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LaunchDarkly

This learning is based on a real production incident that affected customers.

Let’s examine how to properly manage LaunchDarkly flags and their rules, using a real-world example where incorrect flag configuration led to disabled blockchain withdrawals.

The Scenario

A developer updated LaunchDarkly flags to include a new segment. While the changes were tested and reviewed, they inadvertently affected blockchain withdrawal permissions because:

  1. Rules were added as or when they should have been and.
  2. Limited access to LaunchDarkly configuration meant few engineers could review.
  3. Manual testing didn’t catch all the configuration issues.
  4. No standardized practices for flag rule management.
  5. No infrastructure for proper review of flag changes.
launchdarkly.ts
const rules = [
  {
    segment: 'new_segment',
    value: true
  },
  {
    user: 'specific_user',
    value: true
  },
]

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Key Lessons

1. Understanding Flag Rules

  • Know how rules combine (AND vs OR)
  • Document the intended behavior
  • Verify rule interactions
  • Test all affected features

2. Review Process

  • Ensure multiple engineers can review changes
  • Use infrastructure as code for flag management
  • Document review requirements
  • Consider automated testing

3. Best Practices

  • Standardize flag management processes
  • Create clear documentation
  • Implement proper access controls
  • Use infrastructure as code

Tips for Reviewers

1. Ask Rule-Focused Questions

  • How should these rules combine?
  • What’s the intended behavior?
  • How do we verify the logic?
  • Example: “Should these rules work as AND or OR?“

2. Verify Testing Strategy

  • Are all affected features tested?
  • Is there automated testing?
  • Do we have proper test coverage?
  • Example: “Let’s add tests for the flag configurations”

3. Document Dependencies

  • List affected features
  • Note required permissions
  • Document rule interactions
  • Example: “Blockchain withdrawals depend on these flags”

Common Pitfalls to Avoid

1. Focusing Only on Changes

  • ❌ “The flag changes look good, let’s merge.”
  • ✅ “The flag changes look good, but let’s verify the rule logic.”

2. Assuming Manual Testing is Sufficient

  • ❌ “We tested it manually, so it’s good.”
  • ✅ “We tested it manually, but let’s add automated tests.”

3. Missing Rule Interactions

  • ❌ “These are just flag changes.”
  • ✅ “These flag changes affect multiple features, let’s verify them all.”

Remember: A good code review considers both the technical implementation and the business logic. Understanding how flag rules work and their impact helps prevent issues before they reach production!

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