TrueTime
API returns [earliest, latest] time interval. Uncertainty typically 1-7ms. Bounded by hardware (GPS + atomic clocks).
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External consistency
Commit-wait: wait until interval passes, then commit. Ensures serialization order matches wall clock. Strict guarantees.
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Multi-Paxos + Colossus
Data replicated via Paxos. Storage in Colossus (Google's distributed file system). Hierarchical + sharded.