Scalability

In telecommunications and software engineering, scalability is a desirable property of a system, a network or a process, which indicates its ability to either handle growing amounts of work in a graceful manner, or to be readily enlarged.André B. Bondi, 'Characteristics of scalability and their impact on performance', Proceedings of the 2nd international workshop on Software and performance, Ottawa, Ontario, Canada, 2000, ISBN 1-58113-195-X, pages 195 - 203 For example, it can refer to the capability of a system to increase total throughput under an increased load when resources (typically hardware) are added. An analogous meaning is implied when the word is used in a commercial context, where scalability of a company implies that the underlying business model offers the potential for economic growth within the company.

Scalability as a property of systems in general is difficult to define,See for instance, Mark D. Hill, 'What is scalability?' in ACM SIGARCH Computer Architecture News, December 1990, Volume 18 Issue 4, pages 18-21, (ISSN 0163-5964) and Leticia Duboc, David S. Rosenblum, Tony Wicks, 'Doctoral symposium: presentations: A framework for modelling and analysis of software systems scalability' in Proceeding of the 28th international conference on Software engineering ICSE '06, May 2006. ISBN 1-59593-375-1, pages 949 - 952 and in any particular case it is necessary to define the specific requirements for scalability on those dimensions which are deemed important. It is a highly significant issue in electronics systems, database, routers, and networking. A system whose performance improves after adding hardware, proportionally to the capacity added, is said to be a scalable system.

Scalability can be measured in various dimensions, such as:

Load scalability - A distributed system should make it easy for us to expand and contract its resource pool to accommodate heavier or lighter loads.
Geographic scalability - A geographically scalable system is one that maintains its usefulness and usability, regardless of how far apart its users or resources are.
Administrative scalability - No matter how many different organizations need to share a single distributed system, it should still be easy to use and manage.

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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