Which mechanism lets a globally distributed database order transactions consistently across machines on several continents, so that externally consistent reads and writes are possible at scale?
Show answer and explanation
Answer: A
Spanner relies on TrueTime, a clock API backed by GPS receivers and atomic clocks in each data center. It exposes a bounded uncertainty interval, and Spanner waits out that uncertainty when committing, which gives externally consistent ordering across regions. Sharding and replication are necessary too, but the global ordering guarantee comes from the bounded time source.
Why the other options are wrong
- B. Caching rows on application nodes would produce stale reads across machines and does not order writes globally. Consistency would have to be managed by the application, which defeats the purpose of a managed database.
- C. A single primary region would bottleneck writes and reintroduce one point of coordination, which contradicts the goal of spreading writes across continents. Buffering also adds data loss risk.
- D. Periodic snapshots provide backup and point-in-time recovery, not transaction ordering or write consistency. Five-minute copies would also leave a gap in what is recoverable.