THE IMPORTANCE OF FUNCTIONAL AND NON-FUNCTIONAL REQUIREMENTS IN SOFTWARE DEVELOPMENT
Requirements specification is an important document that should ideally be described for each product. Well-described requirements simplify the work of the entire development team and make the customer happy. It simplifies the understanding of how the product should be implemented and how to properly design the architecture. The ability to write and test requirements well is an indicator of competence and professionalism. In this article, we are going to research the difference between functional and non-functional requirements and why both are so important to start a software development.
Originally, this material was published by Tonya Smyrnova on UppLabs blog.
SOFTWARE REQUIREMENTS SPECIFICATION (SRS DOCUMENT)
The first step is to identify the business requirements that the company wants to implement for the product. These requirements will be used to assess the success of the product.
Another group of requirements is stakeholder requirements that reflect their needs and how they want to interact with the system. To ensure effective work at this stage, a number of techniques have to be developed: brainstorming and focus group, techniques of active and passive observation, business process modeling, analysis of documents and business rules, and many others. This allows taking into account the features of the project, organization, material, regulatory and technical resources.
One of the first documents created during any project to develop an information system is a concept document, which describes at a high level
- the purpose of the system,
- the context of its use,
- approaches to solving problems,
- limitations, and
- strategic decisions on project implementation.
This document sets the boundaries of the project and provides a common vision of the product.
In order to unambiguously understand the requirements for the software product, a team creates a Software Requirements Specification (SRS), which contains highly detailed functional and non-functional requirements, behavioral models, user interface, etc. The SRS document is the most widely used form for software requirements. By agreement with the Customer, different types of specifications can be used: Use Case Specification, User Story, Specification by Example, and others.
Why do we need the Requirements:
- We can get an agreed list of business requirements, stakeholder requirements, and decision requirements;
- We can obtain the ability to determine the priorities by which the system will be developed and implemented;
- We will create a base for estimating the complexity and overall costs of the project.
What do we need the Requirements Specifications:
- It’s a complete description of the developed system’s behavior;
- We’ll get the fully formulated functional and non-functional software requirements;
- There is a possibility of accurate assessment of stages and plans, terms of the project;
This is the ground base for starting work on software development.
TWO CATEGORIES OF REQUIREMENTS
There are many requirements for the characteristics, quality of software products, information systems. However, they can be divided into only two major categories — functional and non-functional requirements. It is important to mark the difference between them.
- Functional requirements describe what specifically needs to be implemented in a system or product, what actions should be performed by users about this development.
- Non-functional requirements describe exactly how the created system or software product works, what properties and characteristics a particular development has.
WHAT ARE FUNCTIONAL REQUIREMENTS?
These requirements include all the necessary functionalities that the end-user specifically demands as basic facilities for the system to offer. They are usually written in the contract and represented in the form of input to be given to the system. They are the requirements stated by the user which one can check and see directly in the final product, unlike the non-functional requirements.
In other words, a functional requirement will represent a particular behavior of function of the system, for example: “Create a new account for a new customer”.
Some of the more typical functional requirements include:
- Business Rules
- Transaction corrections, adjustments, and cancellations
- Administrative functions
- Certification Requirements
- Reporting Requirements
- Historical Data
- Authorization levels
- Audit Tracking
- External Interfaces
- Legal or Regulatory Requirements
WHAT ARE NON-FUNCTIONAL REQUIREMENTS?
The non-functional requirements primarily include various attributes of product quality properties — the requirements that determine the qualitative characteristics of development (software, information system) such as reliability, scalability, product performance, etc.
Non-functional requirements try to meet the user’s expectations, as they are product properties.
Some typical non-functional requirements are:
- Performance (Response Time, Utilization, Throughput, Utilization, Static Volumetric)
- Data Integrity
As for the non-functional category of requirements, it is important to involve not only analysts, but also key product developers, system architects, and a group of testers. An architect, for example, will define the non-functional requirements as input for selecting and designing a program architecture while the testing team will plan the appropriate load testing scenarios.
DIFFERENCE BETWEEN FUNCTIONAL AND NON-FUNCTIONAL REQUIREMENTS
Functional and non-functional requirements are usually interrelated, and it is necessary to track the dependencies of the requirements. In requirements management systems, there exist so-called matrix traces, where arrows graphically show the relationships between requirements.
EXAMPLES OF FUNCTIONAL AND NON-FUNCTIONAL REQUIREMENTS
Functional Requirements Examples:
- The user tries to log into the system and he/she needs to go through the authentication.
- The system shut down because of a cyberattack.
- The user registers for the first time and he/she gets the verification email.
Non-functional Requirements Example:
- Restrictions, like the development, should be conducted only on platform A.
- Only biometric identification techniques should be used for user information authentication.
- Each request should be processed within 20 seconds.
- The website loads in 3 seconds when the number of simultaneous users is more than a thousand.
HOW TO TEST FUNCTIONAL AND NON-FUNCTIONAL REQUIREMENTS?
Testing the requirements is a necessary step to improve them by clarifying, detailing, as well as ensuring mutual understanding between team members and avoiding different interpretations. In addition, testing the requirements will help to understand whether they can be implemented in general in terms of resources, time, budget, or technologies involved.
There are the following testing techniques:
1. Mutual review
- Quick review — the author of the requirements provides a document for a quick review to colleagues who give their comments, recommendations, ask questions in the form of a simple informal discussion.
- Technical review — the requirements are provided by the author for review to a group of experts that document all comments.
2. Ask questions
If questions arise during the study of requirements — all contradictory issues are clarified with experienced colleagues or the customer.
3. Verification of requirements
To be able to test, you can try to create a checklist or test case for a specific requirement. If you can quickly come up with tests for a checklist or test case — it’s not bad.
4. Test the behavior of the implemented system
The user’s work with the system created according to the tested requirements. You may notice unclear or ambiguous points in working with the system.
5. Graphic visualization and prototyping
Depicting information in the form of drawings, diagrams, prototyping the system (user interface) helps to better analyze the information in the specification, to find inconsistencies and inaccuracies.
SOME TIPS ON COMMENTING ON THE REQUIREMENTS
- Do not change the format of the documentation file. You do not need to delete or change the source text, you need to leave comments on the text or suggest edits. The document must remain in an editable format (TXT / Excel / DOC), .pdf format or images will not work.
- The comments should include only problem areas. You don’t need to note well-formulated requirements. This only complicates the work with comments, because among all the comments you will have to choose the ones that need to be corrected.
- Do not describe the same issue in several places. If there is a need to write the same remark to the same information, it is better to make this remark at the end of the document.
- Indicate the exact place of the remark’s referring. It is not necessary to highlight the whole paragraph if the remark refers to one sentence.
- If it is necessary to add a clarifying question, it should be formulated very precisely and thoughtfully.
- There is no need to write very long comments. It’s much easier to perceive short, formulated, and structured text without spelling mistakes.
- There should be only constructive comments. Do not write comments in the form of criticism of the text or its author or categorical remarks like “it is impossible to implement”.
- Do not edit the requirements without agreement. It is possible to make changes to the specification only after coordination with the responsible persons. Otherwise, there can be an extremely serious situation when something in the product is not implemented as planned, due to uncoordinated changes in requirements.
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