Synchronization among old and new

A friend at MSDN asked how to design a synchronization mechanism for legacy db and "new" db. What I thought first was the data transportation. I mean, your application could construct such a chain-of-responsibility and proxy like structures which can switch the DB on-the-fly. You can construct an application logic such as trying to extract data from legacy if it fails then check on application db on read cycles. And, try to insert on both parties by firing just one command on write cycles. But be sure to keep log on operations and use transactions on both reading and writing. Such an approach might help you to run reliable.

The last but the most important thing is to keep in mind that such transportation tasks (r/w from some data source) usually introduces so many trade-offs. For instance, you'd probably have comm and resource mngmt costs on the solution above. But, hey, you've to choose an end on trade-off (performance vs. reliability)