My personal favorite is materialized views laid on top of already existing tables at the source site. However, I have only done this in situations where the network was known and controlled. Doing this over a public network using SSL is an unknown to me. I have no idea what the performance would be like. But the reason that I like materialized views is that that you can take as long as you need to transfer the data. The only customer outage is when you switch the IP address for the customers' access method and this is usually a matter of minutes.