Database / Google Spanner Database Interview questions
What is the purpose of TrueTime in Spanner?
TrueTime is a globally synchronized clock API that Spanner uses to assign commit timestamps to transactions. Instead of returning a single instant, TrueTime returns an interval [earliest, latest] that is guaranteed to contain the actual current time, using GPS and atomic clock references distributed across Google's data centers.
Spanner uses this bounded uncertainty to enforce external consistency: before committing a write, it waits out the remaining uncertainty window ("commit wait") so that any transaction that starts after another one commits is guaranteed to see a later timestamp. This lets Spanner order transactions correctly across data centers without needing a central coordinator for every commit, which is what makes global strong consistency practical at low latency.
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