I have not worked on this project, so I don't have details. Mongo is in a nutshell to replace a database trader setup under their desk and that grew quite important. There is nothing pretty about that. I'm quite sure they will have issues and workarounds for years to come :(
Incredible -- was this under-the-desk database fairly recent or quite a while ago? I'd love to hear the story if you or someone else familiar with it is willing to share.