Feature flag (feature toggle)

A feature flag (also called feature toggle, feature switch, or feature flipper) is a software development technique that allows you to enable or disable features in your application without deploying new code. By wrapping features in conditional statements, teams can control who sees what, test features with limited audiences, and quickly disable problematic features without emergency deployments.

Why it matters

Feature flags have become essential for modern software development:

How feature flags work

Basic implementation

\\\`javascript

if (featureFlags.isEnabled('new-checkout-flow')) {

showNewCheckout();

} else {

showOldCheckout();

}

\\\`

Flag evaluation

Flags can be evaluated based on:

Types of feature flags

Release flags

Control when features become available:

Lifecycle: Temporary (remove after full rollout)

Experiment flags

Support A/B testing and experimentation:

Lifecycle: Temporary (remove after experiment concludes)

Operational flags

Control operational aspects of the system:

Lifecycle: Long-lived or permanent

Permission flags

Control feature access by user type:

Lifecycle: Permanent (often replaced by entitlements system)

Feature flag use cases

Gradual rollout

Release a feature to increasing percentages:

Monitor metrics at each stage; roll back if issues arise.

Canary releases

Test in production with minimal risk:

A/b testing

Compare variants scientifically:

Beta programs

Give early access to engaged users:

Kill switches

Disable features instantly:

Trunk-based development

All developers work on main branch:

Feature flag best practices

Naming conventions

Use clear, consistent names:

Default states

Choose defaults carefully:

Flag documentation

Maintain flag documentation:

Regular cleanup

Remove obsolete flags:

Testing

Test both states:

Monitoring

Monitor flag-related metrics:

Feature flag risks

Flag proliferation

Too many flags create complexity:

Mitigation: Regular cleanup, flag limits, ownership requirements

Configuration drift

Production differs from other environments:

Mitigation: Environment-specific testing, flag parity

Testing challenges

Multiple flag combinations multiply test cases:

Mitigation: Test critical paths, use default states, limit flag interactions

Performance impact

Flag evaluation can add latency:

Mitigation: Local caching, simple rules, performance monitoring

Feature flag architecture

Client-side flags

Evaluated in the browser/app:

Server-side flags

Evaluated on the server:

Hybrid approach

Combine both:

Feature flag tools

Self-hosted

Managed services

Klero integration

Klero complements feature flags by: