Functional and non-functional requirements: Definitions and Examples

Functional business requirement is a list of services that the system must perform, and it should be indicated how the system responds to certain input data, how it behaves in certain situations, etc. In some cases, it is indicated that the system should not do.

Standard types of functional requirements:

• outline of objects / functions;
• outline of input data and their sources;
• outline of the output data with an indication of their destination;
• outline of what is needed to perform the function;
• outline of the preconditions that must be met before calling the function, and a description of the post condition that must be met after the end of the function (if it is a functional specification);
• outline of side effects.

Functional requirement forms the functionality of the software that is necessary to perform user tasks within the framework of business requirements. Such requirements are often formulated using the word “should”, for example: “The system should send the user a receipt for payment.”

Software functional requirements are documented in software requirements specification (SRS), which describes the expected system’s behavior as fully as necessary.

The functional project documentation include:

• Specifications for certification.
• Business rules.
• Specifications for reporting.
• Audit tracking.
• Administrative functions.
• Historical data.
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• Levels of authorization.
• Technical specifications.

Functional documentation

Functional documentation being a detailed outline of the features and capabilities that a product has consists of the following:

• Aim: background information, identifications, and review of a system.
• General description: vision, assumptions and business rules of a product.
• Special requirements: functional and database requirements, and system attributes.

What are non-functional requirements of a system?

 Software non-functional requirements describe the system’s characteristics and the characteristics of its environment. Among the restrictions imposed on the actions and functions worried by the system are the following: time limits, restrictions on the system development process, standards, etc.

Non-functional requirements are associated with system integration features such as reliability, response time, or system size. In addition, non-functional requirements may determine systemic restrictions, such as:

• bandwidth of input-output devices;

• data formats used in the system interface.

Non-functional requirements reflect users’ requirements. Non-functional requirements are based on budgetary constraints. They also take into account the organizational capabilities of the development company, the possibility of interaction of the developed system with other software and computer systems, as well as external factors such as safety regulations, legislation on the protection of intellectual property, etc.

Non-functional requirements characterize quality attributes and goals. Quality parameters are an additional product description features, expressed through a specification of its characteristics that are important to users or developers. These characteristics include:

• easiness & simplicity in usage;
• easiness of movement;
• integrity;
• efficiency & tolerance;
• external interactions between the outside world and the system itself;
• design & implementation constraints. Limitations relate to the choice of the possibility of developing the form and structure of the product.

The differences between functional and non-functional requirements

Functional requirements and non-functional requirements are somewhat similar, but also there is a number of differences. Comparison table is provided below.

Thus, software functional requirements define the object of architecture, which means «what» architecture do; non-functional requirements in SRS determine subject to “what conditions” (for example, the response speed when performing a given operation); Use Case is also the functional requirements.