Variant Application Iteration Server is a free and open source code variation middleware, supporting online controlled experiments (a.k.a. A/B tests) and feature flags at the Internet scale. Variant is for engineers and technical product owners, who build enterprise applications and look for a robust and cost-effective platform for conducting full stack online controlled experiments or manage feature releases at continuous deployment rate. Variant makes running hundreds of concurrent experiments at a time, or hundreds of production deployments daily a matter of daily practice. Architected as client/server middleware, Variant is tightly integrated with the host application’s runtime and operational data. Variant is particularly attractive to distributed and cloud-native host applications.