Glossary

Feature Flag

A feature flag (or feature toggle) is a mechanism for enabling or disabling functionality without a deploy. The code for a new feature is present in production, but activated only for selected users or based on a configuration condition.

Use cases

Progressive rollout — start with 1% of users, then 10%, then 100%
A/B testing — different UI versions for different groups
Trunk-Based Development — unfinished code in main, hidden behind a flag
Kill switch — quickly disable a problematic feature without rolling back the deploy

Implementation

Simple flags — environment variables or config. Complex scenarios — dedicated services: LaunchDarkly, Flagsmith, GrowthBook, Unleash. These support targeting by user_id, country, plan, and more.

Technical debt

A feature flag is a temporary construct. Remove flags after full rollout, or the codebase will turn into a maze of conditionals.