"The lack of joins & transactions of course did factor into the original decision. My point (which perhaps could be clearer) was that MongoDB ended up being used outside of the area in which we originally intended to use it. There was some reluctance to add another technology when we could get by with what we had for what was (initially) only a small use. Additionally, some limitations were not always well understood by web developers (who were new to mongo and enthusiastic to try it). I see this as our mistake. With hindsight, it’s clear we should have introduced an RDBMS immediately and kept MongoDB for managing the crawl data."