Backend Development Patterns

Backend architecture patterns and best practices for scalable server-side applications.

Framework-Specific Guidelines

When working with specific frameworks, combine this skill with framework-specific skills:

Framework	Additional Skill	When to Use
NestJS	nestjs-best-practices skill	Modules, Controllers, Providers, Guards, Interceptors, Pipes, DI
FastAPI	fastapi-templates skill	Routes, Decorators, DI, GraphQL, Microservices
Next.js API	This skill only	Serverless API routes

NestJS Integration

When using NestJS, the patterns in this skill should be adapted to NestJS conventions:

Generic Pattern	NestJS Implementation
Repository Pattern	Use @Injectable() repositories with DI
Service Layer	Use @Injectable() services
Middleware	Use NestJS @Injectable() middleware or Guards/Interceptors
Error Handling	Use Exception Filters (@Catch())
Validation	Use Pipes with class-validator
Auth Middleware	Use Guards (@UseGuards())
Rate Limiting	Use @nestjs/throttler module

Example - NestJS Repository Pattern:

// NestJS style with dependency injection
@Injectable()
export class MarketRepository {
  constructor(
    @InjectRepository(Market)
    private marketRepo: Repository<Market>,
  ) {}

  async findAll(filters?: MarketFilters): Promise<Market[]> {
    const query = this.marketRepo.createQueryBuilder('market');

    if (filters?.status) {
      query.where('market.status = :status', { status: filters.status });
    }

    return query.getMany();
  }
}

// Inject into service
@Injectable()
export class MarketService {
  constructor(private marketRepo: MarketRepository) {}
}

Example - NestJS Error Handling:

// Exception filter instead of middleware error handler
@Catch()
export class AllExceptionsFilter implements ExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse<Response>();

    if (exception instanceof HttpException) {
      return response.status(exception.getStatus()).json({
        success: false,
        error: exception.message,
      });
    }

    return response.status(500).json({
      success: false,
      error: 'Internal server error',
    });
  }
}

Tip: For NestJS-specific decorators, modules, and advanced features (GraphQL, Microservices, WebSockets), refer to the nestjs skill for detailed documentation.

API Design Patterns

RESTful API Structure

// ✅ Resource-based URLs
GET    /api/markets                 # List resources
GET    /api/markets/:id             # Get single resource
POST   /api/markets                 # Create resource
PUT    /api/markets/:id             # Replace resource
PATCH  /api/markets/:id             # Update resource
DELETE /api/markets/:id             # Delete resource

// ✅ Query parameters for filtering, sorting, pagination
GET /api/markets?status=active&sort=volume&limit=20&offset=0

Repository Pattern

// Abstract data access logic
interface MarketRepository {
  findAll(filters?: MarketFilters): Promise<Market[]>;
  findById(id: string): Promise<Market | null>;
  create(data: CreateMarketDto): Promise<Market>;
  update(id: string, data: UpdateMarketDto): Promise<Market>;
  delete(id: string): Promise<void>;
}

class SupabaseMarketRepository implements MarketRepository {
  async findAll(filters?: MarketFilters): Promise<Market[]> {
    let query = supabase.from('markets').select('*');

    if (filters?.status) {
      query = query.eq('status', filters.status);
    }

    if (filters?.limit) {
      query = query.limit(filters.limit);
    }

    const { data, error } = await query;

    if (error) throw new Error(error.message);
    return data;
  }

  // Other methods...
}

Service Layer Pattern

// Business logic separated from data access
class MarketService {
  constructor(private marketRepo: MarketRepository) {}

  async searchMarkets(query: string, limit: number = 10): Promise<Market[]> {
    // Business logic
    const embedding = await generateEmbedding(query);
    const results = await this.vectorSearch(embedding, limit);

    // Fetch full data
    const markets = await this.marketRepo.findByIds(results.map((r) => r.id));

    // Sort by similarity
    return markets.sort((a, b) => {
      const scoreA = results.find((r) => r.id === a.id)?.score || 0;
      const scoreB = results.find((r) => r.id === b.id)?.score || 0;
      return scoreA - scoreB;
    });
  }

  private async vectorSearch(embedding: number[], limit: number) {
    // Vector search implementation
  }
}

Middleware Pattern

// Request/response processing pipeline
export function withAuth(handler: NextApiHandler): NextApiHandler {
  return async (req, res) => {
    const token = req.headers.authorization?.replace('Bearer ', '');

    if (!token) {
      return res.status(401).json({ error: 'Unauthorized' });
    }

    try {
      const user = await verifyToken(token);
      req.user = user;
      return handler(req, res);
    } catch (error) {
      return res.status(401).json({ error: 'Invalid token' });
    }
  };
}

// Usage
export default withAuth(async (req, res) => {
  // Handler has access to req.user
});

Database Patterns

Query Optimization

// ✅ GOOD: Select only needed columns
const { data } = await supabase
  .from('markets')
  .select('id, name, status, volume')
  .eq('status', 'active')
  .order('volume', { ascending: false })
  .limit(10);

// ❌ BAD: Select everything
const { data } = await supabase.from('markets').select('*');

N+1 Query Prevention

// ❌ BAD: N+1 query problem
const markets = await getMarkets();
for (const market of markets) {
  market.creator = await getUser(market.creator_id); // N queries
}

// ✅ GOOD: Batch fetch
const markets = await getMarkets();
const creatorIds = markets.map((m) => m.creator_id);
const creators = await getUsers(creatorIds); // 1 query
const creatorMap = new Map(creators.map((c) => [c.id, c]));

markets.forEach((market) => {
  market.creator = creatorMap.get(market.creator_id);
});

Transaction Pattern

async function createMarketWithPosition(
  marketData: CreateMarketDto,
  positionData: CreatePositionDto
) {
  // Use Supabase transaction
  const { data, error } = await supabase.rpc('create_market_with_position', {
    market_data: marketData,
    position_data: positionData
  })

  if (error) throw new Error('Transaction failed')
  return data
}

// SQL function in Supabase
CREATE OR REPLACE FUNCTION create_market_with_position(
  market_data jsonb,
  position_data jsonb
)
RETURNS jsonb
LANGUAGE plpgsql
AS $$
BEGIN
  -- Start transaction automatically
  INSERT INTO markets VALUES (market_data);
  INSERT INTO positions VALUES (position_data);
  RETURN jsonb_build_object('success', true);
EXCEPTION
  WHEN OTHERS THEN
    -- Rollback happens automatically
    RETURN jsonb_build_object('success', false, 'error', SQLERRM);
END;
$$;

Caching Strategies

Redis Caching Layer

class CachedMarketRepository implements MarketRepository {
  constructor(
    private baseRepo: MarketRepository,
    private redis: RedisClient,
  ) {}

  async findById(id: string): Promise<Market | null> {
    // Check cache first
    const cached = await this.redis.get(`market:${id}`);

    if (cached) {
      return JSON.parse(cached);
    }

    // Cache miss - fetch from database
    const market = await this.baseRepo.findById(id);

    if (market) {
      // Cache for 5 minutes
      await this.redis.setex(`market:${id}`, 300, JSON.stringify(market));
    }

    return market;
  }

  async invalidateCache(id: string): Promise<void> {
    await this.redis.del(`market:${id}`);
  }
}

Cache-Aside Pattern

async function getMarketWithCache(id: string): Promise<Market> {
  const cacheKey = `market:${id}`;

  // Try cache
  const cached = await redis.get(cacheKey);
  if (cached) return JSON.parse(cached);

  // Cache miss - fetch from DB
  const market = await db.markets.findUnique({ where: { id } });

  if (!market) throw new Error('Market not found');

  // Update cache
  await redis.setex(cacheKey, 300, JSON.stringify(market));

  return market;
}

Error Handling Patterns

Centralized Error Handler

class ApiError extends Error {
  constructor(
    public statusCode: number,
    public message: string,
    public isOperational = true,
  ) {
    super(message);
    Object.setPrototypeOf(this, ApiError.prototype);
  }
}

export function errorHandler(error: unknown, req: Request): Response {
  if (error instanceof ApiError) {
    return NextResponse.json(
      {
        success: false,
        error: error.message,
      },
      { status: error.statusCode },
    );
  }

  if (error instanceof z.ZodError) {
    return NextResponse.json(
      {
        success: false,
        error: 'Validation failed',
        details: error.errors,
      },
      { status: 400 },
    );
  }

  // Log unexpected errors
  console.error('Unexpected error:', error);

  return NextResponse.json(
    {
      success: false,
      error: 'Internal server error',
    },
    { status: 500 },
  );
}

// Usage
export async function GET(request: Request) {
  try {
    const data = await fetchData();
    return NextResponse.json({ success: true, data });
  } catch (error) {
    return errorHandler(error, request);
  }
}

Retry with Exponential Backoff

async function fetchWithRetry<T>(
  fn: () => Promise<T>,
  maxRetries = 3,
): Promise<T> {
  let lastError: Error;

  for (let i = 0; i < maxRetries; i++) {
    try {
      return await fn();
    } catch (error) {
      lastError = error as Error;

      if (i < maxRetries - 1) {
        // Exponential backoff: 1s, 2s, 4s
        const delay = Math.pow(2, i) * 1000;
        await new Promise((resolve) => setTimeout(resolve, delay));
      }
    }
  }

  throw lastError!;
}

// Usage
const data = await fetchWithRetry(() => fetchFromAPI());

Authentication & Authorization

JWT Token Validation

import jwt from 'jsonwebtoken';

interface JWTPayload {
  userId: string;
  email: string;
  role: 'admin' | 'user';
}

export function verifyToken(token: string): JWTPayload {
  try {
    const payload = jwt.verify(token, process.env.JWT_SECRET!) as JWTPayload;
    return payload;
  } catch (error) {
    throw new ApiError(401, 'Invalid token');
  }
}

export async function requireAuth(request: Request) {
  const token = request.headers.get('authorization')?.replace('Bearer ', '');

  if (!token) {
    throw new ApiError(401, 'Missing authorization token');
  }

  return verifyToken(token);
}

// Usage in API route
export async function GET(request: Request) {
  const user = await requireAuth(request);

  const data = await getDataForUser(user.userId);

  return NextResponse.json({ success: true, data });
}

Role-Based Access Control

type Permission = 'read' | 'write' | 'delete' | 'admin';

interface User {
  id: string;
  role: 'admin' | 'moderator' | 'user';
}

const rolePermissions: Record<User['role'], Permission[]> = {
  admin: ['read', 'write', 'delete', 'admin'],
  moderator: ['read', 'write', 'delete'],
  user: ['read', 'write'],
};

export function hasPermission(user: User, permission: Permission): boolean {
  return rolePermissions[user.role].includes(permission);
}

export function requirePermission(permission: Permission) {
  return (handler: (request: Request, user: User) => Promise<Response>) => {
    return async (request: Request) => {
      const user = await requireAuth(request);

      if (!hasPermission(user, permission)) {
        throw new ApiError(403, 'Insufficient permissions');
      }

      return handler(request, user);
    };
  };
}

// Usage - HOF wraps the handler
export const DELETE = requirePermission('delete')(async (
  request: Request,
  user: User,
) => {
  // Handler receives authenticated user with verified permission
  return new Response('Deleted', { status: 200 });
});

Rate Limiting

Simple In-Memory Rate Limiter

class RateLimiter {
  private requests = new Map<string, number[]>();

  async checkLimit(
    identifier: string,
    maxRequests: number,
    windowMs: number,
  ): Promise<boolean> {
    const now = Date.now();
    const requests = this.requests.get(identifier) || [];

    // Remove old requests outside window
    const recentRequests = requests.filter((time) => now - time < windowMs);

    if (recentRequests.length >= maxRequests) {
      return false; // Rate limit exceeded
    }

    // Add current request
    recentRequests.push(now);
    this.requests.set(identifier, recentRequests);

    return true;
  }
}

const limiter = new RateLimiter();

export async function GET(request: Request) {
  const ip = request.headers.get('x-forwarded-for') || 'unknown';

  const allowed = await limiter.checkLimit(ip, 100, 60000); // 100 req/min

  if (!allowed) {
    return NextResponse.json(
      {
        error: 'Rate limit exceeded',
      },
      { status: 429 },
    );
  }

  // Continue with request
}

Background Jobs & Queues

Simple Queue Pattern

class JobQueue<T> {
  private queue: T[] = [];
  private processing = false;

  async add(job: T): Promise<void> {
    this.queue.push(job);

    if (!this.processing) {
      this.process();
    }
  }

  private async process(): Promise<void> {
    this.processing = true;

    while (this.queue.length > 0) {
      const job = this.queue.shift()!;

      try {
        await this.execute(job);
      } catch (error) {
        console.error('Job failed:', error);
      }
    }

    this.processing = false;
  }

  private async execute(job: T): Promise<void> {
    // Job execution logic
  }
}

// Usage for indexing markets
interface IndexJob {
  marketId: string;
}

const indexQueue = new JobQueue<IndexJob>();

export async function POST(request: Request) {
  const { marketId } = await request.json();

  // Add to queue instead of blocking
  await indexQueue.add({ marketId });

  return NextResponse.json({ success: true, message: 'Job queued' });
}

Logging & Monitoring

Structured Logging

interface LogContext {
  userId?: string;
  requestId?: string;
  method?: string;
  path?: string;
  [key: string]: unknown;
}

class Logger {
  log(level: 'info' | 'warn' | 'error', message: string, context?: LogContext) {
    const entry = {
      timestamp: new Date().toISOString(),
      level,
      message,
      ...context,
    };

    console.log(JSON.stringify(entry));
  }

  info(message: string, context?: LogContext) {
    this.log('info', message, context);
  }

  warn(message: string, context?: LogContext) {
    this.log('warn', message, context);
  }

  error(message: string, error: Error, context?: LogContext) {
    this.log('error', message, {
      ...context,
      error: error.message,
      stack: error.stack,
    });
  }
}

const logger = new Logger();

// Usage
export async function GET(request: Request) {
  const requestId = crypto.randomUUID();

  logger.info('Fetching markets', {
    requestId,
    method: 'GET',
    path: '/api/markets',
  });

  try {
    const markets = await fetchMarkets();
    return NextResponse.json({ success: true, data: markets });
  } catch (error) {
    logger.error('Failed to fetch markets', error as Error, { requestId });
    return NextResponse.json({ error: 'Internal error' }, { status: 500 });
  }
}

Remember: Backend patterns enable scalable, maintainable server-side applications. Choose patterns that fit your complexity level.

Related Skills

• NestJS Projects: Use the nestjs-best-practices skill for framework-specific features (modules, decorators, DI, GraphQL, microservices)
• ** FastAPI Projects**: Use the fastapi-templates skill for framework-specific features (routes, decorators, DI, GraphQL, microservices)
• Database Design: Use postgres-patterns or sequelize-patterns or sqlalchemy-orm for database-specific patterns
• API Testing: Use playwright-skill for end-to-end API testing