In April 2013 Facebook acquired the Atlas Advertising Suite from Microsoft. The Atlas system was originally an independent company started in 2001, later purchased by aQuantive, and then purchased by Microsoft in 2007 for $6 billion. Atlas is a third party ad server with a focus on campaign management and results tracking. It serves billions of ad impressions each day.

It quickly became apparent that we wouldn't be able to continue to run the Atlas software stack in its current form within Facebook datacenters. At the time, Atlas worked on a closed-source and proprietary Microsoft technology stack. This approach is incompatible with Facebook's commitment to Linux and open source software and hardware solutions. Even if we had been able to move, without a loss of service for our customers, the thousands of physical machines that Microsoft was running the servers on, we would have been left with an operational nightmare of a very aging system. We elected to rebuild the Atlas system using Facebook technologies.

There were three main challenges. First, how do you even begin to understand a software system where portions of the code were written 12 years earlier by engineers that are no longer available. Second, how do you migrate hundreds of ad agencies and users along with their data. And third, how do you rebuild the software in less than 18 months.

In this talk I will describe the entire Atlas architecture and the underlying software components that support the system. I will detail the interesting software systems that those that we released as open source. I will talk about the mistakes that we made during the rebuild and the lessons that we learned in this project.