Feature flags are everywhere. Building a robust system is harder than it looks: targeting rules at evaluation time, gradual rollouts, kill switches, audit trails, multi-region. LaunchDarkly and Unleash exist for a reason.
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Targeting and rules
Flag value = function of (flag, user, context). Rules: user attributes, percentage rollout, segments. Evaluated in app or in SDK with local cache. Cache invalidation strategy matters for low latency.
Rollout and kill
Gradual percentage rollouts. Schedule-based progression. Kill switch (set to false globally) reverts instantly. Logged for incident review.
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Audit and approvals
Every flag change: who, what, when, why. Approval workflow for production flags. Audit log retained. Without this, flag changes become silent prod changes you can't reconstruct.
Targeting + rollout + kill + audit. Either build all four or buy a service that does.