Functional and Non-Functional Requirements

Functional and Non-Functional Requirements

Lesson Objectives

• Describe functional requirements

• Describe non-functional requirements

• Categorize non-functional requirements into 3 main groups

• Give examples of functional requirements

• Give examples of non-functional requirements

What is a requirement?

The requirements for a system are the descriptions of the services that a system should provide and the constraints on its operation. These requirements reflect the needs of customers for a system that serves a certain purpose such as controlling a device, placing an order, or finding information. The process of finding out, analyzing, documenting and checking these services and constraints is called requirements engineering (RE).

User Requirement

User requirements are statements, in a natural language plus diagrams, of what services the system is expected to provide to system users and the constraints under which it must operate. The user requirements may vary from broad statements of the system features required to detailed, precise descriptions of the system functionality.

System Requirements

System requirements are more detailed descriptions of the software system’s functions, services, and operational constraints. The system requirements document (sometimes called a functional specification) should define exactly what is to be implemented. It may be part of the contract between the system buyer and the software developers.

User requirements and system requirements example.

Software system requirements are often classified as:

• functional requirements or

• non-functional requirements:

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.

Here are examples of functional requirements for the Mentcare system, used to maintain information about patients:

• A user shall be able to search the appointments lists for all clinics.

• The system shall generate each day, for each clinic, a list of patients who are expected to attend appointments that day.

• Each staff member using the system shall be uniquely identified by his or her eight-digit employee number.

Open Ended

Give an example of a functional requirement based on your SBA

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 constraints imposed by standards. Non-functional requirements often apply to the system as a whole rather than individual system features or services.

Product requirements

These requirements specify or constrain the runtime behavior of the software. Examples include performance requirements for how

fast the system must execute and how much memory it requires; reliability requirements that set out the acceptable failure rate; security requirements; and usability requirements.

An example:

• The Mentcare system shall be available to all clinics during normal working hours (Mon–Fri, 08:30–17:30). Downtime within normal working hours shall not exceed 5 seconds in any one day.

Organizational requirements

These requirements are broad system requirements derived from policies and procedures in the customer’s and developer’s organizations. Examples include operational process requirements that define how the system will be used; development process requirements that specify the programming language; the development environment or process standards to be used; and environmental requirements that specify the operating environment of the system.

An example:

• Users of the Mentcare system shall identify themselves using their health authority identity card.

External requirements

This broad heading covers all requirements that are derived from factors external to the system and its development process. These may include regulatory requirements that set out what must be done for the system to be approved for use by a regulator, such as a nuclear safety authority; legislative requirements that must be followed to ensure that the system operates within the law; and ethical requirements that ensure that the system will be acceptable to its users and the general public.

An example:

• The system shall implement patient privacy provisions as set out in HStan-03-2006-priv.

Metrics for specifying non-functional requirements:

Testing non-functional requirements

Whenever possible, you should write non-functional requirements quantitatively so that they can be objectively tested. Figure 4.5 shows metrics that you can use to specify non-functional system properties. You can measure these characteristics when the system is being tested to check whether or not the system has met its non-functional requirements.

An example using the metric 'ease of use':

Medical staff shall be able to use all the system functions after two hours of training. After this training, the average number of errors made by experienced users shall not exceed two per hour of system use.

Open Ended

Give an example of a functional requirement based on your SBA