Scalability

Scalability is the capability of a system to handle a growing amount of work by adding resources. A scalable system can maintain performance levels when facing increased demands.

Types of Scaling

Vertical Scaling (Scale Up)

Vertical scaling involves adding more power to an existing machine—more CPU, RAM, or storage.

Code

Before: 4 CPU cores, 8GB RAM
After:  16 CPU cores, 64GB RAM

Pros:

Simple to implement
No application code changes required
No data partitioning complexity

Cons:

Hardware limits (you can only add so much)
Single point of failure
Expensive high-end hardware
Downtime during upgrades

Horizontal Scaling (Scale Out)

Horizontal scaling involves adding more machines to your resource pool.

Code

Before: 1 server
After:  10 servers behind a load balancer

Pros:

Theoretically unlimited scaling
Better fault tolerance
Cost-effective (commodity hardware)
No downtime for scaling

Cons:

Application complexity (distributed systems)
Data consistency challenges
Network latency between nodes
Requires load balancing

Comparison Table

Aspect	Vertical Scaling	Horizontal Scaling
Cost	Expensive	Cost-effective
Complexity	Low	High
Limit	Hardware bound	Virtually unlimited
Downtime	Required	Not required
Failure Impact	High	Low

When to Use Each

Use Vertical Scaling When:

Your application is monolithic
You're in early stages with low traffic
Quick fixes are needed
Database that's hard to distribute

Use Horizontal Scaling When:

You expect significant growth
High availability is critical
You have a distributed architecture
Cost optimization is important

Scaling Patterns

Stateless Services

Design services to be stateless for easy horizontal scaling:

Code

// ❌ Bad: Stateful service
class UserService {
  private sessions = new Map(); // State stored in memory
  
  getUser(sessionId: string) {
    return this.sessions.get(sessionId);
  }
}
 
// ✅ Good: Stateless service
class UserService {
  constructor(private redis: RedisClient) {}
  
  async getUser(sessionId: string) {
    return this.redis.get(`session:${sessionId}`);
  }
}

Database Scaling

For databases, you typically combine both approaches:

Read Replicas: Horizontal scaling for read operations
Larger Instance: Vertical scaling for write-heavy primary
Sharding: Horizontal scaling for write operations

Best Practices

Start Simple: Begin with vertical scaling, move to horizontal when needed
Design for Horizontal: Even if not needed now, architect for future scaling
Monitor and Measure: Know your bottlenecks before scaling
Automate Scaling: Use auto-scaling groups in cloud environments
Test at Scale: Load test to validate scaling strategies