What are vertical and horizontal scaling in databases?

Vertical (scale up) — increasing the capacity of a single server (more CPU, RAM, fast disk). Pros: simple to implement. Cons: there is a limit to scaling, a single point of failure, simple during modernization.

Horizontal (scale out) — distributing load among multiple servers. Pros: high availability, virtually unlimited scalability, resilience to single server failures. Cons: implementation complexity (sharding, replication), data consistency.

Comparison:

Characteristic	Vertical scaling	Horizontal scaling
Implementation	Simpler	More complex
Limit	Restricted	Virtually unlimited
Availability	Lower (single point of failure)	Higher (distributed system)
Cost	Usually higher per unit of capacity in the upper segment	Scales linearly on inexpensive hardware
Data modification	Simpler	More complex (consistency, distributed transactions)