Scalability is a system's ability to handle more workload while meeting performance and reliability requirements.

Vertical scaling

• Increase CPU, RAM, or storage resources on an existing machine.
• Often simpler initially.
• Has machine and service limits; a single machine can remain a failure point.

Horizontal scaling

• Add more machines or application instances.
• Can increase capacity and resilience when designed correctly.
• Requires attention to distributed state, network failures, coordination, and shared dependencies.

Comparison and example

• A small dashboard may work well on one larger server.
• A global commerce system may need multiple app instances, caching, replicas, and data partitioning.
• Choose based on cost, workload, performance, availability, and operational complexity