When organization a continuously expanding system with lots of moving parts, it is critical to break the system into large numbers of small pieces and direct them with plenty of small, devoted teams, counsel Bobby Johnson, executive of engineering for Facebook, at the Use nix Annual Technical meeting in Boston.

The subject of his appearance was "Lessons of Scale at Facebook." Johnson, who starts working at social networking service four years ago, when it had 7 million clients, has seen exponential increase at the corporation.

Facebook has about 400 million users and Johnson supervise about 400 engineers.

One main lesson he has learned above the years: maintain the projects to add new characteristics small, and direct them with small teams that have direct control over the features.

At a high level, Facebook has a easy three-layer architecture, consisting of Web servers that collect pages for users, the store layer, which maintains a lot of the data that is often used, and a database layer, which serves mostly as "constant storage," Johnson said.

Every layer has been scaled horizontally; sense the layers are run crosswise thousands of servers.