Google Spanner: First Globally Scalable Database With External Consistency 49
vu1986 writes with this bit from GigaOm: "Google has made public the details of its Spanner database technology, which allows a database to store data across multiple data centers, millions of machines and trillions of rows. But it's not just larger than the average database, Spanner also allows applications that use the database to dictate where specific data is stored so as to reduce latency when retrieving it. Making this whole concept work is what Google calls its True Time API, which combines an atomic clock and a GPS clock to timestamp data so it can then be synched across as many data centers and machines as needed."
Original paper. The article focuses a lot of the Time API, but external consistency on a global scale seems to be the big deal here. From the paper: "Even though many projects happily use Bigtable, we have also consistently received complaints from users that Bigtable can be difficult to use for some kinds of applications: those that have complex, evolving schemas, or those that want strong consistency in the presence of wide-area replication. ... Many applications at Google have chosen to use Megastore (PDF) because of its semi-relational data model and support for synchronous replication, despite its relatively poor write throughput. As a consequence, Spanner has evolved from a Bigtable-like versioned key-value store into a temporal multi-version database. Data is stored in schematized semi-relational tables; data is versioned, and each version is automatically timestamped with its commit time; old versions of data are subject to configurable garbage-collection policies; and applications can read data at old timestamps. Spanner supports general-purpose transactions, and provides a SQL-based query language." Update: 09/20 17:57 GMT by T : Also in a story at Slash BI.
Re:GPS tampering (Score:2, Informative)
Then you'll be too worried about aeroplanes falling from the sky and ships running aground to be worried about database consistency, I guess.
Re:More to it than NTP... (Score:5, Informative)
GPS/Atomic clock is better than NTP. It's a system to distribute time that will have a 400ns precision (probably a couple microseconds once you reach the actual servers in the data center).
If you use NTP or message passing you can't synchronize data centers more accurately than a couple milliseconds (assuming you have paths that are quite stable between them as transit time can be corrected).
So basically GPS/Atomic clock lets you synchronize 2 systems that are far apart more precisely and without having to make them communicate.
Note that Atomic clocks protect them from GPS outages, so they can really rely on the timestamps.