Software requirements

Objectives
? understand the concepts of user requirements and system requirements and why these requirements should be written in different ways;
? understand the differences between functional and non-functional software requirement

The requirements for a system are the descriptions of the services provided by the system and its operational constraints.

The process of finding out, analyzing, documenting and checking these services and constraints is called requirements engineering

User requirements and system requirements may be defined as follows:

1. User requirements are statements, in a natural language plus diagrams, of what services the system is expected to provide and the constraints under which it must operate.

2. System requirements set out the system’s functions, services and operational constraints in detail. The system requirements document (sometimes called a functional specification) should be precise. It should define exactly what is to be implemented. It may be part of the contract between the system buyer and the software developers.

6.1 Functional and non-functional requirements
1. Functional requirements These are statements of services the system should provide, how the system should react to particular inputs and how the system should behave in particular situations. In some cases, the functional requirements may also explicitly state what the system should not do.

2. Non-functional requirements These are constraints on the services or functions offered by the system. They include timing constraints, constraints on the development process and standards. Non-functional requirements often apply to the system as
a whole. They do not usually just apply to individual system features or services.