Feature Flag Vocabulary Exercises

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A senior engineer says: "The team uses a kill switch for the payment redesign feature."

What does "kill switch" mean in feature flag vocabulary?

A kill switch is a feature flag specifically designed for fast disablement — flipping it to "off" immediately stops the feature for all users without needing a deployment. This is different from a rollback, which reverts code.

Term	Meaning
kill switch	Feature flag with a fast disable path — turns off a feature instantly without deployment
flag toggle	The act of switching a feature flag on or off
kill switch vs. rollback	Kill switch disables the feature; rollback reverts the entire code version
flag management service	Tools like LaunchDarkly, Unleash, or Flagsmith that manage flag state and targeting

2 / 5

A release engineer says: "We're doing a percentage-based rollout — the flag is enabled for 5% of users."

What is the key benefit of a percentage-based (gradual) rollout compared to an immediate full release?

A gradual rollout (percentage-based) limits blast radius — if the feature causes errors, only the small exposed cohort is affected. The team can observe metrics (error rate, latency, conversion) before increasing exposure.

Term	Meaning
exposure percentage	The fraction of users receiving the new feature (e.g. 5%, 20%, 100%)
rollout cohort	The subset of users selected for each rollout increment
blast radius	Scope of impact if the feature fails — gradual rollout minimises this
full general availability (GA)	The point when the feature is enabled for 100% of users

3 / 5

A feature flag has a targeting rule: user.plan === 'enterprise'.

What is this type of flag targeting called?

Flag targeting rules allow you to direct a feature to specific user segments based on attributes: plan type, region, user ID, email domain, or custom properties. This is different from a percentage rollout (random sample) or a canary (traffic percentage).

Term	Meaning
targeting rule	A condition that determines which users receive a feature (attribute-based)
user segment	A group of users sharing common attributes used for targeting
cohort targeting	Enabling a feature for a pre-defined named group or cohort
gradual rollout by segment	Combining segmentation with a percentage rollout within a specific segment

4 / 5

A product manager asks the team to "dark launch" the new search service before the official release.

What does "dark launch" mean in feature flag vocabulary?

A dark launch means the feature code runs in production and processes real requests — but results are not shown to users. The purpose is to test performance, correctness, and production load without any user-facing risk.

Term	Meaning
dark launch	Feature runs in production on real traffic but output is hidden from users
shadow traffic	Real production traffic mirrored to the new service for testing without user impact
traffic mirroring	Duplicating live requests to a new service to validate behaviour under real load
production readiness testing	Validating that a feature handles real-world load and scale before user exposure

5 / 5

A tech lead announces: "The stale flag cleanup sprint removed 47 feature flags from the codebase."

Why is managing the lifecycle of feature flags important?

Flag debt is a real risk: old flags create dead code paths, create conflicting interactions, and nobody knows their original purpose. Security risks arise when old flags bypass access controls that were later added.

Term	Meaning
flag lifecycle	The stages of a feature flag: created → enabled → rolled out → permanent or removed
flag obsolescence	A flag that is no longer needed but remains in the codebase — a source of confusion
temporary vs. permanent flags	Release flags (temporary, remove after full rollout) vs. ops flags (permanent kill switches)
flag cleanup	Removing the flag and the associated conditional code once the rollout is complete