Percentage Rollouts

Percentage rollout uses murmurhash3 for deterministic bucketing: the same user always gets the same result for a given flag, ensuring a consistent experience across requests.

Evaluation Priority

When isEnabled() is called, flags are evaluated through a 6-layer cascade. The first matching layer wins:

Priority	Layer	Description
1	Archived	If the flag has archivedAt set, evaluation always returns false
2	User override	Override matching the current userId (most specific)
3	Tenant override	Override matching the current tenantId
4	Environment override	Override matching the current environment
5	Percentage rollout	Deterministic hash of flagKey + userId (or tenantId) mod 100
6	Global default	The flag's enabled field

CRUD Operations

FeatureFlagService also exposes methods for managing flags programmatically:

// Create a flag
const flag = await this.flags.create({
  key: 'NEW_FEATURE',
  description: 'Enables the new feature',
  enabled: false,
  percentage: 0,
});

// Update a flag
await this.flags.update('NEW_FEATURE', {
  enabled: true,
  percentage: 50,
});

// Archive a flag (soft delete -- evaluations return false)
await this.flags.archive('OLD_FEATURE');

// List all active (non-archived) flags
const allFlags = await this.flags.findAll();

// Manually invalidate the cache
this.flags.invalidateCache();

Caching

Built-in caching is controlled by the cacheTtlMs option (default 30000 ms). Set to 0 to disable caching. You can manually invalidate the cache at any time:

this.flags.invalidateCache();

Events

Enable event emission to observe flag lifecycle changes. Requires @nestjs/event-emitter as an optional peer dependency.

Important: You must import EventEmitterModule.forRoot() in your app module. The feature-flag module reuses the same EventEmitter2 singleton that NestJS manages, so @OnEvent() listeners work out of the box.

Setup

import { EventEmitterModule } from '@nestjs/event-emitter';

@Module({
  imports: [
    EventEmitterModule.forRoot(),   // must be imported
    FeatureFlagModule.forRoot({
      environment: 'production',
      prisma: prismaService,
      emitEvents: true,
    }),
  ],
})
export class AppModule {}

Event types

Event constant	Event string	Payload type
FeatureFlagEvents.EVALUATED	feature-flag.evaluated	FlagEvaluatedEvent
FeatureFlagEvents.CREATED	feature-flag.created	FlagMutationEvent
FeatureFlagEvents.UPDATED	feature-flag.updated	FlagMutationEvent
FeatureFlagEvents.ARCHIVED	feature-flag.archived	FlagMutationEvent
FeatureFlagEvents.OVERRIDE_SET	feature-flag.override.set	FlagOverrideEvent
FeatureFlagEvents.OVERRIDE_REMOVED	feature-flag.override.removed	FlagOverrideEvent
FeatureFlagEvents.CACHE_INVALIDATED	feature-flag.cache.invalidated	{}

Listening to events

import { OnEvent } from '@nestjs/event-emitter';
import { FeatureFlagEvents, FlagEvaluatedEvent } from '@nestarc/feature-flag';

@Injectable()
export class FlagAuditListener {
  @OnEvent(FeatureFlagEvents.EVALUATED)
  handleEvaluation(event: FlagEvaluatedEvent) {
    console.log(`Flag ${event.flagKey} = ${event.result} (source: ${event.source})`);
  }
}