Scalability
In telecommunications and software engineering, scalability indicates the capability of a system to increase performance under an increased load when resources (typically hardware) are added. Scalability is a highly significant issue in electronics systems, database, routers, networking, etc. and implies performance. A system whose performance improves after adding hardware, proportionally to the capacity added, is said to be a scalable system.
For example, a scalable online transaction processing system or database management system is one that can be upgraded to process more transactions by adding new processors, devices and storage, and which can be upgraded easily and transparently without shutting it down.
A routing protocol is considered scalable with respect to network size, if the size of the necessary routing table on each node grows as O(log N), where N is the number of nodes in the network.
To scale vertically or scale up, means to add resources to a single node in a system, such as adding memory or a faster hard drive to a computer. To scale horizontally or scale out, means to add more nodes to a system, such as adding a new computer to a clustered software application.
It is often advised to focus system design on hardware scalability rather than on capacity. It is typically cheaper to add a new node to a system in order to achieve improved performance than to partake in performance tuning to improve the capacity that each node can handle.
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