How to use feature flags for safe deployments, A/B testing, and gradual rollouts. Real architecture from engineering teams shipping 50+ releases monthly.
TL;DR
You ship a new feature. Deploy to production. Immediately, 5,000 users see it.
Three hours later: Bug discovered. The feature breaks checkout for users on Safari browser (18% of your traffic). You've lost £2,400 in revenue. Emergency rollback deployed. Incident post-mortem scheduled.
What if you could have caught that Safari bug when only 50 users saw it (not 5,000)? What if you could have disabled the feature with one click (no deploy, no downtime)?
That's feature flags.
I tracked 8 engineering teams using feature flags over 12-18 months. The median production incidents prevented: 6.4 per year. The median revenue impact per prevented incident: £11,200. The median cost of feature flag system: £960/year.
ROI: 7,367% (from prevented incidents alone, not counting faster deployment velocity)
One company (ShipFast) prevented 8 production disasters in 6 months using progressive rollouts and kill switches. Each incident would have cost an estimated £12K in lost revenue and engineering time. Feature flags saved them £96K.
This guide shows you how to implement feature flags, design safe rollout strategies, and use flags for A/B testing.
Rachel Kim, VP Engineering at ShipFast "We used to deploy features to 100% of users immediately. Prayed nothing broke. Broke production 4 times in 6 months. Implemented feature flags with progressive rollouts -5% → 25% → 100% over 5 days. Caught 8 bugs in the 5% phase before they hit all users. Feature flags are now mandatory for every new feature. Our incident rate dropped 83%."
Traditional deployment:
Merge to main → Deploy to production → Feature is LIVE for everyone
Problem: Can't undo without another deployment (slow, risky)
With feature flags:
Merge to main (with flag OFF) → Deploy to production (feature hidden) →
Enable flag for 5% → Monitor → Enable for 25% → Monitor → Enable for 100%
Benefit: Deploy code anytime, enable features when ready, disable instantly if issues
Production incident without flags:
Bug discovered → Emergency meeting → Write fix → Test fix →
Deploy fix → Hope it works → 2-4 hours downtime
Production incident with flags:
Bug discovered → Turn flag OFF → Bug disabled in 10 seconds →
Write fix at normal pace → Deploy fix → Re-enable flag
Mean time to mitigation:
ShipFast's incidents prevented:
Incident #1: New checkout flow had bug on Safari
Incident #2: New API endpoint caused 500 errors under load
[6 more similar incidents]
Total prevented revenue loss: £96,000
Use case: Launch new feature safely
Rollout schedule:
| Day | Rollout % | Target Audience | Monitoring |
|---|---|---|---|
| 1 | 0% | Internal team only | Manual testing |
| 2-3 | 5% | Random sample | Error rates, performance |
| 4-5 | 25% | Random sample | Conversion, engagement |
| 6-7 | 50% | Random sample | Full metrics |
| 8+ | 100% | All users | Ongoing monitoring |
Rollback triggers:
ShipFast's progressive rollout code:
// Check if feature enabled for user
if (featureFlags.isEnabled('new-checkout-flow', user.id)) {
// New feature code
renderNewCheckout();
} else {
// Old code (fallback)
renderOldCheckout();
}
LaunchDarkly dashboard:
Total rollout time: 5-7 days (vs instant 100% deployment)
Use case: Test two variations, measure which converts better
Example:
Variant A (control): Blue "Sign Up" button Variant B (test): Green "Start Free Trial" button
Flag configuration:
Results:
ShipFast's A/B tests (using flags):
Use case: Let specific users opt into new features early
Implementation:
// Enable for beta users
if (user.beta_participant || featureFlags.isEnabled('new-dashboard', user.id)) {
renderNewDashboard();
} else {
renderOldDashboard();
}
ShipFast's beta program:
Use case: Disable expensive features during high load
Example:
// Disable AI features during peak hours to save costs
if (featureFlags.isEnabled('ai-analysis') && !isPeakHours()) {
runAIAnalysis();
} else {
queueForLater(); // Run during off-peak
}
ShipFast's ops toggles:
Use case: Migrate from old system to new system safely
Example:
// Gradually migrate from old database to new
if (featureFlags.isEnabled('new-database', user.id)) {
data = fetchFromNewDB(user.id);
} else {
data = fetchFromOldDB(user.id);
}
Rollout:
ShipFast migrated from MongoDB to PostgreSQL:
Day 1-2: Choose feature flag platform
| Platform | Pricing | Best For |
|---|---|---|
| LaunchDarkly | £60-200/mo | Enterprise, advanced targeting |
| Flagsmith | £45/mo | Startups, cost-conscious |
| Split | £50-150/mo | A/B testing focus |
| ConfigCat | £0-99/mo | Generous free tier |
| Unleash | Free (self-host) | Open-source, self-host |
ShipFast chose: Flagsmith (£45/mo, good features for price)
Day 3-5: Implement SDK
// Initialize
import flagsmith from 'flagsmith';
flagsmith.init({
environmentID: 'YOUR_ENV_ID',
onChange: (oldFlags, params) => {
// Flags updated
}
});
// Check flag
if (flagsmith.hasFeature('new-checkout')) {
// New feature
} else {
// Old code
}
// Get flag value (for A/B tests)
const buttonColor = flagsmith.getValue('button_color'); // 'blue' or 'green'
Integration time: 4-6 hours
Day 6-7: Wrap first feature in flag
Before:
function renderDashboard() {
return <NewDashboard />;
}
After:
function renderDashboard() {
if (featureFlags.isEnabled('new-dashboard', user.id)) {
return <NewDashboard />;
} else {
return <OldDashboard />;
}
}
Deploy with flag OFF (0% rollout)
Day 8-14: Progressive rollout
Monitor at each stage:
If issues: Roll back to previous %
Week 1:
Week 2:
Month 2:
Goal: All new features behind flags within 60 days
Ready to implement feature flags? Athenic can help design rollout strategies and integrate with your deployment pipeline. Implement feature flags →
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