The Kill Switch Pattern: Your Safety Net in Production

Every production system needs a panic button. The kill switch pattern is your emergency brake for when things go wrong, and they will go wrong.

What Is a Kill Switch?

A kill switch is a feature flag with superpowers. It's designed specifically for emergency situations where you need to instantly disable functionality across your entire system.

// Server-side kill switch
const flagswift = new FlagSwiftServer({
  apiKey: process.env.FLAGSWIFT_API_KEY
})

app.post('/api/payment', async (req, res) => {
  // Check kill switch first
  if (!await flagswift.isEnabled('payment-processing')) {
    return res.status(503).json({ 
      error: 'Payment processing temporarily unavailable' 
    })
  }
  
  await processPayment(req.body)
})

When You Need Kill Switches

Third-Party API Failures

Your payment processor goes down. Rather than letting errors cascade through your system, flip the kill switch and show a maintenance message. Your users see a clean error instead of cryptic failures.

Resource Overload

A new feature is hammering your database. CPU at 98%. Kill switch OFF. Crisis averted. Investigate later when things are calm.

Discovered Security Vulnerabilities

Security researcher reports a vulnerability in your new feature. Disable it in 2 seconds while you patch. No emergency deployment needed.

Cascading Failures

One microservice is struggling and dragging down everything else. Kill switches let you isolate the problem instantly.

Implementation Best Practices

Server-Side Only

Kill switches should always be checked server-side. Client-side checks can be bypassed or cached. For critical systems, the server must enforce the switch.

// Node.js example
const flagswift = require('@flagswift/node-server')

const flags = new flagswift.FlagClient({
  apiKey: process.env.FLAGSWIFT_SERVER_KEY,
  environment: 'production'
})

async function criticalOperation() {
  if (!await flags.isEnabled('critical-feature')) {
    throw new Error('Feature temporarily disabled')
  }
  
  // Proceed with operation
}

Default to Safe

If your flag service is unreachable, default to the safe state (usually OFF). It's better to show a maintenance message than to let a broken feature run wild.

Clear Error Messages

When a kill switch is active, tell users what's happening. "Payment processing is temporarily unavailable" is much better than a generic 500 error.

Monitor Everything

Track when kill switches are flipped. Set up alerts so your team knows immediately when a critical feature gets disabled.

Kill Switches vs Regular Flags

Kill switches are a special category of feature flags designed for emergencies:

• Always server-side: Cannot be bypassed by clients
• Default to safe: If unreachable, assume OFF
• Fast evaluation: Cached aggressively for speed
• Logged heavily: Every state change is recorded
• Alert on change: Team gets notified immediately

Real-World Example

A FlagSwift user runs an e-commerce platform. Black Friday arrives and their new recommendation engine starts making excessive database queries. Site slows to a crawl.

Rather than panic, they:

1. Toggle the ai-recommendations kill switch OFF
2. Site speed returns to normal in 5 seconds
3. Users see static recommendations instead
4. Team investigates and fixes the query optimization
5. Toggle back ON after 2 hours

No deployment. No downtime. No lost revenue.

Setting Up Kill Switches with FlagSwift

// Install server SDK
npm install @flagswift/node-server

// Initialize
const { FlagClient } = require('@flagswift/node-server')

const flags = new FlagClient({
  apiKey: process.env.FLAGSWIFT_SERVER_KEY,
  environment: 'production'
})

// Protect critical endpoints
app.use(async (req, res, next) => {
  const apiEnabled = await flags.isEnabled('api-access')
  
  if (!apiEnabled) {
    return res.status(503).json({
      error: 'API temporarily unavailable',
      status: 'maintenance'
    })
  }
  
  next()
})

Testing Your Kill Switches

Don't wait for an emergency to test your kill switches. Practice in staging:

1. Deploy code with kill switch checks
2. Verify feature works when switch is ON
3. Toggle switch OFF in dashboard
4. Verify graceful degradation
5. Check error messages are clear
6. Toggle back ON and verify recovery

Conclusion

Kill switches are your production safety net. They turn potential disasters into minor incidents. Every critical system should have them.

With FlagSwift's server-side SDKs, implementing kill switches takes minutes. The first time you need one, you'll be grateful you set it up.

Ready to add safety switches to your production system? Try FlagSwift's server SDKs today.