Scalability is the property of a system to handle a growing amount of work by adding resources to the system. In system design, we generally talk about two primary ways to scale: Vertical Scaling (Scaling Up) and Horizontal Scaling (Scaling Out).

Vertical Scaling (Scaling Up)

Vertical scaling means adding more power (CPU, RAM, Disk) to an existing machine.

• Pros: Simple to implement (no code changes), lower network latency (all components on one box), easier to manage.
• Cons: Has a hard upper limit (you can only buy so much RAM), single point of failure (if the machine dies, everything dies), expensive (high-end hardware is pricey).

Horizontal Scaling (Scaling Out)

Horizontal scaling means adding more machines to your pool of resources.

• Pros: Practically limitless (just add more servers), high availability (if one node fails, others continue), cost-effective (use commodity hardware).
• Cons: Increases complexity (requires load balancers, service discovery), data consistency issues, network latency between nodes.

Elasticity vs Scalability

While often used interchangeably, they are different:

• Scalability: The capacity to handle more load.
• Elasticity: The ability to automatically scale up or down based on current demand (common in cloud environments).