[!todo] Seed note. A starting point, not a finished note yet.
External consistency is the strongest ordering guarantee a distributed database can offer: if transaction A commits before transaction B starts in wall-clock time, then every observer sees A ordered before B, with no way to catch the system in a stale or reordered view. It is stronger than serializability, which only requires some valid serial order, not one that matches real time. Google’s Spanner achieves it with TrueTime, a clock API that exposes bounded uncertainty and waits out that uncertainty before committing, so commit timestamps never disagree with real-time order. This is the substrate Google Zanzibar builds on: because Spanner writes carry causally correct timestamps, a zookie can encode a single timestamp and trust that everything before it is visible and everything after it is not. Seeded from Google Zanzibar.