OpenGrok has been very useful when I want to understand a new codebase. The distributed / REST aspect is going to make it harder to use code tools, though.
OpenGrok seems like a light version of DMS as I understand. It was presented at Google in 2010 [1]. It's a software analysis tool (control/data flow, copy/paste detection, ast levenstein distance), refactor and translator between several languages. IIRC they do distributed systems.
[1] https://www.youtube.com/watch?v=C-_dw9iEzhA