Backend Patterns Skill

Purpose

Implement production-ready backend services with proper patterns.

Framework Selection

Error Handling Pattern

// Error hierarchy
class AppError extends Error {
  constructor(
    public code: string,
    message: string,
    public status: number = 500,
  ) {
    super(message);
    this.name = this.constructor.name;
  }
}

class NotFoundError extends AppError {
  constructor(resource: string, id: string) {
    super('NOT_FOUND', `${resource} ${id} not found`, 404);
  }
}

class ValidationError extends AppError {
  constructor(public errors: { field: string; message: string }[]) {
    super('VALIDATION_ERROR', 'Validation failed', 400);
  }
}

// Global handler
app.use((err, req, res, next) => {
  if (err instanceof AppError) {
    return res.status(err.status).json({
      type: `https://api.example.com/errors/${err.code.toLowerCase()}`,
      title: err.message,
      status: err.status,
    });
  }
  logger.error(err);
  res.status(500).json({ title: 'Internal error', status: 500 });
});

Middleware Pattern

// Request ID middleware
const requestId = (req, res, next) => {
  req.id = req.headers['x-request-id'] || crypto.randomUUID();
  res.setHeader('X-Request-ID', req.id);
  next();
};

// Logging middleware
const requestLogger = (req, res, next) => {
  const start = Date.now();
  res.on('finish', () => {
    logger.info({
      method: req.method,
      path: req.path,
      status: res.statusCode,
      duration: Date.now() - start,
      requestId: req.id,
    });
  });
  next();
};

// Error boundary
const asyncHandler = (fn) => (req, res, next) => {
  Promise.resolve(fn(req, res, next)).catch(next);
};

Validation Pattern

import { z } from 'zod';

const CreateUserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1).max(100),
  password: z.string().min(12),
});

function validate(schema) {
  return (req, res, next) => {
    const result = schema.safeParse(req.body);
    if (!result.success) {
      throw new ValidationError(
        result.error.issues.map(i => ({
          field: i.path.join('.'),
          message: i.message,
        }))
      );
    }
    req.body = result.data;
    next();
  };
}

app.post('/users', validate(CreateUserSchema), createUser);

Graceful Shutdown

const server = app.listen(3000);

async function shutdown() {
  console.log('Shutting down...');

  // Stop accepting new connections
  server.close();

  // Close database connections
  await db.destroy();

  // Close Redis
  await redis.quit();

  process.exit(0);
}

process.on('SIGTERM', shutdown);
process.on('SIGINT', shutdown);

Unit Test Template

describe('Backend Pattern: Error Handling', () => {
  it('should return proper error response', async () => {
    const res = await request(app)
      .get('/users/invalid-id')
      .expect(404);

    expect(res.body).toMatchObject({
      type: expect.stringContaining('not-found'),
      status: 404,
    });
  });
});

Troubleshooting

Quality Checklist

• Error handling standardized
• Request logging enabled
• Input validation implemented
• Graceful shutdown configured
• Health check endpoint