What does a feature flag do?

A feature flag is a mechanism that lets you turn a specific piece of functionality on or off in an application without changing code or restarting it.

#Why it's needed:

1. A gradual feature rollout You can roll out new functionality to 1% of users first, then 10%, then everyone.
2. A/B tests It's easy to split users into groups and show each one its own version of the feature.
3. A fast switch-off when something breaks Something broke, the flag gets turned off, the feature disappears. No need to rush a fix out.
4. Flexible configuration Features can be enabled by role, geography, time, browser, ID, and so on.
5. Working in production without a rollout The code is already deployed to production, but the flag is off. Users don't see it. You change the value, the feature activates.

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Summary: a feature flag is like a switch above every feature. Turn it on, it works. Turn it off, it's gone. You control the product in real time.