What is the difference between horizontal and vertical scaling?

Vertical scaling (Scale Up) is increasing resources (CPU, RAM, disk) for an existing server. Horizontal scaling (Scale Out) is adding new servers to the cluster.

Main differences:

• Vertical:

  • Increases the capacity of one server.
  • Requires rebooting or stopping the service (for most systems).
  • Has physical limits on the available resources on a single server.
  • Easier to implement for monolithic applications.
  • Does not increase fault tolerance (single point of failure).

• Horizontal:

  • Increases the number of servers (instances).
  • Potentially has no upper limit on scaling (can add many servers).
  • Improves fault tolerance (if one server fails, others remain accessible).
  • Requires application architecture supporting distributed operation (stateless, microservices).
  • Requires mechanisms for load distribution (Load Balancer) and data synchronization.

Examples:

• Vertical: Moving from a server with 4 CPU cores and 8GB RAM to a server with 8 CPU cores and 16GB RAM.
• Horizontal: Adding a second web server to an existing one behind a load balancer.

Choosing between vertical and horizontal scaling depends on the application's architecture, budget, fault tolerance requirements, and management simplicity. Often, a combination of both approaches is used.