Senior Backend Engineer

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

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

references/api_design_patterns.md

references/database_optimization_guide.md

references/backend_security_practices.md

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

Troubleshooting

api_scaffolder.py

operationId

operationId

database_migration_tool.py

CREATE TABLE

;

/* ... */

curl

--no-verify-ssl

--concurrency

unknown

$ref

components/schemas

$ref

VARCHAR(255)

varchar(255)

--timeout

--duration

--timeout 5 --duration 30

$ref

Success Criteria

• API p99 latency under 200ms at production concurrency levels, verified by

  api_load_tester.py

• Zero N+1 query patterns detected in schema analysis via

  database_migration_tool.py --analyze

• All foreign key columns indexed with no "missing index" warnings from the migration tool
• 100% of generated routes include input validation middleware (Zod schemas auto-generated from OpenAPI spec)
• Success rate above 99.5% during sustained load tests at target concurrency for 60+ seconds
• Every migration paired with a rollback script to enable zero-downtime deployment reversals
• API response format consistency across all endpoints following the standardized

  data

  /

  error

  /

  meta

  envelope pattern

Scope & Limitations

What this skill covers:

• REST API design, scaffolding, and OpenAPI-driven code generation for Express, Fastify, and Koa
• PostgreSQL schema analysis, index optimization, migration generation with rollback support
• HTTP load testing with latency percentile analysis, throughput measurement, and endpoint comparison
• Backend security patterns including JWT configuration, rate limiting, input validation, and security headers

What this skill does NOT cover:

• Frontend development, UI components, or client-side state management -- see

  senior-frontend

• Infrastructure provisioning, container orchestration, or CI/CD pipeline setup -- see

  senior-devops

• GraphQL schema design, resolvers, or subscriptions -- see

  senior-fullstack

• Application performance monitoring (APM), distributed tracing, or log aggregation -- see

  senior-secops

Integration Points

senior-fullstack

senior-devops

migrations/

senior-security

senior-qa

senior-frontend

types.ts

code-reviewer

Tool Reference

api_scaffolder.py

Purpose: Generate Express.js/Fastify/Koa route handlers, Zod validators, and TypeScript types from an OpenAPI specification.

Usage:

python scripts/api_scaffolder.py <spec> [flags]

Flags:

spec

--output

-o

./generated

--framework

-f

express

express

fastify

koa

--types-only

false

--verbose

-v

false

--json

false

Example:

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

API Scaffolder - Express
Spec: openapi.yaml
Output: src/routes/
--------------------------------------------------
Loaded: User Service API v1.0.0
  Generated: src/routes/types.ts
  Generated: src/routes/validators.ts
  Generated: src/routes/users.routes.ts (5 handlers)
  Generated: src/routes/index.ts
--------------------------------------------------
Generated 5 route handlers
Generated 3 type definitions
Output: src/routes/

Output Formats: Human-readable console output by default. Add

--json

status

generated_files

routes_count

types_count

database_migration_tool.py

Purpose: Analyze SQL schema files for issues, compare schemas to generate migrations with rollback scripts, and suggest missing indexes.

Usage:

python scripts/database_migration_tool.py <schema> [flags]

Flags:

schema

--analyze

false

--compare

--suggest-indexes

false

--output

-o

--verbose

-v

false

--json

false

Example:

python scripts/database_migration_tool.py schema.sql --analyze --verbose

Database Migration Tool
Schema: schema.sql
--------------------------------------------------
Parsed 8 tables

Analysis Results:
  Tables: 8
  Errors: 1
  Warnings: 3
  Suggestions: 7

ERRORS:
  [audit_log] Table 'audit_log' has no primary key
    Suggestion: Add a primary key column (e.g., 'id SERIAL PRIMARY KEY')

WARNINGS:
  [orders] Foreign key column 'user_id' is not indexed
    Suggestion: CREATE INDEX idx_orders_user_id ON orders(user_id);

Output Formats: Human-readable console output by default. Add

--json

issues_detail

--compare --output

_migration.sql

_migration_rollback.sql

api_load_tester.py

Purpose: Perform HTTP load testing with configurable concurrency, measuring latency percentiles (p50/p90/p95/p99), throughput, error rates, and optional endpoint comparison.

Usage:

python scripts/api_load_tester.py <urls...> [flags]

Flags:

urls

--method

-m

GET

GET

POST

PUT

PATCH

DELETE

--body

-b

--header

-H

"Name: Value"

--concurrency

-c

10

--duration

-d

10.0

--timeout

-t

30.0

--compare

false

--no-verify-ssl

false

--verbose

-v

false

--json

false

--output

-o

Example:

python scripts/api_load_tester.py https://api.example.com/users \
  --method GET \
  --header "Authorization: Bearer tok_abc123" \
  --concurrency 50 \
  --duration 30 \
  --verbose

============================================================
LOAD TEST RESULTS
============================================================

Target: https://api.example.com/users
Method: GET
Duration: 30.2s
Concurrency: 50

THROUGHPUT:
  Total requests: 14,832
  Requests/sec: 491.1
  Successful: 14,710 (99.2%)
  Failed: 122

LATENCY (ms):
  Min: 11.3
  Avg: 92.4
  P50: 71.2
  P90: 165.8
  P95: 201.3
  P99: 387.6
  Max: 1,102.5
  StdDev: 89.2

TRANSFER:
  Total bytes: 45,291,520
  Throughput: 12.01 Mbps

RECOMMENDATIONS:
  Warning: P99 latency (388ms) exceeds 500ms
    Consider: Connection pooling, query optimization, caching
  Performance looks good for this load level
============================================================

Output Formats: Human-readable console report by default with latency distribution and recommendations. Add

--json

--output results.json

--compare