backend-patterns

name	backend-patterns
type	reference
description	Applies production backend patterns: middleware, error handling, auth, database integration, and API design. Use when working with backend service files or when the user mentions Express, Fastify, NestJS, backend patterns, or service architecture.
paths	["**/*.ts","**/*.js","**/server.*","**/app.*","**/routes/**"]
effort	3
allowed-tools	Read, Glob, Grep, Write, Edit, Bash
context	fork
agent	backend-architect
user-invocable	true
when_to_use	When building Node.js backend services with Express, Fastify, or similar frameworks

Backend Patterns

Critical rules (non-obvious)

• Always handle async errors in Express: unhandled promise rejections crash the process; use express-async-errors or wrap every async handler
• Never trust req.body size: set limit on body-parser; default 100kb is too large for some, too small for others
• process.env access at import time: if accessed before dotenv.config(), value is undefined; call config() first in entry file
• Connection pool misconfiguration: default pool size (10) will exhaust under load; set pool.max based on (num_cores * 2) + effective_spindle_count
• res.json() after res.send(): causes "Cannot set headers after they are sent" — always return after sending response

Express: production setup

import express from "express";
import "express-async-errors";  // patches async error handling globally
import helmet from "helmet";
import { rateLimit } from "express-rate-limit";

const app = express();

app.use(helmet());
app.use(express.json({ limit: "10kb" }));
app.use(rateLimit({ windowMs: 15 * 60 * 1000, max: 100 }));

// Routes
app.use(, userRouter);
app.(, productRouter);


app.( res.().({ :  }));


app.( {
   status = err   ? err. : ;
  res.(status).({ : err. });
});