Etsy is one of the Web s biggest marketplaces. The vintage and homemade goods online store will push code to production upwards of 50 times a day. Etsy relies on what it calls Feature Flags, code wrappers that allow security engineers to easily find particular functionality in the code tree, fix it if necessary, and roll it out incrementally to specific segments of Etsy users. Feature Flags also gives security engineers the ability to add logging tags that were previously left off to old, legacy code.
Source: https://threatpost.com/application-security-the-etsy-way/105344/