Skill: Scalable Thinking

Purpose: Design for scale while keeping implementation simple (KISS) Priority: HIGH Auto-invokes: Architecture, API design, data modeling, database schema

Core Philosophy

"Think Scalable, Build Simple"

Where to Apply

1. Data Models

❌ Non-Scalable:

interface User {
  id: string
  preferences: string  // JSON string - can't query
}

✅ Scalable (Simple Implementation):

interface User { id: string; name: string; email: string }
interface UserPreference { userId: string; key: string; value: string }

// Simple queries, can index by key, can partition by userId

2. API Design

❌ Non-Scalable:

POST /api/process { everything... }

✅ Scalable (Simple):

POST /api/v1/users/:userId/processes
GET /api/v1/processes/:processId

Benefits: Versionable, RESTful, cacheable, evolvable

3. Database Schema

❌ Non-Scalable:

CREATE TABLE orders (
  id UUID PRIMARY KEY,
  items JSON  -- Can't filter by item
);

✅ Scalable (Simple):

CREATE TABLE orders (
  id UUID PRIMARY KEY,
  user_id UUID REFERENCES users(id),
  INDEX idx_user_id (user_id)
);

CREATE TABLE order_items (
  order_id UUID REFERENCES orders(id),
  product_id UUID,
  INDEX idx_order_id (order_id)
);

4. File Structure

❌ Non-Scalable:

src/utils.ts      # 5000 lines
src/components.tsx # All components

✅ Scalable:

src/features/auth/hooks/, components/, api/
src/features/posts/hooks/, components/, api/
src/shared/hooks/, components/, utils/

Scalable Patterns

Always Plan For Pagination

// ❌ Returns all records
GET /api/users → [...50,000 users]

// ✅ Cursor-based
GET /api/users?limit=20&cursor=abc123
→ { data: [...], pagination: { nextCursor, hasMore } }

Centralized Configuration

const config = {
  api: { baseUrl: process.env.API_URL, timeout: 5000 },
  features: { /* feature flags */ },
  limits: { maxFileSize: 10 * 1024 * 1024 }
}

Structured Error Handling

class AppError extends Error {
  constructor(
    message: string,
    public code: string,
    public statusCode: number = 500
  ) { super(message) }
}

throw new AppError('User not found', 'USER_NOT_FOUND', 404)

Anti-Patterns to Avoid

Checklist

Before Designing

• Can data model support 10x more records?
• Can I query the data I'll need?
• Are there natural partition keys (userId, orgId)?
• Is API versioned (/v1)?
• Does pagination exist for lists?

During Implementation

• Am I using standard patterns?
• Is this the simplest solution?
• Can a junior dev understand this?
• No abstractions until 3+ examples?

When to Scale Up

Add Complexity When:

• Database queries > 2s consistently
• API response > 500ms at p95
• Error rate > 1%
• User growth > 10x in 3 months
• Team size > 5 developers

Until Then: Keep it simple, measure everything

Summary

1. Think Scalable: Data models that grow, APIs that evolve, architecture that supports change
2. Build Simple: Simplest implementation first, standard patterns, no premature optimization
3. Iterate: Add complexity based on data, refactor when patterns emerge

Remember: Scalable ≠ Complex. Scalable = Can grow. KISS = Simple to maintain.

Applied In: Phase 2 (Design), Phase 5 (Implementation)