senior-backend

What this skill does

# 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: "zstringmin1max100"
  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":