What is a feature flag?

A feature flag is a mechanism that lets you turn certain application features on or off without changing the code and without a new release. It gives the team control over who sees new functionality and when.

#How it works

• In the code, the new feature is wrapped in a condition:

  python

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  if feature_flag("new_ui"): show_new_interface() else: show_old_interface()

• The flag is stored in configuration (a file, a database, or a service like LaunchDarkly, Unleash).

• Developers or product managers can turn on the feature selectively - for example, for only 10% of users or a specific group.

#Why feature flags are needed

1. Safe rollout A new feature can be enabled gradually (rollout), while tracking metrics and errors.
2. A/B testing One flag, different interface variants for different users.
3. Fast shutdown of problematic features If a new feature causes bugs, it can be turned off instantly without rolling back the release.
4. Release flexibility Development and deployment are decoupled: the code can be in production but "asleep" until the flag is turned on.

#Usage examples

• A "dark theme" is enabled only for internal testers.
• A new recommendation algorithm is enabled for 20% of users.
• After a release, a failed feature is turned off with one click.

#Conclusion

A feature flag is a "switch" inside the product that lets you manage functionality without new releases or risk. It makes the system more flexible, safer, and better suited for Continuous Delivery.