What Are Feature Flags? Safe Deployments Guide

Is deploying a new feature stressful? Hard to roll back? Feature flags let you deploy code but toggle features on or off at runtime — no redeployment needed.

What Is a Feature Flag?

A feature flag (feature toggle) is a technique that allows you to enable or disable a feature at runtime. The code is live, but the feature can remain hidden.

if (featureFlags.isEnabled('new-checkout')) {
  showNewCheckout();
} else {
  showOldCheckout();
}

Why Feature Flags?

1. Safe deployment — Roll out to 1% of users, monitor, then increase
2. Trunk-based development — No long-lived feature branches; everyone merges to main
3. A/B testing — Show different experiences to different user groups
4. Kill switch — Instantly disable a feature without redeploying

Types of Feature Flags

| Type | Lifetime | Purpose | |------|----------|---------| | Release | Short (weeks) | Toggle new features | | Experiment | Medium (months) | A/B testing | | Ops | Long | Performance/maintenance | | Permission | Permanent | User-specific features |

Simple Implementation

const flags = {
  'new-dashboard': true,
  'dark-mode': true,
  'beta-ai-chat': (user) => user.plan === 'premium',
  'experimental-editor': (user) => user.email.endsWith('@company.com'),
};

function isEnabled(flagName: string, user?: User): boolean {
  const flag = flags[flagName];
  if (typeof flag === 'function') return user ? flag(user) : false;
  return !!flag;
}

React Usage

function CheckoutPage() {
  const { isEnabled } = useFeatureFlags();

  return isEnabled('new-checkout')
    ? <NewCheckoutFlow />
    : <LegacyCheckoutFlow />;
}

Canary Release Strategy

Day 1:  1% of users   → Feature on → Monitor metrics
Day 2:  5% of users   → No issues  → Continue
Day 3:  25% of users  → No issues  → Continue
Day 5:  50% of users  → No issues  → Continue
Day 7:  100% of users → Full rollout → Remove flag

Feature Flag Tools

| Tool | Type | Highlights | |------|------|-----------| | LaunchDarkly | SaaS | Enterprise, rich SDKs | | Unleash | Open source | Self-hosted, free | | ConfigCat | SaaS | Simple, affordable | | Flagsmith | Hybrid | Open source + SaaS |

Best Practices

1. Clean up flags — Remove flags that are no longer needed (tech debt!)
2. Naming convention — Use feature.checkout.new-flow style
3. Default off — New flags should be disabled by default
4. Monitor — Log and track flag changes
5. Test both paths — Write tests for enabled and disabled states
6. Limit active flags — More than 30-50 creates complexity

Conclusion

Feature flags are essential for modern software development. They enable safe deployments, A/B testing, and instant rollbacks. But without careful management, they create technical debt — always clean up your flags.

Learn feature flags and deployment strategies on LabLudus.