Overview
Feature flags (also called feature toggles) are conditional statements in code that enable or disable functionality without deploying new code. They decouple deployment from release, allowing teams to ship code to production in a dormant state and activate it independently. This pattern supports progressive rollouts, A/B testing, kill switches for problematic features, and trunk-based development. Feature flag systems range from simple environment variables to sophisticated platforms with targeting rules, percentage rollouts, and audit trails.
Key Concepts
- Release flags — Control when a feature becomes visible to users. Typically short-lived and removed after full rollout.
- Operational flags — Kill switches for features under load or experiencing errors. Should be fast to toggle.
- Experiment flags — Enable A/B testing by routing different users to different code paths.
- Permission flags — Gate features behind entitlements or user segments (beta users, premium tier).
- Flag lifecycle — Flags should be temporary. Stale flags become technical debt and increase code complexity.
- Targeting rules — Logic that determines which users see which variant (by percentage, attribute, cohort).
Trade-offs
- Flags add conditional complexity to the codebase
- Stale flags accumulate as technical debt if not cleaned up
- Testing combinatorial flag states is exponentially complex
- Flag evaluation latency matters for hot code paths
Related
- Blue-Green Deployment is an alternative deployment strategy for risk reduction
- CD pipelines integrate with feature flag systems for progressive delivery
- Software Testing must account for multiple flag states