Contract
- Input: features to control, rollout requirements, environments.
- Output: flag lifecycle + targeting rules + inventory.
- Side effects: none (design only; execution requires flag platform).
- Dependencies: feature flag platform (LaunchDarkly / Split / Unleash / Flagsmith / in-house).
- Stop condition: lifecycle and targeting rules documented.
- Risk: medium — flag mishandling can expose unready features or cause outages.
- Boundary: designs flag system; does not deploy flags unless explicitly instructed.
Feature Flag System Design
Design a feature flag system — rollouts, targeting, kill switches — with lifecycle management and operational runbooks.
Process
1. Flag types
- Release flag: hide unfinished features from users; enable in dev/staging.
- Experiment flag: A/B test; percentage split; metric tracking.
- Ops flag: kill switch / circuit breaker (e.g. disable expensive recommendation engine).
- Permission flag: gradual rollout to user segments (beta users, tiers, regions).
Completion criterion: flag types mapped to features.
2. Lifecycle design
Every flag goes through:
- Created: default off.
- Gradual rollout: 1% → 10% → 50% → 100%.
- Full release: flag kept for quick rollback or removed if not needed.
- Retired: removed from code within N sprints.
Completion criterion: lifecycle documented per flag type.
3. Targeting rules
- Percentage-based (random or deterministic).
- User segment (tier, region, language, plan type).
- Environment (dev / staging / prod).
- Custom rules (date-based, device type, account age).
Completion criterion: targeting rules template saved.
4. Kill switches and emergency rollback
- Ops flags: what to do if the flag causes an outage? (disable immediately; have runbook)
- Rollback time: must be < 5 minutes for critical paths.
- Alert: flag change should trigger an alert to on-call.
Completion criterion: kill switch runbook saved.
5. Flag inventory
Per flag: name, owner, purpose, type, created date, retirement date, dependencies.
Completion criterion: inventory saved and reviewed quarterly.
6. Deliver
Flag lifecycle document + targeting rules template + flag inventory.