Senior Backend

Senior Backend Engineer

Backend development patterns, API design, database optimization, and security practices.

Core rules

1. Before changing backend code for a substantial task, first create or update a backend codemap that maps stories to modules, files, interfaces, and data flow.
2. Treat the codemap as the implementation contract for the backend phase; coding starts only after the target module boundaries are explicit.
3. Keep the codemap synchronized when the implementation plan changes materially during the phase.

Table of Contents

• Quick Start
• Tools Overview
  • API Scaffolder
  • Database Migration Tool
  • API Load Tester
• Backend Development Workflows
  • API Design Workflow
  • Database Optimization Workflow
  • Security Hardening Workflow
• Reference Documentation
• Common Patterns Quick Reference

Quick Start

# Generate API routes from OpenAPI spec
python scripts/api_scaffolder.py openapi.yaml --framework express --output src/routes/

# Analyze database schema and generate migrations
python scripts/database_migration_tool.py --connection postgres://localhost/mydb --analyze

# Load test an API endpoint
python scripts/api_load_tester.py https://api.example.com/users --concurrency 50 --duration 30

Tools Overview

1. API Scaffolder

Generates API route handlers, middleware, and OpenAPI specifications from schema definitions.

Input: OpenAPI spec (YAML/JSON) or database schema Output: Route handlers, validation middleware, TypeScript types

Usage:

# Generate Express routes from OpenAPI spec
python scripts/api_scaffolder.py openapi.yaml --framework express --output src/routes/

# Output:
# Generated 12 route handlers in src/routes/
# - GET /users (listUsers)
# - POST /users (createUser)
# - GET /users/{id} (getUser)
# - PUT /users/{id} (updateUser)
# - DELETE /users/{id} (deleteUser)
# ...
# Created validation middleware: src/middleware/validators.ts
# Created TypeScript types: src/types/api.ts

# Generate from database schema
python scripts/api_scaffolder.py --from-db postgres://localhost/mydb --output src/routes/

# Generate OpenAPI spec from existing routes
python scripts/api_scaffolder.py src/routes/ --generate-spec --output openapi.yaml

Supported Frameworks:

• Express.js (--framework express)
• Fastify (--framework fastify)
• Koa (--framework koa)

2. Database Migration Tool

Analyzes database schemas, detects changes, and generates migration files with rollback support.

Input: Database connection string or schema files Output: Migration files, schema diff report, optimization suggestions

Usage:

# Analyze current schema and suggest optimizations
python scripts/database_migration_tool.py --connection postgres://localhost/mydb --analyze

# Output:
# === Database Analysis Report ===
# Tables: 24
# Total rows: 1,247,832
#
# MISSING INDEXES (5 found):
#   orders.user_id - 847ms avg query time, ADD INDEX recommended
#   products.category_id - 234ms avg query time, ADD INDEX recommended
#
# N+1 QUERY RISKS (3 found):
#   users -> orders relationship (no eager loading)
#
# SUGGESTED MIGRATIONS:
#   1. Add index on orders(user_id)
#   2. Add index on products(category_id)
#   3. Add composite index on order_items(order_id, product_id)

# Generate migration from schema diff
python scripts/database_migration_tool.py --connection postgres://localhost/mydb \
  --compare schema/v2.sql --output migrations/

# Output:
# Generated migration: migrations/20240115_add_user_indexes.sql
# Generated rollback: migrations/20240115_add_user_indexes_rollback.sql

# Dry-run a migration
python scripts/database_migration_tool.py --connection postgres://localhost/mydb \
  --migrate migrations/20240115_add_user_indexes.sql --dry-run

3. API Load Tester

Performs HTTP load testing with configurable concurrency, measuring latency percentiles and throughput.

Input: API endpoint URL and test configuration Output: Performance report with latency distribution, error rates, throughput metrics

Usage:

# Basic load test
python scripts/api_load_tester.py https://api.example.com/users --concurrency 50 --duration 30

# Output:
# === Load Test Results ===
# Target: https://api.example.com/users
# Duration: 30s | Concurrency: 50
#
# THROUGHPUT:
#   Total requests: 15,247
#   Requests/sec: 508.2
#   Successful: 15,102 (99.0%)
#   Failed: 145 (1.0%)
#
# LATENCY (ms):
#   Min: 12
#   Avg: 89
#   P50: 67
#   P95: 198
#   P99: 423
#   Max: 1,247
#
# ERRORS:
#   Connection timeout: 89
#   HTTP 503: 56
#
# RECOMMENDATION: P99 latency (423ms) exceeds 200ms target.
# Consider: connection pooling, query optimization, or horizontal scaling.

# Test with custom headers and body
python scripts/api_load_tester.py https://api.example.com/orders \
  --method POST \
  --header "Authorization: Bearer token123" \
  --body '{"product_id": 1, "quantity": 2}' \
  --concurrency 100 \
  --duration 60

# Compare two endpoints
python scripts/api_load_tester.py https://api.example.com/v1/users https://api.example.com/v2/users \
  --compare --concurrency 50 --duration 30

Backend Development Workflows

API Design Workflow

Use when designing a new API or refactoring existing endpoints.

Step 1: Define resources and operations

# openapi.yaml
openapi: 3.0.3
info:
  title: User Service API
  version: 1.0.0
paths:
  /users:
    get:
      summary: List users
      parameters:
        - name: limit
          in: query
          schema:
            type: integer
            default: 20
    post:
      summary: Create user
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/CreateUser'

Step 2: Generate route scaffolding

python scripts/api_scaffolder.py openapi.yaml --framework express --output src/routes/

Step 3: Implement business logic

// src/routes/users.ts (generated, then customized)
export const createUser = async (req: Request, res: Response) => {
  const { email, name } = req.body;

  // Add business logic
  const user = await userService.create({ email, name });

  res.status(201).json(user);
};

Step 4: Add validation middleware

# Validation is auto-generated from OpenAPI schema
# src/middleware/validators.ts includes:
# - Request body validation
# - Query parameter validation
# - Path parameter validation

Step 5: Generate updated OpenAPI spec

python scripts/api_scaffolder.py src/routes/ --generate-spec --output openapi.yaml

Database Optimization Workflow

Use when queries are slow or database performance needs improvement.

Step 1: Analyze current performance

python scripts/database_migration_tool.py --connection $DATABASE_URL --analyze

Step 2: Identify slow queries

-- Check query execution plans
EXPLAIN ANALYZE SELECT * FROM orders
WHERE user_id = 123
ORDER BY created_at DESC
LIMIT 10;

-- Look for: Seq Scan (bad), Index Scan (good)

Step 3: Generate index migrations

python scripts/database_migration_tool.py --connection $DATABASE_URL \
  --suggest-indexes --output migrations/

Step 4: Test migration (dry-run)

python scripts/database_migration_tool.py --connection $DATABASE_URL \
  --migrate migrations/add_indexes.sql --dry-run

Step 5: Apply and verify

# Apply migration
python scripts/database_migration_tool.py --connection $DATABASE_URL \
  --migrate migrations/add_indexes.sql

# Verify improvement
python scripts/database_migration_tool.py --connection $DATABASE_URL --analyze

Security Hardening Workflow

Use when preparing an API for production or after a security review.

Step 1: Review authentication setup

// Verify JWT configuration
const jwtConfig = {
  secret: process.env.JWT_SECRET,  // Must be from env, never hardcoded
  expiresIn: '1h',                 // Short-lived tokens
  algorithm: 'RS256'               // Prefer asymmetric
};

Step 2: Add rate limiting

import rateLimit from 'express-rate-limit';

const apiLimiter = rateLimit({
  windowMs: 15 * 60 * 1000,  // 15 minutes
  max: 100,                   // 100 requests per window
  standardHeaders: true,
  legacyHeaders: false,
});

app.use('/api/', apiLimiter);

Step 3: Validate all inputs

import { z } from 'zod';

const CreateUserSchema = z.object({
  email: z.string().email().max(255),
  name: z.string().min(1).max(100),
  age: z.number().int().positive().optional()
});

// Use in route handler
const data = CreateUserSchema.parse(req.body);

Step 4: Load test with attack patterns

# Test rate limiting
python scripts/api_load_tester.py https://api.example.com/login \
  --concurrency 200 --duration 10 --expect-rate-limit

# Test input validation
python scripts/api_load_tester.py https://api.example.com/users \
  --method POST \
  --body '{"email": "not-an-email"}' \
  --expect-status 400

Step 5: Review security headers

import helmet from 'helmet';

app.use(helmet({
  contentSecurityPolicy: true,
  crossOriginEmbedderPolicy: true,
  crossOriginOpenerPolicy: true,
  crossOriginResourcePolicy: true,
  hsts: { maxAge: 31536000, includeSubDomains: true },
}));

Reference Documentation

Common Patterns Quick Reference

REST API Response Format

{
  "data": { "id": 1, "name": "John" },
  "meta": { "requestId": "abc-123" }
}

Error Response Format

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid email format",
    "details": [{ "field": "email", "message": "must be valid email" }]
  },
  "meta": { "requestId": "abc-123" }
}

HTTP Status Codes

Database Index Strategy

-- Single column (equality lookups)
CREATE INDEX idx_users_email ON users(email);

-- Composite (multi-column queries)
CREATE INDEX idx_orders_user_status ON orders(user_id, status);

-- Partial (filtered queries)
CREATE INDEX idx_orders_active ON orders(created_at) WHERE status = 'active';

-- Covering (avoid table lookup)
CREATE INDEX idx_users_email_name ON users(email) INCLUDE (name);

Common Commands

# API Development
python scripts/api_scaffolder.py openapi.yaml --framework express
python scripts/api_scaffolder.py src/routes/ --generate-spec

# Database Operations
python scripts/database_migration_tool.py --connection $DATABASE_URL --analyze
python scripts/database_migration_tool.py --connection $DATABASE_URL --migrate file.sql

# Performance Testing
python scripts/api_load_tester.py https://api.example.com/endpoint --concurrency 50
python scripts/api_load_tester.py https://api.example.com/endpoint --compare baseline.json