Spanner: TrueTime and external consistency | Google Cloud

TrueTime is a highly available, distributed clock that is provided to applications on all Google servers1. TrueTime enables applications to generate monotonically increasing timestamps: an application can compute a timestamp T that is guaranteed to be greater than any timestamp T' if T' finished being generated before T started being generated. This guarantee holds across all servers and all timestamps. This feature of TrueTime is used by Spanner to assign timestamps to transactions. Specifically, every transaction is assigned a timestamp that reflects the instant at which Spanner considers it to have occurred. Because Spanner uses multi-version concurrency control, the ordering guarantee on timestamps enables clients of Spanner to perform consistent reads across an entire database (even across multiple Cloud regions) without blocking writes. Spanner provides clients with the strictest concurrency-control guarantees for transactions, which is called external consistency2. Under externa

TrueTime is a highly available, distributed clock that is provided to applications on all Google servers1. TrueTime enables applications to generate monotonically increasing timestamps: an application can compute a timestamp T that is guaranteed to be greater than any timestamp T' if T' finished being generated before T started being generated. This guarantee holds across all servers and all timestamps. This feature of TrueTime is used by Spanner to assign timestamps to transactions. Specifically, every transaction is assigned a timestamp that reflects the instant at which Spanner considers it