What are Functional & Non-Functional Requirements?

When gathering requirements for software application/module development, requirements gathering remain as one of the key challenges for stakeholders at different levels including project manager, tech lead, software developers and test engineers. Interestingly, due to inefficient requirement gathering, most of the project fails.

Following is definition of functional requirements as per wikipedia:

In software engineering, a functional requirement defines a function of a software system or its component. A function is described as a set of inputs, the behavior, and outputs (see also software). Functional requirements may be calculations, technical details, data manipulation and processing and other specific functionality that define what a system is supposed to accomplish. Read further here.

Following is definition of non-functional requirements as per wikipedia:

In systems engineering and requirements engineering, a non-functional requirement is a requirement that specifies criteria that can be used to judge the operation of a system, rather than specific behaviors. Read further here.

Software requirements shall be gathered keeping in mind following:

• Stated needs: There are requirements that are specifically mentioned at the time of requirements gathering.
• Implied needs: These represent the requirements which are implicit and need to be confirmed with the end customers.

Requirements can be gathered keeping in mind following aspects of software quality:

1. Functional suitability: This is covered under functional requirements.
2. Maintainability: This is covered under non-functional requirements.
3. Efficiency: This is covered under non-functional requirements.
4. Security: This is mostly covered under non-functional requirement. However, at times, this also forms part of functional requirements.
5. Reliability: Non-functional
6. Usability (or Operability): Non-functional
7. Compatibility: Non-functional
8. Portability: Non-functional

If one sees above, its makes much more important for both, software and the test engineers have clear understanding about all the non-functional characteristic of software to be able to deliver software that can match or exceed customer satisfaction and achieve “customer delight”.