>One of my clients use a finished app on more than 5 independent locations. There is no connection between these locations (they don't want to use online
I'm offering the following so you don't re-create the wheel or spend too much time engineering your application to fit this weird transaction scheme:

Sybase makes a product called SQL Anywhere (previously known at Watcom SQL) which includes a SQL Server application (theirs, not MicroSoft) for your backroom and components to perform replication like you are talking about.

Replication can be performed thru MAPI, SMTP, or through a file connection. This product does replicate using the transaction logs from the Server. There are also triggers for preventing collisions between nodes.

This product is not aimed at the market that needs full-time high-speed high-volume connections, as long as the company has an e-mail connection the connected databases can by synchronized. If your companies do share a file network this product can use that connection instead of e-mail (or even if you could dial up and start a RAS connection between the server nodes)...