Why does Spanner use TrueTime for consistency?

Spanner needs to order transactions correctly across machines that may be thousands of miles apart, and clocks on separate servers naturally drift, so a plain local timestamp can't be trusted to reflect real-world order. TrueTime solves this by exposing clock uncertainty explicitly, as an interval, instead of pretending clocks are perfectly synchronized.

When a transaction commits, Spanner picks a timestamp and then waits until it is certain that timestamp has passed everywhere, the "commit wait" period equal to the uncertainty bound. This guarantees that if transaction B starts after transaction A commits, B's timestamp will be greater than A's, which is the definition of external consistency. Without TrueTime, Spanner would need a slower consensus round trip on every single commit just to establish ordering, which is why TrueTime is central to keeping global transactions both correct and fast.

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