My understanding is that Spanner is optimistically immediately consistent, with very clever logic for deciding on whether to bounce the attempted transaction (T

My understanding is that Spanner is optimistically immediately consistent, with very clever logic for deciding on whether to bounce the attempted transaction (TrueTime). But strictly speaking, even inside a single multicore CPU, there is no such thing as immediate consistency. The universe doesn't allow you to update information in two places simultaneously. You can only propagate at the speed of light. Oh, and the concept of "simultaneous" is suspect too. Our hardware cousins have striven mightily and mostly successfully for decades to create the illusion that code runs in a Newtonian universe. But it is very much a relativistic one.