FEAT - Transaction Challenge Solution - Joshua Navarro

.gitignore

@@ -102,3 +102,6 @@ dist
 
 # TernJS port file
 .tern-port
+
+# WebStorm IDE files
+.idea

anti-fraud-service/.dockerignore

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+node_modules/
+.dist/

anti-fraud-service/.prettierrc

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+{
+  "singleQuote": true,
+  "trailingComma": "all",
+  "printWidth": 120
+}

anti-fraud-service/Dockerfile

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+FROM node:22-alpine as development
+
+WORKDIR /app
+
+COPY package.json yarn.lock ./
+
+RUN yarn install --frozen-lockfile
+
+COPY . .
+
+RUN yarn build
+
+FROM node:22-alpine AS production
+
+ARG NODE_ENV=production
+ENV NODE_ENV=${NODE_ENV}
+
+WORKDIR /app
+
+COPY package.json yarn.lock ./
+
+RUN yarn install --frozen-lockfile --production
+
+COPY . .
+
+COPY --from=development /app/dist ./dist
+
+EXPOSE 3000
+
+CMD ["yarn", "run", "start:prod"]

anti-fraud-service/README.md

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+# Anti-Fraude Service
+
+## Description
+
+The Anti-Fraud Service is responsible for validating financial transactions based on predefined business rules. It is
+part of a distributed, event-driven architecture and communicates asynchronously with other services using Kafka.
+
+## Features
+
+- Listen for `transaction_created` events to validate transactions.
+- Applies anti-fraud validates logic (e.g., rejecting transaction over a certain threshold).
+- Emits either `approve_transaction` or `reject_transaction` events based on validation results.
+
+## Tech Stack
+
+- Node.js - using the NestJS framework
+- Kafka - for event-driven communication
+- Jest - for unit testing
+
+## Validation Rules
+
+- Transactions with a `value > 1000` are automatically **rejected**.
+- All other transactions are **approved**.
+
+## Kafka Topics
+
+- **Produces:**
+    - `approve_transaction` - emitted to update the status of a transaction to `APPROVED`.
+    - `reject_transaction` - emitted to update the status of a transaction to `REJECTED`.
+- **Consumes**:
+    - `transaction_created` - Received when a new transaction is created, to validate it.
+
+## Environment Variables
+
+| Variable                   | Description                  |
+|----------------------------|------------------------------|
+| `KAFKA_BROKER_0`           | Kafka broker address         |
+| `KAFKA_CLIENT_ID_CONSUMER` | Kafka client ID for consumer |
+| `KAFKA_CLIENT_ID_PRODUCER` | Kafka client ID for producer |
+| `KAFKA_GROUP_ID`           | Kafka consumer group ID      |
+
+## Running the Service
+
+To run the service in **development** mode:
+
+```bash
+yarn start:dev
+```
+
+To run the service in **production** mode:
+
+```bash
+yarn start:prod
+```
+
+## Running Tests
+
+To execute the test suite:
+
+```bash
+yarn test
+```

anti-fraud-service/eslint.config.mjs

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+// @ts-check
+import eslint from '@eslint/js';
+import eslintPluginPrettierRecommended from 'eslint-plugin-prettier/recommended';
+import globals from 'globals';
+import tseslint from 'typescript-eslint';
+
+export default tseslint.config(
+  {
+    ignores: ['eslint.config.mjs'],
+  },
+  eslint.configs.recommended,
+  ...tseslint.configs.recommendedTypeChecked,
+  eslintPluginPrettierRecommended,
+  {
+    languageOptions: {
+      globals: {
+        ...globals.node,
+        ...globals.jest,
+      },
+      sourceType: 'commonjs',
+      parserOptions: {
+        projectService: true,
+        tsconfigRootDir: import.meta.dirname,
+      },
+    },
+  },
+  {
+    rules: {
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/no-floating-promises': 'off',
+      '@typescript-eslint/no-unsafe-argument': 'warn',
+      '@typescript-eslint/unbound-method': 'off',
+    },
+  },
+);

anti-fraud-service/nest-cli.json

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+{
+  "$schema": "https://json.schemastore.org/nest-cli",
+  "collection": "@nestjs/schematics",
+  "sourceRoot": "src",
+  "compilerOptions": {
+    "deleteOutDir": true
+  }
+}

anti-fraud-service/package.json

@@ -0,0 +1,84 @@
+{
+  "name": "anti-fraud-service",
+  "version": "0.0.1",
+  "description": "",
+  "author": "Joshua Navarro <joshua.navarro35@gmail.com>",
+  "engines": {
+    "node": ">=22.0.0"
+  },
+  "private": true,
+  "license": "UNLICENSED",
+  "scripts": {
+    "build": "nest build",
+    "format": "prettier --write \"src/**/*.ts\" \"test/**/*.ts\"",
+    "start": "nest start",
+    "start:dev": "nest start --watch",
+    "start:debug": "nest start --debug --watch",
+    "start:prod": "node dist/main",
+    "lint": "eslint \"{src,apps,libs,test}/**/*.ts\" --fix",
+    "test": "jest",
+    "test:watch": "jest --watch",
+    "test:cov": "jest --coverage",
+    "test:debug": "node --inspect-brk -r tsconfig-paths/register -r ts-node/register node_modules/.bin/jest --runInBand",
+    "test:e2e": "jest --config ./test/jest-e2e.json"
+  },
+  "dependencies": {
+    "@nestjs/common": "^11.0.1",
+    "@nestjs/config": "^4.0.2",
+    "@nestjs/core": "^11.0.1",
+    "@nestjs/microservices": "^11.1.3",
+    "@nestjs/platform-express": "^11.0.1",
+    "joi": "^17.13.3",
+    "kafkajs": "^2.2.4",
+    "reflect-metadata": "^0.2.2",
+    "rxjs": "^7.8.1"
+  },
+  "devDependencies": {
+    "@eslint/eslintrc": "^3.2.0",
+    "@eslint/js": "^9.18.0",
+    "@nestjs/cli": "^11.0.0",
+    "@nestjs/schematics": "^11.0.0",
+    "@nestjs/testing": "^11.0.1",
+    "@swc/cli": "^0.6.0",
+    "@swc/core": "^1.10.7",
+    "@types/express": "^5.0.0",
+    "@types/jest": "^29.5.14",
+    "@types/node": "^22.10.7",
+    "@types/supertest": "^6.0.2",
+    "eslint": "^9.18.0",
+    "eslint-config-prettier": "^10.0.1",
+    "eslint-plugin-prettier": "^5.2.2",
+    "globals": "^16.0.0",
+    "jest": "^29.7.0",
+    "prettier": "^3.4.2",
+    "source-map-support": "^0.5.21",
+    "supertest": "^7.0.0",
+    "ts-jest": "^29.2.5",
+    "ts-loader": "^9.5.2",
+    "ts-node": "^10.9.2",
+    "tsconfig-paths": "^4.2.0",
+    "typescript": "^5.7.3",
+    "typescript-eslint": "^8.20.0"
+  },
+  "jest": {
+    "moduleFileExtensions": [
+      "js",
+      "json",
+      "ts"
+    ],
+    "rootDir": "src",
+    "testRegex": ".*\\.spec\\.ts$",
+    "transform": {
+      "^.+\\.(t|j)s$": "ts-jest"
+    },
+    "collectCoverageFrom": [
+      "**/*.(t|j)s"
+    ],
+    "coverageDirectory": "../coverage",
+    "testEnvironment": "node",
+    "coveragePathIgnorePatterns": [
+      ".*\\.module\\.ts$",
+      ".*main\\.ts$"
+    ]
+  }
+}

anti-fraud-service/src/anti-fraud/anti-fraud.controller.spec.ts

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+import AntiFraudService from './anti-fraud.service';
+import TransactionDto from './dtos/transaction.dto';
+import AntiFraudController from './anti-fraud.controller';
+import { Test } from '@nestjs/testing';
+
+describe('AntiFraudController', () => {
+  let controller: AntiFraudController;
+  let mockAntiFraudService: jest.Mocked<AntiFraudService>;
+
+  beforeEach(async () => {
+    mockAntiFraudService = {
+      validateTransaction: jest.fn(),
+      emitTransactionStatusUpdate: jest.fn(),
+    } as unknown as jest.Mocked<AntiFraudService>;
+    const module = await Test.createTestingModule({
+      controllers: [AntiFraudController],
+      providers: [{ provide: AntiFraudService, useValue: mockAntiFraudService }],
+    }).compile();
+    controller = module.get<AntiFraudController>(AntiFraudController);
+  });
+
+  describe('transactionCreated', () => {
+    it('should validate transaction and emit status update', () => {
+      const transaction: TransactionDto = new TransactionDto('trx-id-01', 1200);
+
+      mockAntiFraudService.validateTransaction.mockReturnValue(true);
+
+      controller.transactionCreated(transaction);
+      expect(mockAntiFraudService.validateTransaction).toHaveBeenCalledWith(transaction);
+      expect(mockAntiFraudService.emitTransactionStatusUpdate).toHaveBeenCalledWith(transaction.id, true);
+    });
+  });
+});

anti-fraud-service/src/anti-fraud/anti-fraud.controller.ts

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+import { Controller, Inject, Logger } from '@nestjs/common';
+import { MessagePattern, Payload } from '@nestjs/microservices';
+import TransactionDto from './dtos/transaction.dto';
+import AntiFraudService from './anti-fraud.service';
+
+@Controller()
+export default class AntiFraudController {
+  private readonly logger = new Logger(AntiFraudController.name);
+
+  constructor(@Inject() public readonly antiFraudService: AntiFraudService) {}
+
+  @MessagePattern('transaction_created')
+  transactionCreated(@Payload() transaction: TransactionDto): void {
+    this.logger.log('Transaction created event received');
+    const isFraudulent = this.antiFraudService.validateTransaction(transaction);
+
+    this.logger.log('Transaction validation completed');
+    this.antiFraudService.emitTransactionStatusUpdate(transaction.id, isFraudulent);
+  }
+}

anti-fraud-service/src/anti-fraud/anti-fraud.module.ts

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+import { Module } from '@nestjs/common';
+import AntiFraudController from './anti-fraud.controller';
+import AntiFraudService from './anti-fraud.service';
+import { ClientsModule, Transport } from '@nestjs/microservices';
+import { ANTI_FRAUD_EVENT_PRODUCER } from './tokens';
+import { ConfigService } from '@nestjs/config';
+import { KafkaConfig } from '../config/kafka.config';
+
+@Module({
+  imports: [
+    ClientsModule.registerAsync([
+      {
+        name: ANTI_FRAUD_EVENT_PRODUCER,
+        inject: [ConfigService],
+        useFactory: (configService: ConfigService) => {
+          const kafkaConfig = configService.get<KafkaConfig>('kafka');
+          return {
+            transport: Transport.KAFKA,
+            options: {
+              client: {
+                clientId: kafkaConfig?.client.producerId,
+                brokers: kafkaConfig?.brokers as string[],
+              },
+            },
+          };
+        },
+      },
+    ]),
+  ],
+  controllers: [AntiFraudController],
+  providers: [AntiFraudService],
+})
+export class AntiFraudModule {}

anti-fraud-service/src/anti-fraud/anti-fraud.service.spec.ts

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+import AntiFraudService from './anti-fraud.service';
+import { ClientKafka } from '@nestjs/microservices';
+import { Test, TestingModule } from '@nestjs/testing';
+import { ANTI_FRAUD_EVENT_PRODUCER } from './tokens';
+import TransactionDto from './dtos/transaction.dto';
+import { APPROVE_TRANSACTION_TOPIC, REJECT_TRANSACTION_TOPIC, TRANSACTION_AMOUNT_THRESHOLD } from './constants';
+
+describe('AntiFraudService', () => {
+  let service: AntiFraudService;
+  let mockClientProducer: ClientKafka;
+
+  beforeEach(async () => {
+    mockClientProducer = {
+      emit: jest.fn(),
+    } as unknown as ClientKafka;
+    const module: TestingModule = await Test.createTestingModule({
+      providers: [
+        AntiFraudService,
+        {
+          provide: ANTI_FRAUD_EVENT_PRODUCER,
+          useValue: mockClientProducer,
+        },
+      ],
+    }).compile();
+
+    service = module.get<AntiFraudService>(AntiFraudService);
+  });
+
+  describe('validateTransaction', () => {
+    it('should return true for transactions above the threshold', () => {
+      const dto: TransactionDto = { id: 'test', amount: TRANSACTION_AMOUNT_THRESHOLD + 1 };
+      expect(service.validateTransaction(dto)).toBe(true);
+    });
+
+    it('should return false for transactions below the threshold', () => {
+      const dto: TransactionDto = { id: 'test', amount: TRANSACTION_AMOUNT_THRESHOLD - 1 };
+      expect(service.validateTransaction(dto)).toBe(false);
+    });
+  });
+
+  describe('emitTransactionStatusUpdate', () => {
+    it('should emit reject transaction event for fraudulent transactions', () => {
+      const trxId = 'fraudulent-transaction';
+      service.emitTransactionStatusUpdate(trxId, true);
+      expect(mockClientProducer.emit).toHaveBeenCalledWith(REJECT_TRANSACTION_TOPIC, { transactionId: trxId });
+    });
+
+    it('should emit approve transaction event for non-fraudulent transactions', () => {
+      const trxId = 'valid-transaction';
+      service.emitTransactionStatusUpdate(trxId, false);
+      expect(mockClientProducer.emit).toHaveBeenCalledWith(APPROVE_TRANSACTION_TOPIC, { transactionId: trxId });
+    });
+  });
+});