my world just got complicated... onsite database / central database question

[Travis Hamrick]

Ok, so how do you handle this beauty?

We have a lot of clients who sometimes share customers and would like to share information with each other. Currently, our product has a database on-site that stores customer information.

Here's what our clients want:
-- Each site should be able to sign in new customers and store data for customer visits offline. (as our clients often work away from internet access)
-- Each site should be able to get online and then pull customer data and customer visit data that was created at another site. (we can host a central database for client data at our office)
-- Each site can't store all data from all sites, because there's just way too much data.
-- They would all like for their customers to be able to go to a website to see their data.

I can understand replicating data from each of our clients to a central server, but I have trouble with best-practices for pulling data from site A's replicated data to site B, and then getting the changes back to A's slave and A's master. My thought was each site should be the master, and just replicate when they can to the central server, but what happens when data has to go the other way?

Or am I just way off track in how to do this?

Thanks

Travis