Glossary
Feature flag
Also: Feature toggle
A switch in the code that turns functionality on and off at runtime, without shipping a new release.
Feature flags decouple two things that usually coincide: shipping code and releasing a feature. Unfinished code can travel to production safely, and a new feature can be opened to individual customers first.
For migrations they are the tool of choice: the new path first serves one percent of traffic, then ten, and if in doubt you close it again in seconds.
How you notice it
- Deployment and release should be separated.
- A change needs to go live for a subset of users.
- A rollback must not require a new deployment.
Frequently asked
How do we avoid flags piling up?
Give every flag an owner and a removal date when it is created. Flags outlive their purpose by default, and a codebase with forty stale flags has more untested combinations than tests. Removing them is part of finishing the feature.