Feeds

Facebook reveals TAO - the data store for its social graph

Calmly serves ONE BEEEELION reads per second

Gartner critical capabilities for enterprise endpoint backup

USENIX Facebook has revealed details about Tao, its multi-petabyte data store for the company's social graph.

Though Facebook's social network may have little relevance for IT pros, its internal infrastructure does, because here the social network is dealing with quantities of information so vast that it has to come up with new ways to store, compute, and manage the data.

So the publication of details about Tao at the USENIX conference on Wednesday is novel for two reasons: one, it shows off the scale at which future enterprises are going to have to operate, and two, it highlights some of the design methodologies that modern data systems have brought about within sophisticated tech companies.

"A system like TAO is likely to be useful for any application domain that needs to efficiently generate fine-grained customized content from highly interconnected data," Facebook's employees write in the paper. "The application should not expect the data to be stale in the common case, but should be able to tolerate it. Many social networks fit in this category."

Other applications of a system like Tao could be large data sets relating to wildlife populations over time, or other complex systems with many agents whose relationship to one another is defined by a variety of actions. For the tin foil hat-aficionados, Tao would also seem to deal with the problems an intelligence agency would run into when trying to keep tabs on all its citizens.

Tao is a read-optimized data store that is deployed at Facebook as a single geographically distributed instance. It lets Facebook engineers access and write information across the company's "social graph" which stores all information about objects on Facebook (people, brands, comments and such), and associations (likes, pokes, tags).

It has been built to deal with over a billion reads per second across a data set "of many petabytes," Facebook said. Tao was designed by Facebook to better link together data kept in its main data store (MySQL) and caching layer (memcache), while being able to deal with unpredictable queries on objects.

"The fact Tao is using MySQL is completely hidden away from the client," Facebook director of engineering Venkat Venkataramani, tells The Register. "We haven't found anything that is better than MySQL, we are constantly looking at that."

Its API is mapped to a small amount of SQL queries, which ease communication with the underlying MySQL database. As Facebook's dataset is too large for a single database it has instead split data into logical shards which are handled by database servers.

TAO also has an eventually consistent caching layer which is built via a similar principle and filled with objects, associations, and association counts. The caching layer is crucial for allowing Facebook to speedily load the hundreds of objects and associations that populate any one page on the site.

Because Facebook's dataset is so large, the cache is split into a two-level hierarchy of a few "leader" caches which deal with writes and a subsidiary "follower" cache that helps with reads, which dramatically outnumber writes – Tao typically experiences a billion reads per second versus "millions of writes".

Data is cached in such a way that objects and associations have proximity to one another, Venkataramani says. "An important design decision is to keep the locality the system tries to exploit similar to the locality the workload has," he says. "That was one of the fundamental decisions that allowed us to scale."

Barack Obama's Facebook page, for instance, will generate vast numbers of reads at unpredictable times, and so many of Tao's design considerations revolve around guaranteeing read access to objects – hence its adoption of eventual consistency and high availability, over strong consistency and higher latency.

"Nothing before Facebook has seen this kind of workload," Venkataramani says. "When people think of web-scale apps people think of email, but the workload was very different because everyone checks their own email - you're not looking at other emails. The problem is very different when you take a social network because there are extremely high fan-outs."

Though the number of companies likely to deal with data in this way is quite small for now, studying Tao gives insights into the problems a company will run into when it gets really big, and shows that behind the blue and white bazaar of Facebook there's a rather sophisticated underlay.

"As the world moves more and more to cloud and a lot of data is being managed in bigger data centers I think this may be the starting of an era for new backend architectures," Venkataramani says. ®

Secure remote control for conventional and virtual desktops

More from The Register

next story
The Return of BSOD: Does ANYONE trust Microsoft patches?
Sysadmins, you're either fighting fires or seen as incompetents now
Microsoft: Azure isn't ready for biz-critical apps … yet
Microsoft will move its own IT to the cloud to avoid $200m server bill
US regulators OK sale of IBM's x86 server biz to Lenovo
Now all that remains is for gov't offices to ban the boxes
Flash could be CHEAPER than SAS DISK? Come off it, NetApp
Stats analysis reckons we'll hit that point in just three years
Oracle reveals 32-core, 10 BEEELLION-transistor SPARC M7
New chip scales to 1024 cores, 8192 threads 64 TB RAM, at speeds over 3.6GHz
Object storage bods Exablox: RAID is dead, baby. RAID is dead
Bring your own disks to its object appliances
Nimble's latest mutants GORGE themselves on unlucky forerunners
Crossing Sandy Bridges without stopping for breath
prev story

Whitepapers

Implementing global e-invoicing with guaranteed legal certainty
Explaining the role local tax compliance plays in successful supply chain management and e-business and how leading global brands are addressing this.
Top 10 endpoint backup mistakes
Avoid the ten endpoint backup mistakes to ensure that your critical corporate data is protected and end user productivity is improved.
Top 8 considerations to enable and simplify mobility
In this whitepaper learn how to successfully add mobile capabilities simply and cost effectively.
Rethinking backup and recovery in the modern data center
Combining intelligence, operational analytics, and automation to enable efficient, data-driven IT organizations using the HP ABR approach.
Reg Reader Research: SaaS based Email and Office Productivity Tools
Read this Reg reader report which provides advice and guidance for SMBs towards the use of SaaS based email and Office productivity tools.