These features are enabled by the fact that Spanner as- signs globally-meaningful commit timestamps to trans- actions, even though transactions may be distributed. The timestamps reflect serialization order. In addition, the serialization order satisfies external consistency (or equivalently, linearizability [20]): if a transaction T1 commits before another transaction T2 starts, then T1’s commit timestamp is smaller than T2’s. Spanner is the first system to provide such guarantees at global scale. The key enabler of these properties is a new TrueTime API and its implementation. The API directly exposes clock uncertainty, and the guarantees on Spanner’s times- tamps depend on the bounds that the implementation pro- vides. If the uncertainty is large, Spanner slows.
