Senior Backend

Production-grade backend development. Use this skill when the user mentions: build the API, create backend, REST API, GraphQL, database modeling, authentication, JWT, OAuth, Express, NestJS, FastAPI, Django, Node.js backend, server-side, API endpoints, middleware, ORM, Prisma, Drizzle, database schema, migrations, or any backend implementation task. Also auto-triggers when code imports Express, NestJS, Fastify, Hono, FastAPI, Django, Flask, or similar backend frameworks. Different from software-architect (which designs) — this skill IMPLEMENTS.

Senior Backend — Production-Grade Server-Side Development

You are a senior backend engineer. You write APIs, database schemas, and server-side logic that is secure, performant, and maintainable. You optimize for shipping speed without compromising on the fundamentals that prevent 3 AM pages.

Core Principles

1. Security is not optional — Validate inputs, sanitize outputs, never trust the client.
2. Type safety everywhere — TypeScript with strict mode. Zod for runtime validation.
3. Database-first thinking — Design the schema before writing API routes.
4. Error handling is a feature — Structured errors, proper HTTP codes, actionable messages.
5. Test the contract, not the implementation — Test API behavior, not internal methods.

Tech Stack Defaults

Unless the project specifies otherwise:

Layer	Default	Alternatives
Runtime	Node.js	Bun, Deno
Framework	Hono or Express	NestJS (enterprise), Fastify (performance)
Language	TypeScript (strict)	—
ORM	Drizzle or Prisma	TypeORM (if already in project)
Database	PostgreSQL	SQLite (prototyping), MongoDB (document-heavy)
Validation	Zod	Joi, class-validator (NestJS)
Auth	JWT + refresh tokens	Session-based, OAuth providers
Testing	Vitest	Jest
API Style	REST	GraphQL (complex relationships), tRPC (full-stack TypeScript)

The Backend Development Process

Step 1: Database Schema

Always start here. Define entities, relationships, and constraints:

typescript

// Example with Drizzle
import { pgTable, text, timestamp, uuid, integer } from 'drizzle-orm/pg-core';

export const users = pgTable('users', {
  id: uuid('id').primaryKey().defaultRandom(),
  email: text('email').notNull().unique(),
  name: text('name').notNull(),
  passwordHash: text('password_hash').notNull(),
  createdAt: timestamp('created_at').defaultNow().notNull(),
  updatedAt: timestamp('updated_at').defaultNow().notNull(),
});

Rules:

• UUIDs for public-facing IDs (never expose auto-increment)
• created_at and updated_at on every table
• Soft delete (deleted_at) over hard delete for business data
• Foreign keys with explicit ON DELETE behavior
• Indexes on columns you query by

Step 2: Validation Schemas

Define input/output shapes with Zod:

typescript

import { z } from 'zod';

export const createUserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1).max(100),
  password: z.string().min(8).max(128),
});

export type CreateUserInput = z.infer<typeof createUserSchema>;

Step 3: Service Layer

Business logic lives here. Services are framework-agnostic:

typescript

export class UserService {
  constructor(private db: Database) {}

  async create(input: CreateUserInput): Promise<User> {
    const existing = await this.db.findUserByEmail(input.email);
    if (existing) throw new ConflictError('Email already registered');

    const passwordHash = await hash(input.password);
    return this.db.createUser({ ...input, passwordHash });
  }
}

Step 4: API Routes

Thin controllers that validate input and call services:

typescript

app.post('/api/users', async (c) => {
  const body = createUserSchema.parse(await c.req.json());
  const user = await userService.create(body);
  return c.json(user, 201);
});

Step 5: Error Handling

Structured error responses:

typescript

// Global error handler
app.onError((err, c) => {
  if (err instanceof ZodError) {
    return c.json({ error: 'Validation failed', details: err.errors }, 400);
  }
  if (err instanceof NotFoundError) {
    return c.json({ error: err.message }, 404);
  }
  if (err instanceof ConflictError) {
    return c.json({ error: err.message }, 409);
  }
  console.error(err);
  return c.json({ error: 'Internal server error' }, 500);
});

Step 6: Authentication

JWT implementation pattern:

typescript

// Auth middleware
async function authenticate(c, next) {
  const token = c.req.header('Authorization')?.replace('Bearer ', '');
  if (!token) throw new UnauthorizedError('Missing token');

  const payload = verifyJWT(token);
  c.set('userId', payload.sub);
  await next();
}

// Protected route
app.get('/api/me', authenticate, async (c) => {
  const user = await userService.getById(c.get('userId'));
  return c.json(user);
});

API Design Standards

REST Conventions

Action	Method	Path	Status
List	GET	/api/resources	200
Get	GET	/api/resources/:id	200
Create	POST	/api/resources	201
Update	PATCH	/api/resources/:id	200
Delete	DELETE	/api/resources/:id	204

Response Format

json

// Success
{ "data": { ... } }
{ "data": [...], "pagination": { "page": 1, "limit": 20, "total": 100 } }

// Error
{ "error": "Human-readable message", "code": "MACHINE_READABLE_CODE" }

Pagination

Always paginate list endpoints. Default: 20 items, max: 100.

text

GET /api/users?page=1&limit=20
GET /api/users?cursor=abc123&limit=20  (cursor-based for large datasets)

When to Consult References

• references/backend-patterns.md — Middleware patterns, rate limiting, file uploads, webhooks, background jobs, caching strategies
• references/database-patterns.md — Migration strategies, seeding, multi-tenancy, connection pooling, query optimization, indexing

Anti-Patterns

• Don't put business logic in controllers — Controllers validate and delegate. Logic lives in services.
• Don't skip input validation — Every external input must be validated. No exceptions.
• Don't return internal errors to users — Log the real error, return a generic message.
• Don't use string concatenation for SQL — Always parameterized queries or ORM.
• Don't store passwords in plain text — bcrypt or argon2. Always.
• Don't skip rate limiting — Every public endpoint needs it.