---

name: scalability-patterns description: Master scalability patterns with load balancing, caching, database scaling, microservices, and horizontal scaling strategies.

Scalability Patterns

Implement scalability patterns to handle growth, improve performance, and maintain reliability under increasing load.

When to Use This Skill

• Performance bottlenecks
• User growth
• Data volume increase
• High traffic events
• Geographic expansion
• Cost optimization
• Architecture modernization
• SLA requirements

Core Concepts

1. Horizontal vs Vertical Scaling

Copy**Vertical Scaling (Scale Up):**
- Add more CPU, RAM to existing server
- Limits: Hardware ceiling, single point of failure
- Use case: Databases, legacy apps

**Horizontal Scaling (Scale Out):**
- Add more servers
- Benefits: No limit, fault tolerance
- Requires: Stateless design, load balancing
- Use case: Web servers, app servers

**Recommendation:** Design for horizontal scaling

2. Caching Strategies

Copy## Cache Architecture

**Cache-Aside (Lazy Loading):**

1. App checks cache
2. If miss, read from DB
3. Write to cache
4. Return data

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**Write-Through:**

1. App writes to cache
2. Cache writes to DB
3. Return success

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**Write-Behind:**

1. App writes to cache
2. Return success immediately
3. Cache async writes to DB (eventual consistency)

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**Use Cases:**
- CDN: Static assets (images, CSS, JS)
- Redis: Session data, API responses
- Browser cache: User-specific data
- Application cache: Configuration, reference data

**Example - Redis Caching:**
```javascript
async function getUser(userId) {
  // Try cache first
  let user = await redis.get(`user:${userId}`);
  
  if (!user) {
    // Cache miss - get from database
    user = await database.query('SELECT * FROM users WHERE id = ?', [userId]);
    // Store in cache (TTL: 1 hour)
    await redis.setex(`user:${userId}`, 3600, JSON.stringify(user));
  }
  
  return JSON.parse(user);
}

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### 3. Database Scaling

```markdown
## Database Scaling Strategies

**Read Replicas:**
- Master: Write operations
- Replicas: Read operations
- Reduces load on master
- Eventual consistency

**Sharding (Horizontal Partitioning):**
- Split data across multiple databases
- Shard key (e.g., user_id % num_shards)
- Challenges: Joins, resharding

**Vertical Partitioning:**
- Split tables by columns
- Separate hot/cold data
- Example: User profile vs user activity logs

**CQRS (Command Query Responsibility Segregation):**
- Separate read and write models
- Optimized for different use cases
- Event sourcing integration

4. Load Balancing

Copy## Load Balancer Strategies

**Round Robin:**
Server 1 → Server 2 → Server 3 → Server 1...

**Least Connections:**
Route to server with fewest active connections

**IP Hash:**
Route based on client IP (sticky sessions)

**Weighted:**
More requests to more powerful servers

**Health Checks:**
- Monitor server health
- Remove unhealthy servers
- Automatic failover

**Example Architecture:**

Internet → CloudFlare CDN → AWS ALB (Application Load Balancer) → Auto Scaling Group → EC2 Instance 1 → EC2 Instance 2 → EC2 Instance 3

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Best Practices

1. Stateless design - Store state externally (Redis, DB)
2. Async processing - Use message queues for heavy tasks
3. Cache aggressively - Multiple cache layers
4. Database optimization - Indexes, query optimization
5. Monitor metrics - CPU, memory, response time, error rate
6. Auto-scaling - Scale based on metrics
7. Graceful degradation - Reduce functionality vs complete failure
8. Load testing - Identify bottlenecks before production

Resources

• Scalability Rules: Martin Abbott
• High Performance Browser Networking: Ilya Grigorik