Fundamentals of functional and non-functional requirements

Just like house construction, any software development project must be based on a solid foundation. Clear objectives and accurate requirements cement parts of your solution into a coherent whole.

Conversely, inaccurate specifications can cripple your project and incur additional costs. Moreover, high failure rates are often the result of a misty project vision. Thus, around 66% of software projects fail, according to the Standish Group’s 2020 CHAOS report.

To avert failures, any experienced software team compiles a list of functional and non-functional guidelines. The latter establishes a shared vision between stakeholders and maps project milestones.

Today, we’ll look into these project specifications and explore the functional vs nonfunctional requirements difference.

Why do you need requirements for your project?

Knowing the background is half the battle in software development. To do that, teams create guidelines, identify requirements, and set clear goals. This slew of instructions is meant to put everyone on the same line and facilitate communications. Without them, the project is doomed to failure.

And here’s why:

  • A lack of congruence between business and project objectives causes 44 percent of initiatives to fail.
  • The number of failed initiatives is pretty high—a staggering 70% of all projects fail to deliver on what was promised to clients.
  • Also, 47% of projects fail to meet their goals due to poor management of requirements.

To prevent defeat, teams lay out detailed functional and nonfunctional requirements that bring the following benefits:

Ensure project success

Clearly documented processes have more chances to go as planned. Thorough guidelines lay the groundwork for the product’s vision, scope, cost, and timeline. The latter syncs with the project specifics.

Make the system work as anticipated

The final product may not come out as planned since software development is a flexible process with multiple iterations and improvements. Established requirements ensure that the product preserves high-level functionality and goals. Otherwise, stakeholders’ expectations will turn out as an unpleasant surprise.

Keep spending within the budget

Among other things, software requirements establish a set of must-have features. It means that teams can estimate the total more accurately, while the customer can plan their budget. Also, it enables the customer to change the functionality according to their budget.

Assign clear roles

Requirements help to guarantee that the development team and stakeholders are on the same wavelength in order to avoid future misunderstandings. Documented requirements also allow project managers to better allocate resources and assemble a team.

To sum up, explicit requirements form a common ground for all parties, save resources as well as map milestones and deliverables.

Now let’s have a closer look at the differences between functional vs non-functional requirements.

Difference 1: They serve different purposes

Functional requirements list a set of characteristics the system is expected to have. They also specify how the system reacts to certain inputs and how it behaves in specific conditions.

In simple words, functional requirements define the functionality of the software that developers must build so that users can accomplish their tasks within the business requirements. Sometimes called behavioral requirements, they contain clauses with the traditional “should” or “must”.

Non-functional requirements have nothing to do with the system functionality. Instead, they list the system’s operational capabilities and constraints such as security, reliability, and others. 

Thus, they feature goals and quality attributes that serve as additional descriptions of a product’s functions. For example, they can specify the exact number of password symbols that are necessary to enable authentication.

Difference 2: They have a different hierarchy

Both types of guidelines are based on different prerequisites. Functional ones stem from and include:

  • Business requirements – define high-level goals of a client.
  • User story — specify the value that the system yields for the end-user.
  • Use cases — outline the scenarios of interactions between the system and the user.
  • Wireframes — support the requirements with visual representations.

NFRs rely on a different set of documentation:

  • Business rules – business-related constraints that limit system development (regulations, standards, and policies).
  • Quality attributes – describe additional system characteristics linked with interoperability, stability, and others.
  • Constraints – outline conditions that limit the number of possible solutions. These may include system performance, maintenance, and others.

Difference 3: Functional requirements are easier to set

Functional guidelines refer to the technical features of a system. Thus, they are easier to discern and document. Non-functional ones prioritize user expectations and improve the usability of the system. Therefore, non-functional requirements are hard to measure, making evaluation difficult. As a rule, teams apply testing procedures to evaluate these properties.

Difference 4: The wording is different

Also, both requirements denote different aspects of the system which results in a specific choice of words. Thus, functional requirements have more verbs, while non-functional ones include verbs that are accompanied by adjectives, numerals, and others.

Let’s look at the example of a NFR:

  • When a user logs in, the application runs a dashboard with real-time updates.

On the contrary, non-functional ones have another wording:

  • When a user logs in, the application instantly runs a dashboard.

Now that you’re aware of the fundamental distinctions between the two, let’s see what you need to factor in when writing your NFRs.

Main criteria for non-functional requirements

Writing NFRs is often confusing since there are no clear metrics that establish them. Yet, there are some basics you need to include in your specification:

  • Usability. These requirements focus on the end value that your user interface brings. Usability criteria describe the look and feel of your solution.
  • Availability. These criteria define system availability if it has to be reachable at all times.
  • Scalability. This component establishes whether the system can manage evolving business needs.
  • Performance. These metrics demonstrate whether the system offers uninterrupted resiliency and enhanced stability.
  • Maintenance. In this section, specialists specify whether the support can be accessed in-house or remotely.
  • Security. Here, teams set the security measures for the system to prevent breaches and leaks.

The Final Word

Laying solid ground is crucial for any undertaking. In software development, accurate and shaped requirements are what set up the project for success. Therefore, defined functional and non-functional requirements are mandates for any tech success. While the former outline the technical limitations and stimuli for the system, the latter further deepens the inner workings of any application.

Interesting Related Article: “Seven Stages of Software Development