What is the difference between horizontal and vertical scaling?

The distinction between horizontal and vertical scaling lies primarily in how resources are added to a system to handle increased loads.

Horizontal scaling, also known as scaling out, refers to the process of adding more machines or instances to a system. This means increasing the number of servers, which can improve the capacity and reliability of applications by distributing the load among multiple resources. It is often preferred because it can provide fault tolerance and increase performance by spreading workloads.

Vertical scaling, on the other hand, involves upgrading the existing machines by enhancing their hardware capabilities, such as increasing CPU power, memory, or disk space. This approach is beneficial for applications that require higher compute resources on a single machine, but it generally has limits and may lead to a single point of failure.

The correct answer accurately captures this relationship by stating that horizontal scaling involves adding more machines, while vertical scaling entails upgrading those existing machines. Understanding this difference is crucial for designing scalable systems that can efficiently handle varying loads.