OpenX is a system that was designed to scale as is evident in some key design decisions found throughout the platform. The multi-server setup that was chosen at Mail.com allowed for a distributed server architecture which separated the front end web delivery nodes from the backend MySQL Database master which replicated data back to each front end delivery node mysql slave. This eased the ability to horizontally scale as needed due to the ingenious separation of the ad impressions data tables on the delivery nodes which were being collected locally on each ad impression and then processed periodically back to to the master database which replicated the aggregated statistics back to the delivery nodes. The ability to load balance across the front end web delivery nodes, add caching at many different layers, utilize a CDN for the static ad images, implement PHP accelerators, and hit memcached instead of the MySQL master made the OpenX platform a service that was very resilient to failures.
The ability to optimize the platform is also something that OpenX does very well. This is evident in many of the config key parameters available. In addition to OpenX specific tunable's, optimizations can be made at many different levels in which the system sits. These include hardware and operating system level optimizations, tcp and networking stack optimizations, web server/php-cgi configurations, and database (MySQL InnoDB) tunable's. There are many different optimization knobs that can be tuned to help scale the application for the best performance possible
The documentation was simple, to the point, and well written which led to an easy initial implementation and roll out of a multi-server setup. The installation and upgrade procedures were straight forward even for the complexities of the multi-server setup. I also found the OpenX team to be accessible and even got a chance to meet some of the team at their Up Close and Personal events at Cal Tech in Pasadena. Core developers were on hand to answer technical questions and also made themselves available to us via email and irq channels.
The open nature of the OpenX platform allows for flexibility in the choice of web serving platform to use (Apache, nginx, Lighttpd) and also the database management systems (MySQL, PostGreSQL)