scalability
What is scalability?
In information technology, scalability (frequently spelled scaleability) has two uses: for a computer application to function with change in size and volume, and to take advantage of rescaling.
Continual application function with size and volume change
The ability of a computer application or product, hardware or software, to continue to function well when it, or its context, is changed in size or volume in order to meet a user need. Typically, the rescaling is to a larger size or volume. The rescaling can be of the product itself, such as a line of computer systems of different sizes in terms of storage and RAM, or in the scalable object's movement to a new context, such as a new operating system (OS).
Example
John Young, in his book Exploring IBM's New-Age Mainframes, describes the RS/6000 SP OS as one that delivers scalability as he states it has "the ability to retain performance levels when adding additional processors."
As another example, in printing, scalable fonts can be resized without losing quality.
Taking advantage of rescaling
Scalability is to function well in the rescaled situation, but to also take full advantage of it. It is usually easier to have scalability upward rather than downward since developers often must make full use of a system's resources, such as the amount of disk storage available, when an application is initially coded. Scaling a product downward may mean trying to achieve the same results in a more constrained environment.
Example
An application program would be scalable if it could be moved from a smaller to a larger OS and take full advantage of the larger OS in terms of performance, user response time, and the larger number of users that could be handled.
