What is the difference between horizontal and vertical scaling?

Horizontal scaling and vertical scaling are two approaches to increasing a system’s capacity, but they work in fundamentally different ways. Horizontal scaling, often called “scaling out,” involves adding more machines or instances to a system to handle increased load. Vertical scaling, or “scaling up,” focuses on upgrading the resources of an existing machine, such as adding more CPU, RAM, or storage. The key difference lies in whether you expand by adding more nodes (horizontal) or enhancing existing nodes (vertical).

Horizontal scaling is commonly used in distributed systems where workloads can be split across multiple servers. For example, a web application facing high traffic might deploy additional servers behind a load balancer to distribute incoming requests. This approach improves fault tolerance because if one server fails, others can take over. However, it requires applications to be stateless or designed to share state externally (e.g., using a centralized database or caching layer). Technologies like Kubernetes or cloud auto-scaling groups automate horizontal scaling by dynamically adding or removing instances based on demand. A limitation is that not all applications can be easily partitioned to run across multiple nodes, especially those relying on single-machine resources like local disk storage.

Vertical scaling simplifies architecture by avoiding the complexity of distributed systems. For instance, a database server struggling with query performance might be upgraded from 8GB to 32GB of RAM to handle larger datasets in memory. This approach works well for monolithic applications or legacy systems that aren’t designed for horizontal scaling. However, vertical scaling has physical limits (e.g., maximum CPU a machine can support) and creates a single point of failure. Upgrading hardware also often requires downtime, whereas horizontal scaling can be done with minimal disruption. Developers might choose vertical scaling for quick, short-term fixes but opt for horizontal scaling when long-term scalability and resilience are priorities. For example, a video rendering service might vertically scale a server to process 4K videos faster, while a social media platform would horizontally scale to manage millions of concurrent users.