Feature Flagging and Rollout System

Shopify has evolved its system for safely deploying new features using beta flags and phased rollouts. This includes defining primitives like Subject, BetaIdentifier, BetaFlag, and BetaRollout, and implementing a digest-based percentage rollout mechanism for consistent growing sets of subjects. The system also introduces a higher-level 'Feature' abstraction to manage complex qualification changes and provide kill switches for immediate rollback. Considerations for data structure differences, rollback disruption, and avoiding BetaIdentifier reuse are also addressed.