What are Functional vs Non-Functional Requirements?

Are you interested in knowing functional vs non-functional requirements? If yes, then you probably consider it highly important to get the requirements right for your software development project.

An effective functional specification document is an important one among your deliverables. Having said that, you can’t focus on functional requirements alone. You need to focus on non-functional requirements (NFRs) too.

What are the differences between a functional and non-functional requirement? We discuss that in this article.

We briefly recap what functional and non-functional requirements are. Subsequently, we discuss how they differ. Finally, we talk about what you need to do differently for each of them.

The various kinds of system requirements: A brief overview

We first set the context by briefly summarizing the two kinds of requirements for a software project. A requirements document should include the following:

1. Functional specification details

You need to define what your proposed software system will offer. You do that as a part of the functional requirements, where you define the functionality of the system.

As a part of the software development process, you create a functional specification document (FSD). This defines inputs, system behaviors, and outputs. You need to work with the end-user community to elicit these business requirements.

Creating functional requirements involves providing plenty of details. You need to provide mockups, wireframes, user stories, use cases, etc., and the FSD should describe the user interface in great detail.

The following are a few examples of functional requirements:

• User authentication rules;
• The description of the different user roles and their authorization levels;
• Transaction initiation, validation, approval, and cancellation rules;
• Detailed descriptions of system dialogs;
• Error reporting requirements.

2. Non-functional requirements (NFRs)

Unlike functional requirements that describe “what” a software system should do, NFRs describe “how well” the system should work. Non-functional requirements (NFRs) focus on the qualitative attributes of a software application.

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You can create application software that only delivers its functionality but not NFRs. Such a system would meet its functional spec. However, it wouldn’t deliver the right user experience.

A few examples of NFRs are as follows:

• Availability;
• Reliability;
• Security;
• Performance;
• Scalability;
• Usability;
• Maintainability.

The differences between functional vs non-functional requirements

The differences between a functional requirements document and NFRs are as follows:

1. Functionality vs quality attribute

The functional design of your proposed software system describes its functionality. It will specify the various user inputs. Furthermore, the FSD will describe the behavior of the system. Finally, it will describe the outputs of the application.

There lies the most crucial difference between functional vs non-functional requirements. NFRs don’t describe what an application will do. Instead, NFRs describe how well it should work. While functional specs deal with the inputs and outputs, NFRs are about qualitative attributes.

2. The dependence on use cases vs benchmarks

Are you undertaking a complex software development project? You need to provide as much detail in your functional design as you can.

Typical functional design documents are text-intensive, and you would need to do more. You need to ensure that both business stakeholders and the development team understand the requirements specification. Diagrams can help.

Use cases are use-case models that include diagrams and flowcharts. This makes it easier to understand them. For complex software development projects, use cases can enhance your functional design document.

There lies another key difference between functional requirements vs NFRs. You can’t depend on use cases to explain NFRs like maintainability and scalability. Instead, you need to rely on industry benchmarks.

E.g., are you creating a SaaS (Software-as-a-Service) product in the financial services sector? You need to study industry benchmarks to understand how scalable your proposed SaaS product should be. Your project manager needs to get hold of authoritative industry benchmarking studies for this.

3. The degree of importance during the product development lifecycle

Functional requirements are important during one stage of the development lifecycle. NFRs have their importance in subsequent stages, and there are reasons for that.

Functional requirements help you to define the project scope. Your project management processes should use the FSD to prevent scope creep. On the other hand, you need to meet the business rules and requirements mentioned in the FSD. End-users won’t accept the software system without that.

Assume that you are developing a “Minimum Viable Product” (MVP). Users will accept it only if the application meets the functional requirements. At that stage, your organization might not focus on NFRs like scalability.

Now, assume that you have progressed beyond the MVP. You are now launching a full-fledged product. Scalability becomes immensely important at this stage. Functional requirements will still have their importance. However, the success of your business will depend on your delivery of the NFRs.

4. The dependence on business analysts vs software architects

When you elicit functional requirements from your end-users, you have a heavy dependence on business analysts. You should form a team with a project manager, a software architect, and business analysts for gathering functional requirements.

Business analysts (BAs) interview all relevant stakeholders including end-users. They might need to conduct structured “discovery” sessions.

As we have explained in our guide to creating MVPs, BAs might need to use tools like the “pain and gain map”. This will help to map the planned features against the pain points highlighted by end-users.

BAs might also need to use tools like “prioritization matrix”. This helps them to prioritize the planned features. As you can see, you have a high dependence on BAs.

You will find the scenario quite different in the case of NFRs. End-users typically belong to organizational departments concerned with specific functionality. E.g., you might need to work with the procurement manager if you are building a supply chain management (SCM) solution. End-users won’t have the necessary knowledge to provide you with the NFRs.

A procurement manager will probably not know about the industry benchmarks for response time or scalability. As a result, you can’t rely on BAs for NFRs.

You need a knowledgeable software architect in your team. Working with the project manager, the architect will need to analyze industry benchmark data. This will help you to determine requirements like availability, scalability, performance, etc.

5. The use of a functional specifications document vs NFRs

You need both functional requirements and NFRs for your project. However, they are used differently. Consider the following examples:

Your development team will use the functional specification document to code the application system that provides the functionality. On the other hand, the team will use NFRs to optimize design, code, etc.

Testers will use the FSD to create functional testing cases. They will use the NFRs to create performance test cases, stress test cases, etc.

Architects use the FSD to make architectural decisions concerning interfaces, dependencies, etc. On the other hand, they use NFRs to choose a suitable architecture pattern.

For example, consider a proposed system that offers completely separate services with different peaks and troughs in demand. An architect might choose to use the microservices pattern for this.

What’s different when creating a functional specification document vs NFRs?

Now that you understand the differences between functional vs non-functional requirements, let’s analyze the commonalities and differences when creating them. We start with the commonalities, which are as follows:

• The team: In both cases, you need a competent team. Onboard a project manager, software architect, and business analyst with sufficient skills and competencies.
• Tools and templates: You need appropriate tools like Microsoft Office in both cases. You would also want a useful template to create requirement documents.
• The requirements management process: Whether you create an FSD or NFR, you need to follow an established requirements management process. This involves a series of steps. You need to have the requirements reviewed and approved. Finally, you need to use a change management process to prevent scope creep. These aspects remain the same irrespective of the development methodology you use.

What would you do differently to create an FSD vs NFRs? These are as follows:

1. Elicit requirements vs study industry benchmarks

To gather functional requirements, your team needs to engage with the business stakeholders including end-users. You need to have structured interview sessions. You might choose to use a template with a questionnaire.

These structured interviews will help you to understand the required functionality. You will get to understand the journey of the end-user through your proposed application. These interviews will enable you to understand the inputs, workflows, and outputs. In other words, this is a highly interactive process.

You will follow a markedly different process for gathering NFRs. Business stakeholders are experts in their functional areas, however, they might not know the industry standards.

E.g., an end-user will know which data security-related regulatory requirements the system will need to meet. However, nearly every industry is rapidly changing as far as information security is concerned.

Companies are adopting new information security standards. Some of these standards might be even more stringent than what the regulations require. Your company needs to remain competitive, therefore, it needs to follow these standards. You need knowledgeable architects that keep themselves up-to-date with these shifts.

2. Incorporating use cases, use-case models, etc. in an FSD vs text-intensive documentation for NFRs

We want to clarify upfront: A functional specification document is text-intensive. You might wonder what’s different when documenting NFRs. After all, NFRs are text-intensive too.

Well, you don’t just depend on texts alone with creating an FSD for a complex software development project. You provide plenty of information on user stories. You create use cases with flowcharts and schematic diagrams. Furthermore, you can create use-case models.

These aids enhance your FSD significantly. Both business stakeholders and the development team can understand it better.

Documenting NFRs is a different ball game though. You won’t create flowcharts and schematic diagrams to document NFRs like maintainability, scalability, etc.

Note: You will notice one common aspect while documenting a functional design vs NFRs. Both of them can use tables extensively. Consider the pre-conditions and post-conditions of transaction execution when you document functional requirements. Tables can help. When you document NFRs like performance requirements during peaks and troughs, tables can help once again.

3. You need to use different templates for a functional specification document vs NFRS

The differences in the content between an FSD and NFRs require you to use different templates. You can study templates in the Smartsheet repository of functional specification templates.

FSD templates in this repository include sections for use cases, and you can include user stories. The template for the generic software development projects is very different. It enables you to write descriptive NFRs as well as functional requirements.

4. Effort and expertise requirements when documenting an FSD vs NFRs

Assuming you have a team of experienced business analysts, you might not need too long to document functional requirements. Expert BAs know the tools and techniques. They know the right questions to ask. They also use a user-friendly template to create an FSD.

You need much more skills and expertise when documenting NFRs though. This task requires plenty of effort from software architects too, and not just from BAs. Documenting NFRs is inherently more complex.

Consider NFRs like maintainability or information security. You can’t just quantify these requirements in a straightforward manner. Documenting these NFRs and reviewing them takes time.    

Important considerations to keep in mind when documenting system requirements

Whether a functional specification document or NFRs, you need to document them effectively. Gathering and managing requirements make a big difference in executing a software development project successfully. Keep the following key considerations in mind:

• Onboard a project manager with deep knowledge of the requirements management process.
• Find a software architect with in-depth knowledge of key architecture patterns. The architect needs experience in creating technical solutions for different kinds of software projects.
• Onboard business analysts with deep knowledge of the industry. BAs also need familiarity with popular requirements management tools. Finally, they need the capabilities to collaborate with end-users and the development team.
• Form an effective team and foster collaboration.
• Plan for a thorough review of the FSD and NFRs. You need sufficiently experienced people for this.
• Get approvals for the functional spec and NFRs from relevant stakeholders. Use effective version-control tools, which will ensure that the entire team has the approved version of business requirements.
• Follow an effective change management process so that requirements can’t be changed without a proper review and estimation. This will prevent scope creep.

Summing up functional vs non-functional requirements

You need to elicit business requirements. You need to document them effectively in a functional specification document. Delivering a superior user experience requires you to deliver non-functional requirements, and you need to document them effectively.

Functional vs non-functional requirements have a few differences. We discussed them. We also discussed what you need to do differently while documenting them. Finally, we reviewed the key considerations while managing system requirements.

If you, as a business CTO, do not find the relevant software engineering skills in your project team to efficiently gather functional and non-functional requirements, we would advise you to partner with a reputed software development agency.

DevTeam.Space can also help you here with its field-expert software developers community. You can write to us your initial product specifications and we will get back to you instantly to see how we can get your software development project up and running.

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