Lockless algorithms for mere mortals

Spanner is not hypothetical. Google is both using it internally, and selling it on GCP (at an extortionate rate, but it is there if you really want it). It actually works as I described. The simple reality is that relativistic effects are too small to matter when you're operating on the scale of milliseconds, on a single planet. If you want to set up datacenters on Mars, or synchronize things at the nanosecond level, then you might have a problem.

(Disclaimer: I work for Google. I do *not* get paid to promote Spanner or any other Google product.)