Feature Flags vs Entitlements

Cocoar.Configuration has built-in support for two related but distinct concepts: feature flags and entitlements.

What They Are

Feature flags and entitlements are computed values, not stored values. They don't exist as their own entries in a config file — they are derived from configuration at runtime.

A flag or entitlement is a function that reads one or more configuration values and returns a result. Sometimes this is a simple pass-through (a single boolean from config), sometimes it's a computation combining multiple config sources:

// Simple: passes through a single config value
public FeatureFlag<bool> NewDashboard => () => Config.UseNewDashboard;

// Computed: combines multiple values into a decision
public FeatureFlag<UserContext, bool> BetaCheckout => user =>
    Config.BetaEnabled && Config.BetaRegions.Contains(user.Region);

The key insight: you cannot set a flag directly. You change the underlying configuration values, and the flag recomputes. This keeps the config layer as the single source of truth.

Feature Flags

Feature flags answer: "Does this code run?"

They represent temporary, operational toggles — rollouts, A/B tests, kill switches. They are owned by engineering/ops and must have an explicit expiration date.

public partial class BillingFlags : IFeatureFlags<BillingConfig>
{
    public DateTimeOffset ExpiresAt => new(2026, 6, 1, 0, 0, 0, TimeSpan.Zero);

    /// <summary>Enables the redesigned billing dashboard.</summary>
    public FeatureFlag<bool> NewDashboard => () => Config.UseNewDashboard;

    /// <summary>Gates beta checkout for specific users.</summary>
    public FeatureFlag<UserContext, bool> BetaCheckout => user => Config.BetaEnabled && user.IsBeta;
}

The partial class implements IFeatureFlags<TConfig>, and the source generator produces the constructor and Config property. Config reads IReactiveConfig<TConfig>.CurrentValue, so it always reflects the latest configuration.

After the expiration date, the flags keep working — but the health API reports Degraded, signaling that cleanup is overdue.

Entitlements

Entitlements answer: "May this actor do this?"

They represent permanent business logic — plan tiers, feature availability, permission limits. They are owned by product/business and have no expiration date.

public partial class PlanEntitlements : IEntitlements<PlanConfig>
{
    /// <summary>Maximum allowed team members.</summary>
    public Entitlement<int> MaxUsers => () => Config.UserLimit;

    /// <summary>Whether this plan can export data.</summary>
    public Entitlement<bool> CanExport => () => Config.Tier != "free";
}

The Litmus Test

A feature flag without an expiration date is an entitlement in disguise.

If you're unsure which to use, ask: will this toggle ever be removed? If yes, it's a feature flag. If it's permanent product logic, it's an entitlement.

How They Work

Both are pure functions over configuration state:

1. Each property is a delegate that reads from Config (the source-generated property backed by IReactiveConfig<T>.CurrentValue)
2. When configuration changes, the next invocation returns the new result
3. No per-request state, no caching — just a function call
4. No constructor needed — the source generator handles wiring

Why Delegates?

Flag and entitlement properties use delegate types (FeatureFlag<T>, Entitlement<T>) instead of plain properties or methods. This is a deliberate architectural constraint:

Enforced simplicity. A FeatureFlag<TResult> takes zero parameters. A FeatureFlag<TContext, TResult> takes exactly one. You cannot accidentally add extra parameters — the type system prevents it.

REST API compatibility. The REST evaluation pipeline maps directly to delegates:

• FeatureFlag<TResult> → GET /flags/{Class}/{Property} — no input needed
• FeatureFlag<TContext, TResult> → POST /flags/{Class}/{Property} — request body is deserialized, passed through a Context Resolver, and the resolved context becomes the single delegate argument

Pure functions. Flags must not inject services or call databases. All they receive is configuration (via Config) and optionally a context object. This keeps evaluation fast, deterministic, and debuggable — no hidden I/O, no async, no side effects.

Flags are pure — side effects belong in resolvers

If a flag needs data from a database or external service, that logic belongs in a Context Resolver. The resolver hydrates a rich context object; the flag makes a decision based on it.

// ✗ WRONG — DB call in a flag
BetaCheckout = async user => await db.Users.IsBeta(user.Id);

// ✓ RIGHT — Resolver fetches data, flag decides
// Resolver:
public async Task<UserContext> ResolveAsync(UserIdRequest req)
    => new UserContext(await db.Users.FindAsync(req.UserId));

// Flag: pure function
BetaCheckout = user => Config.BetaEnabled && user.IsBeta;

With Context

Both support contextual evaluation — flags and entitlements that depend on runtime context (current user, tenant, request):

/// <summary>Gates beta checkout for specific users.</summary>
public FeatureFlag<UserContext, bool> BetaCheckout => user => Config.BetaEnabled && user.IsBeta;

The context is resolved via Context Resolvers — a bridge between HTTP request data and your domain model.

When to Use Cocoar Flags vs. Dedicated Flag Services

Cocoar flags and dedicated feature flag services (LaunchDarkly, Unleash, Flagsmith, etc.) solve different problems. Understanding the trade-offs helps you pick the right tool — or use both.

Cocoar flags are a good fit when:

• Flags are derived from your own configuration — plan tiers, tenant settings, deployment environment. The flag is a pure function over config you already manage.
• You need tight integration with your config layer — flags recompute automatically when configuration changes, share the same health monitoring, and follow the same lifecycle.
• You're an ISV controlling feature rollout across your own instances — each instance has its own config, and flags reflect that instance's state.
• You want type-safe, compile-time validated flags — the source generator catches missing flags, wrong return types, and expired flags at build time.

Dedicated flag services are a better fit when:

• You need a management UI for non-developers — product managers toggling flags without code changes or deployments.
• You need percentage rollouts, A/B testing, or experimentation infrastructure — statistical analysis, cohort management, and gradual rollout are their core competency.
• You need cross-platform flag evaluation — the same flag evaluated consistently across mobile apps, web frontends, and backend services with server-side SDKs.

They can coexist. Use Cocoar flags for config-derived decisions (plan limits, tenant features, deployment-specific toggles) and a dedicated service for user-targeting and experimentation. They solve different problems and don't conflict — a Cocoar entitlement might gate whether a feature is available on a plan, while a LaunchDarkly flag controls whether that feature's new UI is shown to 10% of users.

Comparison

	Feature Flags	Entitlements
Purpose	Temporary operational toggles	Permanent business logic
Expiry	Required (ExpiresAt)	None
Health signal	Degraded when expired	None
Interface	IFeatureFlags<TConfig>	IEntitlements<TConfig>
Property type	FeatureFlag<TResult> or FeatureFlag<TContext, TResult>	Entitlement<TResult> or Entitlement<TContext, TResult>
Owned by	Engineering / Ops	Product / Business
Lifecycle	Create → roll out → expire → remove	Create → keep forever