Functional vs Non-functional Requirements

What is the difference between functional and non-functional requirements? How to define requirements in software development? What decisive aspects do teams have to consider? Finding answers to these and other related questions is really helpful in streamlining the development process and building successful software. This article discusses the topic of functional vs non-functional requirements and introduces essential details on how to impose them regarding your project needs.

List of the Content

• What are requirements in software development?
• What are functional requirements?
• What are non-functional requirements?
• Functional vs non-functional requirements
• How to impose functional vs non-functional requirements?
• In conclusion

WHAT ARE REQUIREMENTS IN SOFTWARE DEVELOPMENT?

When businesses approach software development, they realize how vital requirements can become. Actually, they cover so many aspects and details to impact product delivery and maintenance. It’s related to gaining a vision of the future product, the development strategy, the project scope, required resources, etc. Combining different types of requirements allows the team to lay a solid foundation for further implementation.

Therefore, companies always have to go with a comprehensive requirements analysis. The precise definition greatly helps that the final product comes with no surprises. Experienced teams know how to define requirements to streamline the process and meet the outlined needs. Moreover, it’s done in cooperation with all the stakeholders. The requirements in software development cover even the most minor details regarding business needs.  

Every team should realize that the lack of requirements could lead to project failure. It’s even without mentioning the fact that missing requirements aren’t acceptable in software development at all. Any development process starts with the requirements analysis stage involving all the stakeholders. The primary task remains to discover project needs and carry out detailed requirements analysis.

Business proficiency, tech expertise, and open communication become key components here. At the initial stage, the team agrees on many decisive aspects so that they can later reach outlined expectations. The input from every involved party is an absolute necessity. It’s essential to reach a complete understanding between the product owner and the development team. That involves the assistance of a number of specialists to conduct a comprehensive analysis. They help to get the product vision from different perspectives. Here are some typical representatives to get involved:

• Product owner
• Business analysts
• Project manager
• Tech lead
• Software engineers
• UI/UX designers
• QA specialists
• Marketing managers
• End-user representatives

Encouraging close cooperation between the mentioned specialists allows the team to reach an agreement in all related areas. What’s more, it describes the project on each required level and makes requirements clear for all involved parties. Software requirements aren’t only tech details recognizable to tech specialists. It goes further as chosen requirements give the future product vision and find out the full extent of the project.

The point is that well-defined requirements allow the team to start the project and completely understand it. And it’s commonly specified in terms of expert roles, assigned responsibilities, and tasks. As long as the team outlines business needs, they are ready to proceed with the choice of software features. And these aren’t only certain expectations but explicitly described and documented requirements related to functionality description, work scope, timeline, cost, etc. 

In order to meet specifications in software development, the team has to deal with both functional and non-functional requirements. Even though they are tightly related, these are two different categories of requirements and are often mixed with each other. We’ll continue with the detailed comparison of functional vs non-functional requirements and learn the difference between them.

WHAT ARE FUNCTIONAL REQUIREMENTS

Before we proceed to compare functional vs non-functional requirements, it’s worth addressing these groups separately. The idea is that every made requirement has to be well thought out, understood, and balanced. Thus, the team has to be proficient in recognizing needs and differentiating requirements.

Answering the question of what are functional requirements is possible when you start to specify something software should do. That covers all the necessary details to specify software functionality and behaviour. Functional requirements exactly describe what software has to do or doesn’t have to do. Also, any specifications referred to in this category will define software goals. It means that software won’t work unless functional requirements are met.

In fact, functional requirements are product features and functionality that the team is going to implement. In the light of comparing the functional vs non-functional requirements, the first ones describe the software behaviour under certain conditions. Functional requirements always provide a set of conditions, software behaviour, and a set of responses. 

The team is responsible for collecting and organizing software specifications. They transform the initial customer ideas and expectations into functional solutions. It supports the necessity to make those requirements clear for stakeholders and the development team so that it enables various activities, tasks, user goals, etc. 

The functional requirements describe how functionalities perform under certain conditions. The teams can apply different ways to collect and present these requirements. However, the most common option remains the text form. They also provide use cases, models, diagrams, user stories, and prototypes for better visualization.

Examples of functional requirements include:

• Business rules are constraints to guide system development. They are always must-follow requirements with specific criteria and compliance conditions. For example, “the system must send the confirmation email when the order is placed.” That’s a rule that specifies that all users must receive confirmation emails. At the same time, its criteria can be extended by more business rules to outline this process.
• Authentication is the process of recognizing the users’ identities. Teams can apply different types of credentials to ascertain users. However, the process remains quite similar as it undergoes identification and authentication. Users with granted rights and permissions can provide the required evidence to be authenticated.  
• Authorization levels help to determine software access levels. It covers the extent of system rights and data shared with particular users. Also, it guarantees that only trusted users gain access to the system regarding the chosen levels. That covers such authorization levels as hidden, restricted, and full ones.
• External interfaces allow the team to establish and follow the user’s and software’s interaction logic. It goes with creating user interfaces, screen layouts, buttons, etc. The good practice is to focus on its implementation through wireframing, mockups, and prototypes. Besides, it correlates the work of different components within the system underlying software and hardware needs.

These are just a few of the functional requirements examples, whereas you can continuously extend the list. The main idea is that recognizing the proper requirements guarantees the highest quality project delivery. Besides, their successful fulfillment supports a number of advantages to gain with functional requirements.

• Define the software functionality
• Outline system behaviour
• Examine transaction processing
• Support user expectations, tasks, and activities
• Identify missing requirements

WHAT ARE NON-FUNCTIONAL REQUIREMENTS

Comparing functional vs non-functional requirements, let’s proceed with the second group and learn some tips on recognizing its specifications. The main difficulty is that the non-functional requirements are always harder to capture and define for software development. They are tightly interrelated with functional requirements and sometimes even considered their extensions. 

Therefore, it is essential to draw special attention to what are non-functional requirements. Professional development teams always emphasize the importance of differentiating those categories. The first tip is to approach non-functional requirements as a quality attribute. They focus on capturing detailed descriptions of product properties. In terms of defining functional vs non-functional requirements, the last ones answer the question of how software has to work and perform. They don’t typically impact the outlined software functionality but specify the criteria of required software performance.

As you remember, the software never performs without meeting functional requirements. At the same time, it can function while not satisfying non-functional requirements. That means that the intended purpose can be partially met or result in unacceptable performance. In fact, non-functional requirements become markers to control how the system carries out the defined functionalities. As a result, they are often considered required attributes to verify software performance.

Besides, the team specifies the way they approach requirements analysis. When it handles functional requirements, the main focus is on the client’s needs. In contrast, the non-functional requirements cover user-specific needs. The team takes the responsibility to define and deliver an excellent user experience. They apply the best user-oriented approaches in recognizing these specifications.

As for documenting non-functional requirements, it is handled in parallel with all other requirements. And there won’t be something like a separate format because non-functional requirements bring some measures and objectives to the system behaviour. The most typical format remains the text form. Examining functional vs non-functional requirements examples, the difference could be even hard to notice.

Functional requirement: “the system must send the confirmation email when the order is placed.”

Non-functional requirement: “the system must send the confirmation email within 60 seconds when the order is placed.”

As a result, adding the right attribute helps to enhance the user experience and influence the system’s behaviour. Besides, you can check on more non-functional requirements examples:

• Usability specifies how easy or difficult it will be for users to interact with software. It involves setting time measures to finish tasks, a number of completed transactions without interruptions, event localization, etc.
• Reliability embraces the likelihood of software functioning without failure. Setting reliability measures allows the team to decrease code bugs, hardware and system component issues. The team gathers data on operation efficiency as well as focuses on its improvement.
• Scalability remains a decisive factor for any business growth. These non-functional requirements describe how software could grow without negatively impacting its performance. It defines options to serve more users, complete more transactions, add new servers, etc.
• The performance focuses on software responsiveness and improving user satisfaction. Low-quality performance and long load time negatively influence user experience. Thus, the team needs to form clear performance criteria regarding software behaviour.  
• Security requirements protect software from unauthorized access and guarantee privacy for its users. Defined attributes should cover all the smallest details on how the system data can be added, changed, copied, deleted, etc.

Considering mentioned non-functional requirements examples, you can see how it helps to ensure whether or not the system follows legal, compliance, and adherence regulations. It transforms the user experience and handles it on the highest level. Setting the proper non-functional requirements provides the business with the following benefits:

• Formulate reliable security policies
• Follow legal and compliance requirements
• Provide excellent user experience
• Enhance system performance

FUNCTIONAL VS NON-FUNCTIONAL REQUIREMENTS

The requirements analysis lays the solid foundation for the entire development process and the overall project success. The teams realize the absolute necessity to carry out this stage at the top level. Poor business analysis and poor communication between the business and development teams have serious consequences, including challenging or failed projects.

Knowing the basic principles of each requirements category, we’ll focus on the difference between them. Turning business needs into well-defined requirements is only possible with a clear understanding of functional vs non-functional requirements.

At some point, the non-functional requirements can be considered good extensions to the functional ones. However, the fact they are non-mandatory doesn’t mean that teams can omit them. If the functional requirements specify what the software does, the non-functional ones describe aspects of how the software works. The difference is that the first ones are always mandatory as they enable the system to perform. Even though non-functional requirements aren’t fully satisfied, the software will work. As a result, it becomes harder to meet all the set non-functional specifications but deliver the best quality and performance of the software. The more specific teams address their needs, the better results they can get within their project.

The other distinctive difference in functional vs non-functional requirements is the focus area. If the first group defines the user requirements, the other focuses on user experience. Functional requirements cover the needs of the business and present the necessary features to meet them. The customer and the development team have to reach an understanding of all aspects and turn the primary idea into a workable solution. Functional requirements cover all the details of required software functionality. And it’s worth mentioning that capturing them is easier than other specifications.

Non-functional requirements aim to provide an excellent user experience. The team has to ensure the effectiveness and usability of the software. Users won’t hesitate to reject software even if it has all the necessary features but fails to show strong performance. The business can receive a well-operating product after the comprehensive analysis and definition of non-functional requirements. It’s a unique opportunity to complement the entire system with quality attributes. 

Let’s compare a few more functional vs non-functional requirements examples for better understanding.

Functional requirements examples

“The software must do …”

• The system must authenticate the user when they log into the system.
• The system must send the verification email when the user registers for the first time.
• Users must have the ability to give feedback in the contact form.
• Top-level managers have the right to view revenue data.
• The system must allow the user to change the password when clicking the button “I forgot my password.”

Non-functional requirements examples

“The software should be …” 

• The authentication request should be processed within 15 seconds.
• The verification emails should be sent within 60 seconds.
• When the submit button is clicked, the confirmation message on received feedback should be sent within 2 seconds.
• Any attempt to change the revenue data should be reported to the security administration.
• The system should send the link within 60 seconds to the user’s verified email address to change the password.

HOW TO IMPOSE FUNCTIONAL VS NON-FUNCTIONAL REQUIREMENTS

Documenting the mentioned specifications is an essential step that positively impacts the development process. Imposing functional vs non-functional requirements won’t be completed without comprehensive documentation. It’s a strict rule that the requirements analysis and discovery stage results in documenting all the types of requirements. According to the research, 32% of projects fail due to incomplete requirements and poor business analysis.

Thus, the collaborative process allows the team to keep requirements up-to-date. And the continuous contributions from every stakeholder can evolve into the project’s progress. The primary goal of well-defined requirements is to bring the customer and the development team on the same page. The documented details shed light on the future product and define effective ways to implement it. 

As a result, imposing functional vs non-functional requirements is always worth the effort and time for the business of any specification. Besides the successful product delivery, they have all the chances to balance the budget and timeline. It’s a proven fact that unclear requirements could increase development costs by up to 60%. Close collaboration and detailed analysis help avoid certain misunderstandings and prevent project failure.

The most common approach to collecting and describing requirements is through the Software Requirements Specification (SRS) Document. It covers all the necessary details on what software does and how it should perform. The most challenging task is to create the outline for the software requirements specifications. The outlines might differ from project to project, but the teams commonly follow the next steps to write the SRS.

1. Introduction – It’s an essential part of setting expectations for the entire project. The team focuses on defining the purpose of the software, intended use, and targeted audience. Also, they outline the product scope and define the tech terms so all the involved parties understand the content.
2. Overall description – It’s a stage to outline the key details for every project. It includes software features, system architecture, business logic, design, and implementation constraints. Besides, it involves the definition of user interfaces and hardware interfaces.
3. Specific requirements – That’s a part to detail the enumerated requirements from the previous step. It’s handled to make the development process more efficient and smooth, including well-defined performance criteria. Also, it describes in the smallest detail how the software interacts with other systems and facilitates necessary integrations.
4. References – It collects information on grounds, facts, and evidence to apply specific technologies and development approaches.

Graphic visualization and prototyping are always a good idea in addition to creating the Software Requirements Specification (SRS) Document. The opportunity to depict outlined information in diagrams, schemes, mockups, and prototypes helps to approach your product from different perspectives. The Agile methodology encourages teams to describe software functionality from users’ perspectives by defining use cases and user stories.

Finally, we can say that teams have all the necessary tools and approaches to set the well-outlined requirements for their projects. Understanding functional vs non-functional requirements allows them to carry out the analysis and write documentation more efficiently. Here are a few more useful tips on how to impose the project’s specifications:

• Reflect business objectives
• Define terms and roles
• Have understandable and clear requirements
• Introduce accurate and complete documentation
• Coordinate changes in requirements
• Verify the specifications

IN CONCLUSION

Discussing the question of functional vs non-functional requirements, every team realizes the importance of a comprehensive analysis and detailed specifications for their projects. In fact, the effectiveness of this primary stage helps them to streamline the development process and deliver a valuable product. That’s the combination of functional and non-functional requirements that introduces efficient ways to implement customer expectations into workable solutions.