"For now, we're sending the row-count-queries queries back to the second system, which is otherwise idle, but isn't consistent with the first or third system. "
Can you help us better understand what queries you're talking about? Do you mean, e.g., that any queries that call for *ALL* friends/ids without pagination will use the second inconsistent system? And so the recommended solution would for us to change our queries to use pagination... if we want accurate data?
