Facebook engineer: Going large requires thinking small

IDG News Service - When managing a constantly expanding system with many moving parts, it is crucial to break the system into large numbers of small pieces and manage them with lots of small, dedicated teams, advised Bobby Johnson, director of engineering for Facebook, at the Usenix Annual Technical Conference in Boston.

The topic of his presentation was "Lessons of Scale at Facebook." Johnson, who began working at the social networking service four years ago, when it had 7 million users, has seen exponential growth at the company. Today, Facebook has about 400 million users and Johnson oversees about 400 engineers.

One major lesson he has learned over the years: Keep the projects to add new features small, and manage them with small teams that have direct control over the features.

At a high level, Facebook has a simple three-layer architecture, consisting of Web servers that assemble pages for users, the cache layer, which keeps much of the data that is frequently used, and a database layer, which serves mainly as "persistent storage," Johnson said.

Each layer has been scaled horizontally, meaning the layers are run across thousands of servers.

It is largely an open-source stack, though one that has been heavily modified: The Web server layer runs modified copies of Linux. The cache layer relies on the Memcache, and MySQL powers the databases. The page components (each page is assembled from dozens of smaller components) are written in PHP, though the code has been pre-compiled.

These days the site, which people expect to be up constantly, can get up to 100 million messages per second to the cache layer. The system handles about 1 billion chat messages and 100 million search messages per day. Because the service has a global audience, it remains busy 24 hours a day. And it keeps growing.

"Every week, we have our biggest day ever," he said.

For building in new features and enhancing the old ones, the approach that Facebook has taken has been one of deploying "very small teams who move quickly," Johnson said. "We make small changes frequently," he said, noting that when something goes wrong, they can isolate the problem quickly.

The data set is unusual in that it is highly connected. For instance, Facebook runs the world's largest photo-sharing site, even though it doesn't offer many of the features of other sites, such as Flickr. But the one advantage it does have is that people are tagged in photos.

And this is true overall for Facebook, he added. The true value it adds is connectivity, the ability for users to connect with other users.