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Google unites worldwide data centres with GPS and atomic clocks
The search giant's new Spanner system can juggle data across as many as 10 million servers around the world while minimising the amount of traffic sent between them
Three years ago, a top Google engineer named Vijay Gill was asked what he would do if someone gave him a magic wand.
Gill hesitated before answering. And when he did answer, he was coy. But he seemed to say he would use that magic wand to build a single system that could automatically and instantly juggle information across all of Google's data centres.Then he indicated that Google had already built one. "How do you manage the system and optimise it on a global level?" he said. "That is the interesting part."
But about four months later, Google dropped another hint. At a symposium in the mountains of Montana, Jeff Dean -- one of Google's most important engineers -- revealed that the web giant was working on something called Spanner, describing it as a " storage and computation system that spans all our data centres." He said the plan was to eventually juggle data across as many as 10 million servers sitting in "hundreds to thousands" of data centres across the globe.
According to Google, it's the first database that can quickly store and retrieve information across a worldwide network of data centres while keeping that information "consistent" -- meaning all users see the same collection of information at all times -- and it's been driving the company's ad system and various other web services for years.
but at its heart is something completely new. Spanner plugs into a network of servers equipped with super-precise atomic clocks or GPS antennas
That's right, Google attaches GPS antennas and honest-to-goodness atomic clocks to its servers. "It's a big deal -- and it's really novel," says Andy Gross
"The conventional wisdom -- at least among people with modest resources -- is that time synchronisation like that, on a global scale, that is accurate enough for such a big distributed database, just isn't practical."
The Spanner paper lists many authors, but two stand out: Jeff Dean and Sanjay Ghemawat.
Spanner draws on BigTable, but it goes much further. Whereas BigTable is best used to store information across thousands of servers in a single data centre, Spanner expands this idea to millions of servers and multiple data centres.
The genius of the platform lies in something Google calls the TrueTime API.
TrueTime uses those GPS antennas and atomic clocks to get Google's entire network running in lock step.
"One aspect of our design stands out," the paper reads. "The linchpin of Spanner's feature set is TrueTime."
To understand TrueTime, you have to understand the limits of existing databases.
They work well enough, but they aren't designed to juggle information across multiple data centres
With TrueTime, Spanner solves this problem, taking an approach that no one expected.
Rather than try to improve the communication between servers, Google spreads clocks across its network. It equips various master servers with GPS antennas or atomic clocks,
"A lot of current research [outside of Google] focuses on complicated coordination protocols between machines, but this takes a completely different approach," Gross says. "By using highly accurate clocks and a very clever time API, Spanner allows server nodes to coordinate without a whole lot of communication."
Ordinary servers tap into public atomic clocks in an effort to maintain the correct time. But this method isn't as accurate as it could be, says Gross. Google has gone a step further, installing its own atomic clocks -- and GPS antennas -- directly on its machines.
The system was first used to help serve ads across the Google empire, but it has since expanded to all sorts of other Google services.
The rub is that you can't use Spanner unless you add hardware to your servers.
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