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Version: Config V2

Using ConfigCat's OpenFeature Provider in NestJS

Getting started​

OpenFeature offers a NestJS SDK to streamline the use of OpenFeature in NestJS applications. This SDK is built on top of the OpenFeature JavaScript Node.js SDK.

Since ConfigCat implements a provider for the Node.js SDK, you can use ConfigCat with the OpenFeature NestJS SDK, as explained below.

1. Install the NestJS SDK and the ConfigCat provider​

npm i @openfeature/nestjs-sdk @openfeature/config-cat-provider

2. Initialize the NestJS SDK​

The ConfigCatProvider.create() function takes the SDK key, the desired polling mode (Auto Polling is recommended for this kind of application) and an optional options argument containing additional configuration options for the underlying ConfigCat client:

import { Module } from '@nestjs/common';
import { OpenFeatureModule } from '@openfeature/nestjs-sdk';
import { ConfigCatProvider } from '@openfeature/config-cat-provider';
import { createConsoleLogger, LogLevel, PollingMode } from 'configcat-js-ssr';
import { FeatureFlagService } from './_services/feature-flag.service';

const configCatProvider = ConfigCatProvider.create('#YOUR-SDK-KEY#', PollingMode.AutoPoll, {
  // Specify options for the underlying ConfigCat client
  logger: createConsoleLogger(LogLevel.Info),
  setupHooks: (hooks) => hooks.on('clientReady', () => console.log('Client is ready!')),
  // ...
});

@Module({
  imports: [
    OpenFeatureModule.forRoot({
      defaultProvider: configCatProvider,
      // Set the context for your evaluations
      contextFactory: (request) => ({ targetingKey: 'user-1' })
    }),
  ],
  controllers: [/* ... */],
  providers: [FeatureFlagService],
})
export class AppModule {}

3. Use your feature flag​

To improve maintainability and testability, it's usually a good idea not to directly evaluate your feature flags, but to wrap them in a service:

import { Injectable } from '@nestjs/common';
import { OpenFeatureClient, Client } from '@openfeature/nestjs-sdk';

@Injectable()
export class FeatureFlagService {
  constructor(
    @OpenFeatureClient() private readonly defaultClient: Client,
  ) {
  }

  public async isAwesomeFeatureEnabled() {
    return await this.defaultClient.getBooleanValue('isAwesomeFeatureEnabled', false);
  }
}

Then you can inject this service into your controllers and use it to evaluate feature flags:

import { Controller, Get } from '@nestjs/common';
import { FeatureFlagService } from './_services/feature-flag.service';

@Controller()
export class AppController {
  constructor(
    private readonly featureFlagService: FeatureFlagService
  ) {}

  @Get()
  async welcome() {
    return await this.featureFlagService.isAwesomeFeatureEnabled()
      ? 'Awesome feature is enabled!'
      : 'Awesome feature is disabled.';
  }
}

Evaluation Context​

An evaluation context in the OpenFeature specification is a container for arbitrary contextual data that can be used as a basis for feature flag evaluation.

You can find here how the ConfigCat provider translates these evaluation contexts to ConfigCat User Objects.

Advanced features​

Read the documentation of the OpenFeature NestJS SDK to learn more about the advanced capabilities of the SDK.

Look under the hood​