Feature Flags in the Real World

This article is part of my series on feature flags. Check out the complete article and series on my blog.

Theory and practice often diverge in software development. While the previous articles laid out clean, theoretical approaches to feature flags, production systems demand pragmatic solutions. Let's explore how feature flags work (and sometimes break) in the real world.

The Migration Challenge

One of the most critical uses of feature flags is managing complex system migrations. Consider this common scenario: you need to upgrade your payment processing system without disrupting active transactions. Easy, right? Just flip a switch? Not quite.

// 😱 The complexity spiral
if (flags.isEnabled('new-checkout')) {
  if (flags.isEnabled('payment-provider-v2')) {
    if (flags.isEnabled('advanced-fraud-detection')) {
      if (flags.isEnabled('beta-user-experience')) {
        // Good luck understanding what this does in 6 months!
        return <SuperAdvancedCheckout />;
      }
      return <FraudProtectedCheckout />;
    }
    return <ModernCheckout />;
  }
  return <BasicNewCheckout />;
}
return <LegacyCheckout />;

What started as a simple toggle quickly evolved into a complex web of interdependent flags. Each additional flag multiplies the system's complexity:

• Testing scenarios multiply exponentially
• Documentation struggles to capture all combinations
• Maintenance requires understanding all possible states
• Each evaluation adds latency to the request path

The False Simplicity Trap

"Let's just use one flag for everything!" It's a tempting solution to flag sprawl, but this apparent simplification creates its own problems:

function CheckoutPage({ user }) {
  const showNewCheckout = useFeatureFlag('new-checkout-experience', { user })

  if (showNewCheckout) {
    return (
      <NewCheckoutExperience>
        {/* New UI design */}
        <RedesignedHeader />
        {/* New payment integration */}
        <StripePaymentProcessor />
        {/* New fraud detection */}
        <EnhancedFraudDetection />
        {/* New analytics */}
        <EnhancedTracking />
      </NewCheckoutExperience>
    )
  }

  return <LegacyCheckout />
}

This oversimplified approach means:

• Features can't be enabled independently
• Issues require investigation of the entire feature set
• Rollbacks affect all changes
• Individual feature impacts become unmeasurable

Finding Balance in Production

The key is finding the right balance between granular control and maintainable code. Here's what a more robust approach looks like:

function CheckoutPage({ user }) {
  const flags = useFeatureFlags({
    newDesign: 'checkout-ui-refresh',
    newPayment: 'stripe-payment-integration',
    fraudDetection: 'enhanced-fraud-detection',
    newShipping: 'improved-shipping-calculator'
  }, { user })

  return (
    <CheckoutExperience>
      {/* UI components can be toggled independently */}
      <Header variant={flags.newDesign ? 'modern' : 'classic'} />

      {/* Payment processing can be switched separately */}
      {flags.newPayment ? (
        <ErrorBoundary fallback={<LegacyPaymentProcessor />}>
          <StripePaymentProcessor
            withFraudDetection={flags.fraudDetection}
            onError={(error) => {
              metrics.increment('stripe_payment_error')
            }}
          />
        </ErrorBoundary>
      ) : (
        <LegacyPaymentProcessor />
      )}
    </CheckoutExperience>
  )
}

This approach brings several benefits:

• Each flag controls one specific component or feature
• Error boundaries provide clean fallbacks
• Features can be enabled independently
• Analytics track feature usage separately
• Rollbacks affect only the problematic feature

Want to Learn More?

This post only scratches the surface of managing feature flags in production. In the full article, I dive deeper into:

• Real-world migration patterns that actually work
• Strategies for managing complex dependencies
• Incident response and rollback procedures
• Security considerations in production
• Monitoring and analytics best practices

The article is the final part of a series that takes you from basic concepts to real-world implementation patterns.

Read the full article →

Comments

[Peer Rahne]

Interesting Topic @bdestrempes!
I personally believe feature flags are great for breaking changes - that's why I especially like them to keep Web and Native Apps in sync. However, I agree that they bring way more complexity than you would think in the beginning, so it's always important to ask yourself if you really need them. Also, it makes sense to have incident management processes in place, e.g. alerts if someone who shouldn't have access to a page or API still manages to interact with it.

[Benjamin Destrempes]

Thanks @_sahne_ !

You make some excellent points. The web/native app sync use case is a great example; it's one of those scenarios where feature flags can really help manage the complexity of different release cycles (and platform constraints).

And yeah, it's definitely worth it to weight the operational overhaid against the benefits. Sometimes, a simple feature branch might be more than enough.

As for monitoring, you're right, if a flag is supposed to be off for a specific user segment but you can detect that it's somehow still accessible, that could spell trouble and would merit immediate attention!