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Feature Flags

Feature flags are runtime configuration switches that enable or disable functionality without deploying new code. They decouple deployment from release, allowing gradual rollouts and instant rollbacks, migrations and A/B testing.

Strategies

Environment Variables: Simplest approach. Requires restart to change. Good for operational flags.

Configuration Files: Load from files on startup or reload periodically. No restart needed if hot-reloaded.

Database or Cache: Dynamic flags stored in database or distributed cache. Change instantly across all instances.

Feature Flag Services: Dedicated systems like LaunchDarkly, Split, or Unleash. Advanced targeting and analytics.

Targeting and Segmentation

Flags can target based on:

  • User ID or email for specific users
  • Percentage rollout for gradual releases
  • Geographic region or data center
  • User attributes like subscription tier
  • Environment like staging vs production
  • Random sampling for experiments

Anti-Patterns

Long-Lived Release Flags: Release flags should be temporary. Remove after release complete.

Nested Flags: Flags inside flags create exponential complexity. Avoid deeply nested conditions.

Flag Proliferation: Too many flags make system hard to reason about. Be selective.

Using Flags for Configuration: Feature flags are not general configuration. Use proper config systems for that.

Why you need to know this?

Because if you want to do experiments and A-B testing you will need use feature flags. Feature flags should:

  • Have a clear time to die. i.e this feature flag will last 3 sprints or 2 weeks.
  • Never be nested (make it flat, make it simple)
  • Have a highly descriptive name. i.e sales.report.top.sales.experience.v2 make sure you name your feature flags all with the same pattern.
  • Feature flags should not be confused with the CORE business(backend) or even with permanent configuration. i.e display.theme = dark | light is not a feature flag.
  • Be documented in a central place so everyone knows what they are and what they do. A Catalog for feature flags it’s a good idea.
  • Feature flags must be tested. You must have tests that cover the feature flag being ON and OFF.