I don't think your solution is comparable to Spanner, because you said somewhere you have to write custom merge functions for data. Spanner doesn't impose that on application developers.

Your system sounds really interesting though, thanks for writing it up. For most people, the hardware requirement of a Spanner-like solution does seem impractical, so maybe that's a good tradeoff.

I'm curious what industry this is in? Or what company? 17ms is really low! I wish more sites would put that much effort into their latency.