Good read! That is an ambitious project. Very interesting to note that the implementation didn't take more than a few days.

Thanks! Note that it's just a prototype implementation. We wouldn't want to put it like that in the hands of users, but it's definitely good enough to play with and get a sense of the final result.

In terms of scalability, is SQL an option for you or do you plan to go NoSQL?

SQL should be fine with sharding. Evernote has actually a great article on how they sped up their sync by rethinking the sharding. http://blog.evernote.com/tech/2014/01/28/synchronization-spe...