What are feature flags used for, and what do they cost?

Feature flags, also called toggles, let you change how the app behaves while it runs, without a new deploy. This separates deploy from release. You can ship code to production turned off. Then you turn it on for internal users, for a percentage of users, or for specific tenants (customer accounts).

Common kinds:

• Release toggles hide unfinished work. This keeps the main branch always releasable in trunk-based development.
• Experiment toggles run A/B tests.
• Ops toggles are kill switches. During an incident, you use them to turn off an expensive or failing feature.
• Permission toggles show features only to some plans or roles.

The costs:

• Every flag adds code paths that you must test.
• Combinations of flags can interact in surprising ways.
• Old flags become technical debt.

Keep release flags short-lived. Give each one an owner and a removal date. Check flags in one place, not scattered all over the code.