Back in 1998 or so, a colleague and I were tasked with building a system to target adverts to particular users on a website. It obviously needed to run very efficiently, because it would be invoked on every page load and the server hardware of the time was very underpowered by today’s standards.

The Linux server revolution was still a few years away (at least it was for us – perhaps we were behind the curve), so our webservers were all Sun Enterprise machines running Solaris.

We decided to use a server process that had an in-memory representation of the active users, which we would query using doors from a custom Apache module. (I had read about doors and thought they sounded cool, but neither of us had used them before.)

It worked brilliantly and was extremely fast, though in the end it was never used in production because of changing business priorities.