alt Hacker NewsIf you are an engineer at Google dealing with Spanner, then you can in fact assume clocks are well synchronized and can use timestamps for synchronization. If you get commit timestamps from Spanner you can compare them to determine exactly which commit happened first. That’s a stronger guarantee than the typical Serializable database like postgresql: https://www.postgresql.org/docs/current/transaction-iso.html...
That’s the radical developer simplicity promised by TrueTime mentioned in the article.
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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).
Isn't that because Google has its own atomic clocks, rather than NTP which is (generally) using publicly available atomic clocks?
That’s actually not at all what TrueTime guarantees and assuming they’ve solved a physical impossibility is dangerous technically as a founding assumption for higher level tech (which thankfully Spanner does not do).
What TrueTime says is that clocks are synchronized within some delta just like NTP, but that delta is significantly smaller thanks to GPS time sync. That enables applications to have tighter bounds on waiting to see if a conflict may exist before committing which is why Spanner is fast. CockroachDB works similarly but given the logistical challenge of getting GPS receivers into data centers, they worked to achieve a smaller delta through better NTP-like timestamps and generally get fairly close performance.
https://programmingappliedai.substack.com/p/what-is-true-tim...
> Bounded Uncertainty: TrueTime provides a time interval, [earliest, latest], rather than a single timestamp. This interval represents the possible range of the current time with bounded uncertainty. The uncertainty is caused by clock drift, synchronization delays, and other factors in distributed systems.