FIGURE 1.

Data flow in a hypothetical study involving federation of three separate databases. An application sends query requests to the three databases. Each database processes the query according to its server load, policies, maintenance cycle, and other factors. Because data records need to be merged, a synchronization step is required. Dotted arrows indicate transfers via the internet. The figure depicts a simple architecture involving 2-way merges. The number of steps and the computational cost of a merge is application-dependent, involving buffer lengths, differential latencies, format conversions, semantic translations, and other factors.