Functional vs Non-Functional Requirements
What are functional vs non-functional requirements of software engineering? Why is it essential to have functional and non-functional requirements for your project to succeed? Which is more important? What is the difference between functional and non-functional requirements? This article will answer all of these questions.
The ultimate goal of any project is to deliver a quality product that meets the customer’s expectations. Customers communicate their requirements to project teams through functional requirements. Functional requirements are essential to ensure the development team is progressing in the right direction.
Unclear requirements can lead to a poorly defined scope, which can cause many difficulties from the project’s beginning. A poorly defined scope can lead to delays and rapidly increasing costs. As a result, your customer might not have the funds or time to invest in the high-quality product they desire.
Most customers have both wants and needs. For example, the customer may request to reduce scope once they have received the cost estimate. Most often, removing specific non-functional requirements will reduce the complexity adequately. However, having many non-functional requirements can increase costs. On the other side, not enough non-functional requirements may result in a poor user experience.
Both the client and the supplier will benefit from understanding the differences between non-functional and functional requirements. In addition, understanding the difference allows for better scope definition, cost optimization, and ultimately, happy customers.
A comprehensive and sensible collection of functional and non-functional requirements is the most important thing a project should have to avoid failure.
Every project requires the requirements to be clearly understood, balanced, and well-thought-out by everyone involved. But perhaps the most important thing is to avoid requirements being de-scoped halfway through the project.
But, what does it mean to distinguish between “functional” and “non-functional” requirements? It’s not complicated, and once the distinction is understood, it will become easy to define.
Functional requirements are typically used to specify a behavior or function. Officially, a functional requirement specifies what the system must do.
A perfect example of a functional requirement is: “Display the name of the flash drive, its total size, space, and format.” An additional example would be to “add a customer to a system when they order a product.”
Perhaps an even more relatable functional requirements examples for a fuel canister includes the “ability to contain fluid without leaking.”
What are Functional Requirements?
A functional requirement can be defined as: “Any requirement specifying what the application ought to do.”
A successful product will have strict requirements, which holds true for all projects. Therefore, it is no surprise that 68% percent of projects meeting exact requirements achieve the required quality standard.
Another way to put it, a functional requirement describes a particular system’s behavior when certain conditions have been met, for example, “Send email to a new customer who signs up” or “Open a new bank account.”
A functional requirement for an object many people use daily, such as a thermos, would be: “ability to contain coffee or tea without leaking.”
What are the advantages of Functional Requirements?
These are the advantages of creating a functional requirement document:
- It allows you to determine if the application has all the functionalities specified in the functional requirements.
- You can use a functional requirement document to describe the functionality of a system and its subsystems.
- Both functional requirements and requirement analysis can help you identify missing requirements. This helps define the expected system behavior and service.
- The most cost-effective time to correct errors is during the functional requirement gathering stage.
- Facilitate project management by supporting user goals, tasks, and activities.
- You can express functional requirements in Use Case forms or user stories, provided they show externally visible functional behavior.
What are some examples of Functional Requirements?
Some examples would include:
- When a user attempts to log in, it authenticates them.
- In the event of a cyberattack, the system shuts down.
- An email verification is sent to the user when they register the first time for a particular program.
- The software validates customers automatically against the Salesforce system.
- The Sales system must enable users to track customers’ sales.
- Only employees at the Managerial level can see revenue data.
- The system should be compatible with the bank’s API.
What are the 11 most common types of Functional Requirements?
- Administrative functions
- Authorization levels
- Audit Tracking
- Business Rules
- Certification Requirements
- External Interfaces
- Historical Data
- Legal and Regulatory Requirements
- Reporting requirements
- Transaction cancellations, corrections, and adjustments
What about Non-Functional Requirements, you ask? How do they differ from functional requirements?
The difference between them is non-functional requirements describe the system’s operation while functional requirements describe what the system should be doing.
A non-functional requirement is defined as “how the system should behave” and becomes a limitation on the system’s behavior. Non-functional requirements could be captured as a quality attribute of a system.
One non-functional requirement of a bicycle helmet could be “must not break when dropped from 2 meters.”
What are Non-Functional Requirements?
A non-functional requirement can be defined as: “Any requirement specifying how the system must perform a specific function.” Non-functional specifications specify how a system should behave and determine the limitations on its functionality.
Non-functional requirements encompass all remaining provisions not covered under the functional requirements. These criteria are used to evaluate the system’s operation and not specific behaviors. For example, “users should be able to retrieve data from the system within three seconds.”
For the gas canister previously mentioned, a non-functional requirement would be to: “Contain hot liquid but not heated up to more than 100 degrees Fahrenheit.” Basic functionality is not affected even when a non-functional requirement cannot be met.
Why are nonfunctional requirements necessary if the product’s functionality isn’t dependent on them? It is all about usability, as the user’s experience is affected by non-functional requirements. They define the system’s behavior, general characteristics and inform the features.
When non-functional requirements are clear-cut and properly implemented, they will make the application easier to use and ensure it performs well.
The user’s expectations are the main focus of non-functional requirements. Therefore, the non-functional specifications become the product’s properties.
Let’s look at a simple example of a functional requirement. When a user clicks on a button, the system loads a page. The corresponding non-functional requirement specifies the acceptable response time for the webpage to load. A delay in loading can lead to a poor user experience even if all functional requirements are met.
What are the advantages of Non-Functional Requirements?
There are many benefits to these requirements:
- NFRs are necessary to ensure compliance and legal or regulatory requirements.
- They guarantee the reliability, availability, performance, and security of the application.
- NFRs ensure a great user experience and easy operation of the software.
- They assist in the formulation of security policies for the software system.
What are some examples of Non-functional Requirements?
Some examples of non-functional requirements include:
- Emails should be delivered within 8 hours.
- Each request should be processed in less than 3 seconds.
- After the first login success, customers should immediately change the initial login password. It is vital that the initial password not be repeated.
- A user is not permitted to modify their salary information. The security administrator should be notified if such an attempt is made.
- Audit trails will be kept of every failed attempt to gain access to data items by users.
- Websites should be capable of handling 10 million customers without negatively affecting their performance.
- Software should be easily portable; moving from one OS to another is not a problem.
- Privacy of information, exports of restricted technologies, and intellectual property rights should all be audited. Audits should be conducted monthly.
What are the 25 most common types of Non-Functional Requirements?
- Accessibility – a measure of how easily a solution is available to people with Motor, Cognitive and Visual impairments.
- Reliability, availability, maintainability – How often does the system fail? What are the downtimes, and how often is it possible to prevent them?
- Capacity, current, and forecast.
- Compliance, Regulatory – Describes product conformity to laws.
- Data Integrity – describes how data accuracy, completeness, consistency, and overall accuracy are handled.
- Documentation – Describe what the system accomplishes and how to utilize it.
- Disaster recovery – A collection of policies, tools, and procedures permitting the recovery or continued operation of vital technology infrastructures and systems following an accident or natural disaster.
- Efficiency – how effectively the system uses scarce computational resources like CPU cycles, memory, and disk space.
- Extensibility – ability to extend a system and the level of effort needed to implement the extension.
- Fault tolerance – Ability to meet requirements despite system or hardware failures.
- Interoperability – The ability of software to communicate with other components or systems.
- Manageability – Ability to monitor and manage software to ensure the system is running smoothly and securely.
- Performance – Measure of how well a system responds with data based on utilization and throughput.
- Privacy – what applications will be integrated with, how will data be stored, and who has access to it?
- Portability – Which operating systems, browsers, and hardware do you use to run the software? Is it compatible with other programs and processes in these environments as well?
- Recoverability – The capability of software to restore damaged operational data or achieve the desired level of performance.
- Resilience – the ability of a solution to absorb the effect of a problem within a system and still provide acceptable service levels to the business.
- Response time – measures how long a server takes to reply to every single transaction or query.
- Robustness – the ability of a computer system to deal with errors during execution or with incorrect input.
- Scalability – Assesses the system’s ability to handle high workloads without compromising its performance.
- Security – How do you protect your system and data from hackers?
- Serviceability, Supportability – the ability of a service/technical expert to install the software system in a real-time environment, monitor the system while running, identify any technical issues in the design and provide a solution to resolve the issue.
- Stability – determine if a software application can execute or accomplish tasks within specified conditions or for a set duration.
- Testability – the extent that a system supports testing in the development process.
- Usability – how close does the intended use of the system match the software’s implementation?
Non-functional requirements, as mentioned above, specify the system’s “quality characteristics” or “quality attributes.”
Many stakeholders are interested in ensuring non-functional requirements are met, especially in large systems where the buyer is not necessarily the user.
Non-functional requirements are therefore essential and should not be underestimated. Use non-functional requirement groupings is one way to ensure there are no non-functional requirements left out.
What is the difference between Functional and Non-Functional Requirements?
The table below aims to depict what the differences are by comparing them head-to-head.
|Parameters||Functional Requirements Non-Functional Requirements|
|What it is||Verb||Attributes|
|Requirement||It is mandatory||It is optional|
|Defined by||The subject matter experts||Architect, Technical Leaders, or Software Developers|
|Defined at||Component level||System level|
|Capturing type||It is documented in use cases||It is documented as a quality attribute.|
|End-result||Product feature||Product properties|
|Capturing||Easily captured||Difficult to capture|
|Objective||It helps you verify the functionality of the software||It helps you to verify the performance of the software|
|Area of focus||User requirements||Focus on user expectation|
|Documentation||Describe what the product does||Describes how the product works|
|Type of Testing||Functional Testing like System, Integration, End to End, API testing||Non-Functional Testing like Performance, Stress, Usability, Security testing|
|Test Execution||Test Execution is done before non-functional testing||After the functional testing|
|Product Info||Focuses on Product Features||Focuses on Product Properties|
What is the relationship between Functional and Non-Functional Requirements?
Often, a functional requirement is accompanied by a non-functional one. Keep in mind that the difference is essentially what and how. They are both essential. Here are some examples:
|Functional Requirement||Related NonFunctional Requirements|
|When a new account is created, the server shall send a welcome email.||Sending welcome emails must be done within 5 minutes of registration.|
|If the order status changes to fulfillment, the details will be printed by the company printer.||Packing slips must be printed on both sides of white paper sheets measuring 5″ x 8.5″|
|The system should allow a customer to complete and submit a service request form.||The form must load in 3 seconds after it is requested from the server.|
How to go about gathering requirements?
A guided brainstorming session is a great way to gather requirements and bring all stakeholders together.
Four basic functional requirements can be divided into:
Business requirements: Define the ultimate goal, such as an inventory management system, an e-commerce platform, or a physical product. You can also find things such as approval workflows or authorization levels.
Administrative functions: These are the everyday things the system will do, such as reporting requirements.
System requirements: These could be software engineering or hardware specifications, system reactions, or system action.
Once you have clearly defined the functional requirements, it’s time to start gathering nonfunctional requirements questions.
Usability: This section focuses on the interface’s appearance and how users interact with it. What colors are the screens? What should the size of the buttons be?
Reliability/Availability: What would an acceptable downtime be? What are the uptime requirements?
Scalability: Is the system capable of handling a growing number of simultaneous users? For physical installations, spare hardware is included with the possibility of future expansion.
Performance: What are the acceptable response times?
Supportability: Are you able to access external resources remotely, or can you receive support in-house?
Security: What security requirements are there for both the physical installation as well as cyber-security?
What is the distinction between Business Requirements vs Functional Requirements?
The difference between business requirements and functional requirements is the question they answer. Business requirements define what a business desires (the objective). Contrast that with functional requirements, which deal with how the business can achieve the desire.
How do you write non-functional and functional requirements?
There are many ways to write functional or nonfunctional requirements.
A requirements specification document is the most popular way to describe functional vs nonfunctional requirements. It is an overall description of the functionality required.
It contains the project goal and a brief overview of the project. The requirements specification document also provides applicable constraints and assumptions. To aid non-technical stakeholders’ scope understanding, the requirements specification document should contain visual representations.
The work breakdown structure (WBS) is closely related to a requirements specification document. This allows you to break down the entire process by “decomposing” requirements into their components until they can’t be broken down anymore.
User stories are another approach to writing requirements. A user story is written from the user’s perspective and describes the desired behavior of the system.
An effective User Story states “As a _______, I want to _______.” The beauty of user stories is they do not require technical knowledge to be written. User stories can be used in conjunction with requirements specifications to help an end user define their needs.
Use cases can be compared to User Stories because they don’t require technical knowledge. Instead, these use cases are an overall description of what a user does while they do a task. A good example is “purchase product,” which would describe each step during the purchase process.
What are best practices for functional and non-functional requirements?
Regardless of their form, the key to effective functional requirements is clear, concise, and easily understood. Every audience is different. However, in every case, the more the requirements are in the natural language, the better. Active voice is preferable to passive voice when it comes down to language. By using active voice, you can make requirements more straightforward and concise by ensuring an actor in every requirement statement.
Write precise and complete requirements. Avoid vagueness when writing them. Avoid confusing people by not including unnecessary information. When writing user stories or a requirements document, use “must” rather than “should.” Use the same terminology and units throughout the document, as well as the same format and language.
You must clearly communicate the requirements. What does it mean if stakeholders insist the system should be “fast”? What does it mean when stakeholders state the website must handle “a lot” users? These requirements must be reduced to a number and quantified.
Good requirements can be tested. But, how do you know when the condition was fulfilled? It is crucial to write requirements in a clear way so they are not combined. This will make it easier to turn your needs into reality and help with later testing.
You should ensure your requirements cover every scenario. That means you need to include requirements detailing what the system must not do. However, be careful not to be too specific. Be specific about the features users need. A more extensive product is not always better. Larger projects often result in increased costs, diminished impact, and a product with too many features that is difficult to use and confuses users. It should be possible to track back each requirement to one project objective. It is helpful to compare the industry with your competitors to get a better understanding of their work. Sometimes you have to aim for the same quality as your competition. Other times you reach a point of reference from which you can be better.
When speaking with stakeholders and users, it is crucial to understand better their needs and how they relate to project objectives. Sometimes, they present a solution when they should be offering a problem. Offering the problem allows for a development team to brainstorm the best solution. You should also be aware of their authority to submit requirement requests. A project manager usually has the final say. This should be discussed before adding more requirements. Document and create a requirements traceability matrix. Requirement traceability matrixes help you contact the person right person when questions arise. It is often an iterative process to develop specifications. Ensure the requirements written by you are both feasible and do not conflict.
It can be difficult to define requirements. It is sometimes difficult to define requirements clearly. However, you can make them easily understandable by non-technical stakeholders. To reinforce the information, visuals are encouraged.
Last but not least, don’t consider nonfunctional requirements irrelevant, regardless of their name. For example, although an application that takes 2 minutes to load may meet its functional requirements, it would not be usable.
How to make Non-Functional Requirements Visible in Scrum?
One of the best ways to make scrum non-functional requirements visible is by adding all of your development team’s NFRs to a checklist. This checklist can then be added to each user story and verified prior to moving a user story to Done.
In addition to a checklist, AccelaTest can help answer the question of “how to make non-functional requirements visible in scrum” through the use of Shared Steps. Shared Steps are test steps that can be called in multiple test cases and form the backbone of a successful development team.
How can AccelaTest help with functional vs non-functional requirements?
One of the most critical parts of testing is having traceability between requirements and test cases. AccelaTest is a Free Test Case Management Solution that has integrations with project management systems like Jira to facilitate the traceability between requirements and test cases. In addition to having traceability, AccelaTest offers re-usable test cases that can be used to document and test your Non-Functional Requirements. Get started with documenting your requirements and adding traceability into AccelaTest.
When it comes to functional vs non-functional requirements, both types are critical to a project’s success. Non-functional requirements should not be cut or scrimped on to make a project fit a budget because a project without NFRs will lead to poor quality. Writing solid requirements does not need to be complicated. Every requirement must be clear, concise, and easy to understand.