at all places, not just auth
Basically they wanted the swiss army knife of Jabber servers (think qpsmtpd or mod_perl), but none existed in any language. While some popular Jabber servers let us do pluggable auth, none let us get our hands into roster storage, vcards, avatars, presence, etc.
So they made DJabberd. It's a Jabber server where almost everything defers to hooks to be implemented by plugins. It does the core spec itself (including SSL, StartTLS, server-to-server, etc), but it doesn't come with any way to do authentication or storage or rosters, etc. You'll need to go pick up a plugin to do those.
You should be able to plop DJabberd into your existing systems / userbase with minimal pain. Just find a plugin that's close (if a perfect match doesn't already exist!) and tweak.
The DJabberd module is event-based so we can have really low per-connection memory requirements, smaller than is possible with a threaded jabber server. Because of this, all plugins can operate asynchronously, taking as long as they want to finish their work, or to decline to the next handler. (in the meantime, while plugins wait on a response from whatever they're talking to, the DJabberd event loop continues at full speed) However, that's more work, so some plugins may choose to operate synchronously, but they do so with the understanding that those plugins will cause the whole server to get bogged down. If you're running a Jabber server for 5 users, you may not care that the SQLite authentication backend pauses your server for milliseconds at a time, but on a site with hundreds of thousands of connections, that wouldn't be acceptable. Watch out for those plugins.
$ djabberd --conf=config-file.conf --daemonize