Rethinking multi-grain timestamps

> At which point, don't you now get caught by relativity? :-)

"Relativity" is not some pixie dust you can sprinkle over your argument to handwave away the need to think, unfortunately.

To actually answer the question, yes at some point you need to take relativistic effect into account if you need really accurate time synchronization. Gravitational time dilation, meaning that your clock ticks faster or slower depending on the altitude (strength of the gravitational field), is a thing. Likewise, if two clocks are moving at significant velocity with respect to each other (say, GPS satellites) you start seeing relativistic effects.

But just signals propagating between fixed locations A and B at finite speed does not need any relativity. If you can measure the propagation delay between the two locations, you can agree on a common reference time. That's how e.g. TAI (https://en.wikipedia.org/wiki/International_Atomic_Time ) works, with super accurate atomic clocks spread out all over the world agreeing on a common reference time scale. (Just to clarify, the atomic clocks participating in TAI do account for gravitational time dilation; my point is that fixed clocks separated by some distance is not some unsolvable relativistic mystery.)

> Two events, happening separated by space, you just can NOT always tell which happened first. End of. Tough.

From your, no doubt, extensive studies of relativity you should know that is an ill posed statement. What relativity actually tells us is that there is no absolute time scale in the universe, it's all, drumroll, relative. However, for any particular observer, the order in which the observer sees events IS well defined. And thus two observers, knowing their distance and velocity with respect to each other can agree on a common time scale and they can calculate in which order, and when, the other sees events (which might not be the same in which it itself sees them).

> I think as soon as you have events happening to the same file system, from different computers, you just have to accept that knowing for sure which one happened first is a fool's errand. Some times you just have to accept that the Universe says NO!

Practically speaking, the problem is not so much that relativity is this mysterious force that prevents us from knowing, but rather that things like computers themselves as well as signal propagation in computer networks is subject to a lot of timing variation. Time synchronization protocols like NTP and PTP do a lot of clever filtering etc. to reduce that noise, but obviously can't reduce it to zero.

Another practical problem wrt ordering events is that if you have a bunch of timestamped events (which, as mentioned above, we can agree to a common timescale to a relatively high accuracy) coming in from a number of sources, one must wait for at least the propagation delay before one can be certain about the relative ordering of the events. Well, there are a numbers of approaches to dealing with agreeing upon a common event ordering in a distributed system, like the Google Spanner mentioned in a sibling comment, two-phase commit, and whatnot. They all tend to have drawbacks compared to a purely local system that doesn't need to care about such issues.