Real World Example

Apps like Swiggy and Paytm launch features to a small percentage of users first. A guard reads a @Feature() flag from the handler and checks it against a flag service; disabled features answer 404 so they stay hidden.

src/flags/feature.guard.ts

import { CanActivate, ExecutionContext, Injectable, NotFoundException } from '@nestjs/common';
import { Reflector } from '@nestjs/core';
import { FlagsService } from './flags.service';

export const Feature = Reflector.createDecorator<string>();

@Injectable()
export class FeatureGuard implements CanActivate {
  constructor(private readonly reflector: Reflector, private readonly flags: FlagsService) {}

  async canActivate(context: ExecutionContext): Promise<boolean> {
    const flag = this.reflector.get(Feature, context.getHandler());
    if (!flag) return true;
    const { user } = context.switchToHttp().getRequest();
    if (!(await this.flags.isEnabled(flag, user?.id))) throw new NotFoundException();
    return true;
  }
}

// orders.controller.ts
// @Post(':id/split-payment')
// @Feature('split-payment')
// splitPayment(...) {}