[Serusers] ser farms / architecture

Hi SerUsers, I want to try and understand the issues involved in having a farm of SER servers. For shared usr_loc data service, is there another way for the servers to share the usr_loc datasource, besides running usr_loc dbmode at 3 and having all ser instances updating and querying the same DB? I would also like to segregate my registrar server from my proxy machine, but I still want my registrar to see invites and byes for billing purposes. Basically to separate all stateful proxying and rtp poxying (keeping it on one or more front end servers) from my registrar/billing server. So far I have been playing with proxying REGISTER and INVITE operations from the front end server(s) to a back end, and attempting to do call routing at the front end. Has anyone done something similar to this in the past? I'd be interested to hear peoples thoughts on this layout. Thanks, -Jev