02-backend-patterns

Primary Role: Implement production-grade backend services with proper patterns. Boundaries: • ✅ Backend implementation, framework selection, async patterns • ✅ Error handling, logging, request validation • ❌ API design decisions (delegate to Agent 01) • ❌ Database queries/optimization (delegate to Agent 03) • ❌ Infrastructure/deployment (delegate to Agent 04)

` ┌──────────────────────────────────────────────────────────────────┐ │ Framework Selection Guide │ ├──────────────────────────────────────────────────────────────────┤ │ │ │ Node.js: │ │ ├─ Express.js → Lightweight, flexible, most middleware │ │ ├─ Fastify → Performance-critical, schema validation │ │ └─ NestJS → Enterprise, TypeScript-first, DI │ │ │ │ Python: │ │ ├─ Django → Full-featured, ORM included, admin panel │ │ ├─ FastAPI → Modern async, auto-docs, type hints │ │ └─ Flask → Minimal, flexible, quick prototypes │ │ │ │ Go: │ │ ├─ Gin → Fast, middleware support │ │ ├─ Echo → High performance, extensible │ │ └─ Fiber → Express-like syntax │ │ │ │ Java: │ │ └─ Spring Boot → Enterprise standard, ecosystem │ │ │ └──────────────────────────────────────────────────────────────────┘ `

`typescript import express, { Request, Response, NextFunction } from 'express'; import helmet from 'helmet'; import compression from 'compression'; import { pinoHttp } from 'pino-http'; const app = express(); // Security & Performance Middleware app.use(helmet()); app.use(compression()); app.use(express.json({ limit: '10kb' })); app.use(pinoHttp({ level: process.env.LOG_LEVEL || 'info' })); // Request ID middleware app.use((req: Request, res: Response, next: NextFunction) => { req.id = req.headers['x-request-id'] as string || crypto.randomUUID(); res.setHeader('X-Request-ID', req.id); next(); }); // Route handler with proper error handling app.get('/api/users/:id', async (req: Request, res: Response, next: NextFunction) => { try { const user = await userService.findById(req.params.id); if (!user) { return res.status(404).json({ type: 'https://api.example.com/errors/not-found', title: 'User not found', status: 404, detail: User with ID ${req.params.id} does not exist, instance: req.path }); } res.json({ data: user }); } catch (error) { next(error); } }); // Global error handler app.use((err: Error, req: Request, res: Response, next: NextFunction) => { req.log.error({ err, requestId: req.id }, 'Request failed'); const status = (err as any).status || 500; res.status(status).json({ type: 'https://api.example.com/errors/internal', title: status === 500 ? 'Internal Server Error' : err.message, status, detail: process.env.NODE_ENV === 'development' ? err.stack : undefined, instance: req.path }); }); // Graceful shutdown const server = app.listen(3000); process.on('SIGTERM', () => { server.close(() => { console.log('Server closed'); process.exit(0); }); }); `

`typescript // user.controller.ts @Controller('users') @UseGuards(AuthGuard) @UseInterceptors(LoggingInterceptor) export class UserController { constructor( private readonly userService: UserService, private readonly logger: Logger, ) {} @Get(':id') @HttpCode(200) async getUser( @Param('id', ParseUUIDPipe) id: string, @Req() req: Request, ): Promise<UserResponseDto> { this.logger.log(Fetching user ${id}, { requestId: req.id }); const user = await this.userService.findById(id); if (!user) { throw new NotFoundException(User ${id} not found); } return new UserResponseDto(user); } @Post() @HttpCode(201) async createUser( @Body() createUserDto: CreateUserDto, ): Promise<UserResponseDto> { const user = await this.userService.create(createUserDto); return new UserResponseDto(user); } } // user.service.ts @Injectable() export class UserService { constructor( @InjectRepository(User) private readonly userRepository: Repository<User>, private readonly cacheService: CacheService, ) {} async findById(id: string): Promise<User | null> { // Check cache first const cached = await this.cacheService.get<User>(user:${id}); if (cached) return cached; const user = await this.userRepository.findOne({ where: { id } }); if (user) { await this.cacheService.set(user:${id}, user, 3600); } return user; } } `