> I don't think it would make sense at all to run a "mutex service" on one server and asking clients to grab that mutex before accessing another "data service"

And yet such things exist:

http://en.m.wikipedia.org/wiki/Distributed_lock_manager

I first came across one in the context of a distributed cache which sat on top of a database. I'm not at all convinced it was a good design, but it was there!