[madhouse]

What I did a while ago - which isn't the best way, it's quite crude, but works - is to have the webapp touch a file on the filesystem, which is watched with inotify (or whatever similar feature your OS has), and whenever the file is touched, the backend processor wakes up and starts crunching the new entries in the queue.

Perhaps it might even be possible to construct such a trigger in your database that would wake up an external application - I haven't looked into that possibility yet.