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Kill Switch

A kill switch is a feature flag used as an emergency circuit breaker. When something goes wrong in production, disable the flag and the feature turns off instantly. No code change, no deploy.

Pattern​

Wrap any risky feature path in a flag check. If the flag is off, fall back to the safe path:

const flags = await evaluate(user);

if (flags['new_payment_processor']?.enabled) {
  await processWithNewProvider(order);
} else {
  await processWithLegacyProvider(order);
}

When the new payment processor starts throwing errors, disable the flag. Traffic immediately returns to the legacy path.

Setting up a kill switch flag​

Create the flag with enabled: true and rollout_percentage: 100:

curl -X POST .../flags \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "New Payment Processor",
    "key": "new_payment_processor",
    "enabled": true,
    "rollout_percentage": 100
  }'

Triggering the switch​

When you need to kill the feature, hit the toggle endpoint:

curl -X POST https://api.ffs.adarshrust.com/api/projects/$PROJECT_ID/environments/$ENV_ID/flags/$FLAG_ID/toggle \
  -H "Authorization: Bearer $TOKEN"

Or click the toggle in the dashboard. The change takes effect on the next evaluation call, within seconds for most applications.

Re-enabling​

Toggle it back on when the underlying issue is resolved. Re-enable at 5% first to confirm the fix holds before going back to 100%.

Kill switch vs rollout: key difference​

The global enabled: false switch overrides everything — rules, rollout, all of it. Use it for true emergencies. Note: setting rollout_percentage: 0 does not disable the flag. When rollout is 0 and enabled is true, the flag evaluates to true for all users. The toggle (which flips enabled) is the correct kill switch.

Add it to your runbooks​

Kill switches are only useful if engineers know they exist and know how to trigger them under pressure. Add a section to your incident runbooks:

If [service X] is degraded:
1. Log in to ffs.adarshrust.com
2. Navigate to [project] → production
3. Disable the [flag name] flag
4. Confirm error rate drops in Datadog