Not sure if it was DARPA, but the web server used Tame, a custom event-driven framework at a time where the thread vs. events debate was all the rage in the academic community. (I did a PhD on the topic and that's how I learned about Ok Cupid!)

You can read the paper they published: "events can make sense" https://www.usenix.org/legacy/events/usenix07/tech/krohn.htm...

I met some ex-OkCupid engineers at a later company who said the framework was smart but a pain to maintain in then long run.

I think they're referring to the Internet.