[SR-Users] Best practise for limiting concurrent calls across a cluster

Hey all, So I have a cluster of Kamailio servers ( 4 servers currently, soon to be 8 ), I'm looking for suggestions about the BEST way to achieve concurrent call limiting on a per customer basis, across the whole cluster. Initially I mis-read the dialog module documentation and assumed that dialog would provide me this ability, when used with a database. however it seems that the dialog module does not pull data from the DB after the initial startup. I know I can use sql ops to increment and decrement using event_route[dialog:start] and event_route[dialog:end]. however the database I've chosen ( for other valid reasons ) does not have an atomic increment and decrement. I could add yet another DB, but that just adds more failure points. so lets forget my setup, Im wanting suggestions about the BEST setup for this sort of thing. while remaining fault tolerant, and preferably without relying on any single point of failure. Sincerely Jay