Feature Flag Testing
Feature flags (also called feature toggles) allow code to be deployed but not activated for all users — enabling gradual rollouts, A/B testing, and instant kill-switches for problematic features. Testing feature-flagged code requires deliberate strategies to ensure both flag states (on and off) are tested correctly.
Testing Challenges with Feature Flags
- Combinatorial explosion: Multiple flags create exponential combinations — 10 flags produce up to 1,024 combinations. Automated coverage of all combinations is impractical.
- Flag debt: Old flags that are never removed create ongoing testing burden and code complexity
- Inconsistent states: Users may experience partially-enabled features during rollout if flag logic isn't carefully designed
Testing Strategies
- Always test with flag enabled and disabled — both paths must work correctly
- Use test-specific flag configurations — inject flag states in test environments rather than relying on the production flag service
- Include flag-state assertions in integration tests — verify that flag evaluation is working correctly, not just the resulting behaviour
- Monitor flag-enabled releases — canary deployments with feature flags need active monitoring during rollout
Flag Cleanup
Flags should be temporary. Establish a process for removing flags after features are fully released: add a cleanup ticket at flag creation, review flag age regularly, and treat old flags as technical debt to be eliminated.