Senior Backend Engineer

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

Quick Start

bash
# 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:

bash
# Generate Express routes from OpenAPI spec
python scripts/api_scaffolder.py openapi.yaml --framework express --output src/routes/
# Output: Generated 12 route handlers, validation middleware, and TypeScript types

# 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:

bash
# Analyze current schema and suggest optimizations
python scripts/database_migration_tool.py --connection postgres://localhost/mydb --analyze
# Output: Missing indexes, N+1 query risks, and suggested migration files

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

# 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:

bash
# Basic load test
python scripts/api_load_tester.py https://api.example.com/users --concurrency 50 --duration 30
# Output: Throughput (req/sec), latency percentiles (P50/P95/P99), error counts, and scaling recommendations

# 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

yaml
# 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

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

Step 3: Implement business logic

typescript
// 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

bash
# 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

bash
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

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

Step 2: Identify slow queries

sql
-- 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

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

Step 4: Test migration (dry-run)

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

Step 5: Apply and verify

bash
# 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

typescript
// 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

typescript
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

typescript
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

bash
# 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

typescript
import helmet from 'helmet';

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

Reference Documentation

| File | Contains | Use When | |------|----------|----------| | references/api_design_patterns.md | REST vs GraphQL, versioning, error handling, pagination | Designing new APIs | | references/database_optimization_guide.md | Indexing strategies, query optimization, N+1 solutions | Fixing slow queries | | references/backend_security_practices.md | OWASP Top 10, auth patterns, input validation | Security hardening |

Common Patterns Quick Reference

REST API Response Format

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

Error Response Format

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

HTTP Status Codes

| Code | Use Case | |------|----------| | 200 | Success (GET, PUT, PATCH) | | 201 | Created (POST) | | 204 | No Content (DELETE) | | 400 | Validation error | | 401 | Authentication required | | 403 | Permission denied | | 404 | Resource not found | | 429 | Rate limit exceeded | | 500 | Internal server error |

Database Index Strategy

sql
-- 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

bash
# 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

Assumptions and Verifiable Success Criteria (Karpathy discipline)

Before this skill scaffolds, recommends a pattern, or modifies a schema, the following four assumptions MUST be surfaced. If any are unknown, the skill stops and walks the Forcing-question library instead.

1. Read/write ratio + one-year p99 QPS — drives DB, cache, queue, and partitioning choices. Kleppmann, DDIA (2017).
2. Tenancy model — single-tenant, shared multi-tenant, isolated multi-tenant. Drives data-access pattern.
3. Data sensitivity tier — public / internal / PII / PHI / PCI. Drives compliance floor.
4. SLO + named error-budget consumer — Google SRE Workbook canon. No SLO = no reliability work prioritization.

Verifiable success criteria (Karpathy #4) — every recommendation this skill emits must include:

• Latency targets (p50, p95, p99 in ms)
• Uptime / SLO target
• RPO + RTO

If any of those three is not stated, the recommendation is incomplete — return to Q7 of the forcing-question library.

The scripts/backend_decision_engine.py tool encodes these checks: it refuses to recommend a profile without read/write ratio + QPS + tenancy + data sensitivity + pattern preference.

Customization profiles

Four built-in profiles in profiles/ calibrate every recommendation:

| Profile | When to pick | Pattern | Latency floor (p99) | |---|---|---|---| | node-express | TS team, < 15 eng, customer-facing SaaS | Modular monolith on Postgres | 600ms | | fastapi-python | Python team, < 20 eng, ML-adjacent | Modular monolith on Postgres (async) | 500ms | | django-monolith | Content-heavy CRUD + admin, < 25 eng | Modular monolith on Postgres | 800ms | | go-or-rust-microservice | Extracted service, ≥ 30 eng, platform team, QPS ≥ 1000 | Extracted service | 200ms |

Pick a profile via:

bash
python scripts/backend_decision_engine.py \
  --team-size 8 --qps-p99 50 --read-write-ratio 20 \
  --tenancy shared-multi-tenant --data-sensitivity pii \
  --pattern modular-monolith --language-preference typescript

The tool returns the best-fit profile, runner-up tradeoff (if within 15%), stack picks, anti-patterns, named approvers, and SLO floor. This tool never auto-approves.

To add a custom profile: copy profiles/node-express.json to profiles/<your-org>.json and adjust constraints + success_thresholds + named_approver_chain.

Composition map

This skill does NOT reimplement scope owned by the POWERFUL-tier specialists. It forks into them. See references/composition_map.md for the full routing table. Key forks:

| Concern | Fork into | |---|---| | API contract / breaking-change risk | engineering/skills/api-design-reviewer/ | | Schema design + ERD + indexing | engineering/skills/database-designer/ | | Zero-downtime schema migration | engineering/skills/migration-architect/ | | SLO + SLI + error-budget | engineering/slo-architect/ | | Observability / golden signals | engineering/skills/observability-designer/ | | CI/CD pipeline | engineering/skills/ci-cd-pipeline-builder/ | | Security / threat model | engineering-team/skills/senior-security/, adversarial-reviewer | | Compliance evidence (HIPAA / ISO 27001) | ra-qm-team/ | | Pre-commit Karpathy review | engineering/karpathy-coder/ | | Pre-flight architecture grill | engineering/grill-me/ |

The cs-backend-engineer agent orchestrates these forks via context: fork. Invoke it from another agent with Agent({subagent_type: "cs-backend-engineer", prompt: "..."}) or via /cs:backend-review <your problem>.

Forcing-question library (Matt Pocock grill)

Before locking any backend decision, walk the seven forcing questions in references/forcing_questions.md. Discipline:

1. One question per turn. No bundling.
2. Always recommend the answer with cited canon.
3. Track answers in /tmp/backend-grill-<date>.md.
4. If a kill criterion trips, stop. Don't scaffold around an unresolved gap.
5. After Q7, run backend_decision_engine.py with the seven answers.

Summary:

1. Read/write ratio + p99 QPS forecast?
2. Tenancy model — single / shared / isolated?
3. Sync / async / event-driven — default + exceptions?
4. Data sensitivity tier — PII / PHI / PCI?
5. Monolith / modular monolith / microservices — team-size justification?
6. RPO + RTO?
7. SLO + named error-budget consumer?

Invocation from other agents and skills

Three surfaces:

1. Slash command: /cs:backend-review <prompt> — full grill + decision engine + composition routing.
2. Agent subagent: Agent({subagent_type: "cs-backend-engineer", prompt: "..."}) — forks context, returns ≤ 200-word digest.
3. Direct tool call: python scripts/backend_decision_engine.py ... — deterministic profile match when inputs are known.

See agents/engineering/cs-backend-engineer.md for the full invocation contract.