[idorosen]

Alternatively, you could guarantee the same synchronization using PPS and PTP to each host's DCD pin of their serial port or to specialized hardware such as modern PTP-enabled smart NICs/FPGAs that can accept PPS input. GPS+PPS gets you to within 20-80ns global synchronization depending on implementation (assuming you're all mostly in the same inertial frame), and allows you to make much stronger guarantees than TrueTime (due to higher precision distributed ordering guarantees, which translate to lower latency and higher throughput distributed writes).

[structural]

Of course, you can do this in good conditions. The extremely powerful part that TrueTime brings is how the system degrades when something goes wrong.

If everyone is synced to +/- 20ns, that's great. Then when someone flies over your datacenter with an GPS jammer (purposeful or accidental), this needs to not be a bad day where suddenly database transactions happen out of order, or you have an outage.

The other benefit of building in this uncertainty to the underlying software design is you don't have to have your entire infrastructure on the same hardware stack. If you have one datacenter that's 20yrs old, has no GPS infrastructure, and operates purely on NTP - this can still run the same software, just much more slowly. You might even keep some of this around for testing - and now you have ongoing data showing what will happen to your distributed system if GPS were to go away in a chunk of the world for some sustained period of time.

And in a brighter future, if we're able to synchronize everyone's clocks to +/- 1ns, the intervals just get smaller and we see improved performance without having to rethink the entire design.

[throw0101c]

> Then when someone flies over your datacenter with an GPS jammer (purposeful or accidental), this needs to not be a bad day where suddenly database transactions happen out of order, or you have an outage.

Most NTP/PTP appliances have internal clocks that are OCXO or rubidium that have holdover (even for several days).

If time is that important to you then you'll have them, plus perhaps some fibre connections to other sites that are hopefully out of range of the jamming.

[RossBencina]

> fibre connections to other sites that are hopefully out of range of the jamming.

I guess it's not inconceivable that eventually there's a global clock network using a White-Rabbit-like protocol over dedicated fibre. But if you have to worry about GPS jamming you probably have to worry about undersea cable cutting too.

[throw0101c]

> But if you have to worry about GPS jamming you probably have to worry about undersea cable cutting too.

GPS jamming can be done by a random truck driver:

* https://www.cnet.com/culture/truck-driver-has-gps-jammer-acc...

Cutting cables at the bottom of the sea is an entire different class of attack (anchors notwithstanding).

[namibj]

Good thing cesium fountains are very accurate then...

[idorosen]

In summary, with different business requirements you would build a different technical solution.

[vlovich123]

> and allows you to make much stronger guarantees than TrueTime (due to higher precision distributed ordering guarantees, which translate to lower latency and higher throughput distributed writes).

TrueTime is the software algorithm for managing the timestamps. It’s agnostic to the accuracy of the underlying time source. If it was inaccurate then you get looser bounds and as you note higher latency. Google already does everything you suggest for TrueTime while also having atomic clocks in places.

[idorosen]

Yup! I was referring to the original TrueTime/Spanner papers, not whatever's currently deployed. The original paper makes reference to distributed ordering guarantees at the milliseconds' scale precision, which implies many more transactions in flight in the uncertain state and coarser distributed ordering guarantees than the much tighter upper bound you can set with nanoseconds' precision and microseconds' comms latency...

[vlovich123]

More than a decade of progress, probably in no small part from Google pushing vendors to improve hardware :)

[idorosen]

Amen. :)

[AlotOfReading]

Truetime is based on GPS and local atomic clocks. Google's latest timemasters are even better, around 10ns average.