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Vertical scalability is the addition of resources to a single system node, such as a single computer or network station, which often results in additional CPUs or memory. Vertical scalability provides more shared resources for the operating system and applications.
Vertical scalability may also be referred to as scaling up.
By contrast, horizontal scalability (scaling out) refers to adding more nodes to a system, such as adding computer workstations to a network, which may result in adding multiple computers to accomplish more work in less time. This allows the system to work as a single logical unit, increasing efficiency.
Each model has tradeoffs. In horizontal scalability, adding more nodes increases the network or system complexity, management, programming model, throughput and latency between nodes. It may also produce software application issues that inhibit work throughput, possibly hindering overall system efficiency. In the past, horizontal scalability was considered more cost-effective when applications running on many nodes worked well together.
However, with the recent use of virtualization technology and hypervisors (a host computer running multiple operating systems, also known as guests), the efficiency of a single node has increased significantly via vertical scalability. Thus, in view of horizontal scalability’s decline, both models must be considered and reviewed.