Scalability is the ability of a program to scale. 
For example, an application program would be scalable if it could be moved from a smaller to a larger operating system and take full advantage of the larger operating system in terms of performance (user response time and so forth) and the larger number of users that could be handled. Typically, the rescaling is to a larger size or volume. The rescaling can be of the product itself (for example, a line of computer systems of different sizes in terms of storage, RAM, and so forth) or in the scalable object's movement to a new context (for example, a new operating system). up vote 12 down vote accepted. Scalability is the ability of a program to scale. For example, if you can do something on a small database (say less than 1000 records), a program that is highly scalable would work well on a small set as well as working well on a large set (say millions, or billions of records). Like gap said, it would have a linear growth of resource requirements. It is usually easier to have scalability upward rather than downward since developers often must make full use of a system's resources (for example, the amount of disk storage available) when an application is initially coded. Scalability, as a property of systems, is generally difficult to define and in any particular case it is necessary to define the specific requirements for scalability on those dimensions that are deemed important. 1) It is 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. Scalability is the capability to increase resources to yield a linear (ideally) increase in service capacity. The key characteristic of a scalable application is that additional load only requires additional resources rather than extensive modification of the application itself.
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what does scalability mean in programming