When defining software requirements, various specification types are used to capture critical details. Two of the most common categories are functional and non-functional requirements. Though with distinct focuses, both provide key pieces of the overall product picture.
We’ll compare functional vs non functional requirements in this article, including their purposes, how to write them, and when to apply them in the development process.
With a solid grasp of these two fundamental specification types, you can craft a complete set of product criteria. We’ll also contrast functional and non-functional requirements side-by-side on key factors, so you can determine when each is most appropriate depending on the project need.
What is the Difference Between Functional and Non-functional Requirements?
Bottomline Upfront: The key difference is that functional requirements specify the discrete behaviors and capabilities the system must perform. Non-functional requirements set quality standards and constraints on how the system operates.
Functional scope covers specific functions; non-functional scope addresses cross-cutting quality attributes. These differences are further explained as you read further in this article.
Types of Requirements
There are two main types of requirements in software projects: functional and non-functional.
Functional requirements define the behaviors and capabilities that a system must have. They describe the functionality and features users need the system to provide.
On the other hand, non-functional requirements specify quality attributes and constraints on how the system operates. Non-functional requirements include factors like security, reliability, performance, and maintainability.
While functional requirements focus on what the system should do, non-functional requirements outline how well the system should do it. Both are critical to capturing a complete set of specifications. Understanding the differences helps teams design systems that not only perform necessary functions but also meet quality standards.
What are Functional Requirements?
Functional requirements capture the intended behaviors and capabilities that a system must provide to meet stakeholders’ needs. They describe how the system should react under various conditions and in response to particular inputs.
Functional requirements focus on the business problem and how the software addresses it. They outline the functionality provided to fulfill what the business requires.
Some examples of common functional requirements include:
- User workflows – e.g. user registers account, user searches content
- Input rules and validation – e.g. validates password strength
- Navigation and page flows – e.g. user can return to home page from all screens
- Business processes the system must support – e.g. billing, reporting
- Calculations the system should perform – e.g. taxes, discounts
- Integrations with other systems – e.g. payment gateways, ERP systems
- Reporting needs – e.g. sales reports, operational reports
- Notifications or alerts for users – e.g. email notifications
- Permissions and access levels for users – e.g. restricted access to admin screens
Functional requirements should be written clearly and concisely, and each requirement should describe a single capability instead of bundling multiple needs. Well-defined functional specs enable testers to verify whether the delivered system provides the necessary behaviors and functions.
How to Write Functional Requirements
Crafting well-defined functional requirements is crucial for teams to build the right capabilities into a system. Follow these guidelines to create effective functional specs:
1. Focus on the User Perspective
Describe each requirement from the end user’s point of view, using language that aligns with how they will interact with the system. Avoid technical jargon and keep things user-centered. This aids understanding for both clients and development teams.
2. Isolate Individual Requirements
Decompose needs into singular functional requirements that each capture one specific capability the system must have. Bundling multiple requirements together makes them hard to test and validate, so break them down into concise, standalone specs.
3. Emphasize Problems, Not Solutions
Maintain solution-neutral language that focuses on the problem to be solved, rather than proposing specific implementation details. This gives designers the flexibility to determine optimal technical approaches.
4. Uniquely Identify Every Requirement
Assign each functional requirement a clear, straightforward name and unique ID number or code to reference it unambiguously. This avoids confusion and makes traceability simpler when requirements cross-reference each other.
5. Prioritize by Importance
Indicate priority status like high/medium/low to guide development sequencing. Critical user workflows and functionality should take precedence over nice-to-have capabilities.
6. Note Related Requirements
Call out dependencies between requirements that constrain the delivery order. Some capabilities rely on other prerequisites being built first.
7. Quantify Details Where Possible
Include measurable details like the number of concurrent users supported or maximum acceptable response times. Quantifiable requirements can be verified through testing.
8. Use a Standard Template
Adopt a consistent template for writing requirements to maintain uniform structure and terminology. This improves comprehension across large volumes of requirements.
9. Review with Stakeholders
Involve client stakeholders in validating requirements to achieve agreement and prevent mismatched expectations around functionality. Their feedback improves quality.
With thoughtful requirements development guided by these tips, you can produce functional requirements that clearly articulate the must-have capabilities for end users. The result is a shared understanding across clients and delivery teams.
What are Non-Functional Requirements?
Non-functional requirements define quality attributes and constraints for a system rather than specific behaviors. They set quantifiable metrics and thresholds for how well the system operates, contrasting with functional requirements that cover capabilities.
The focus of non-functional requirements is assessing the quality of system operation through defined standards and limits rather than specific functions. They place restrictions on system implementations regarding performance, scalability, availability, reliability and other operational factors.
These measurable criteria allow evaluation of aspects beyond just whether functionality was delivered. Non-functional requirements set expectations for system qualities like usability, responsiveness, and resilience. They provide a way to evaluate how well a system executes processes, rather than just what discrete capabilities it performs.
Non-functional requirements are key for ensuring systems not only perform functions but also meet quality and experience goals.
What are the 4 Non-Functional Requirements?
As earlier iterated, non-functional requirements define quality attributes and constraints for how a system operates rather than specific behaviors and functions.
Here are 4 key types of non-functional requirements:
1. Performance
This refers to how fast the system responds to user requests and executes processes. Performance requirements set benchmarks like page load times, transaction processing speed, and maximum concurrency supported.
2. Scalability
Scalability requirements outline how well the system can handle growth in workload or users without performance deterioration. They set scale targets like concurrent users supported and database size limits.
3. Availability
Availability requirements specify the percentage of time the system must remain operational and accessible for use. They set uptime goals and allowable periods of downtime for maintenance.
4. Security
Security requirements describe how access to the system and its data should be restricted and controlled. They include needs like user authentication, encryption, and compliance with regulations.
Other common non-functional requirements relate to usability, reliability, capacity, maintainability, and backup/recovery. Non-functional requirements use measurable metrics to define acceptable thresholds. Meeting non-functional criteria is essential for user satisfaction and system operation. However, functional requirements must take priority in the development process.
How to Capture Non-Functional Requirements
Capturing non-functional requirements is crucial for ensuring the overall quality and success of your software project. Follow these guidelines to help you effectively capture non-functional requirements:
1. Define Quality Attributes
Start by identifying the key quality attributes relevant to your project. These may include performance, scalability, security, usability, and reliability. Collaborate with stakeholders to determine which attributes are most critical for your system’s success.
2. Use Measurable Metrics
Transform abstract concepts into measurable metrics. For instance, instead of stating “The system should be fast,” specify “The system must respond to user queries within 2 seconds under normal load conditions.” This approach allows for objective evaluation and testing of the requirements.
3. Conduct Stakeholder Interviews
Engage with various stakeholders, including end-users, Business Analysts, and technical experts. Their insights can help you uncover implicit non-functional requirements that might otherwise be overlooked. Use structured interview techniques to elicit specific information about expected system behavior and performance.
4. Analyze Existing Systems
Examine similar existing systems or previous versions of your product. This analysis can provide valuable insights into non-functional aspects that users expect or areas where improvements are needed. Pay attention to user feedback and performance data from these systems.
5. Utilize Requirement Templates
Employ standardized requirement templates or checklists to ensure comprehensive coverage of non-functional aspects. These tools can guide you through various categories of non-functional requirements, reducing the risk of omitting critical elements.
6. Consider Future Scalability
Anticipate future growth and changes in your system’s usage. Capture requirements that address scalability and maintainability, ensuring your system can adapt to increasing demands and evolving technologies over time.
Functional vs Non-Functional Requirements: Key Differences
Understanding the distinctions between functional and non-functional requirements is crucial for effective project management and software development. Key differences include:
1. Focus and Purpose
Functional requirements center on the specific behaviors and features of a system. They define what the system should do in response to particular inputs or conditions. For instance, a functional requirement might state, “The system shall allow users to reset their passwords via email verification.”
In contrast, non-functional requirements focus on the quality attributes and performance characteristics of the system. They describe how well the system performs its functions rather than what it does. An example of a non-functional requirement is, “The system shall load user profile pages within 2 seconds under normal network conditions.”
2. Measurability and Testability
Functional requirements are typically easier to measure and test. You can often verify them through straightforward functional testing, checking if the system performs the specified action or not. For example, you can test if the password reset feature works as described.
Non-functional requirements often require more complex testing methodologies. They frequently involve quantitative measurements and may need specialized tools or environments for accurate assessment. Testing performance, scalability, or security requirements often demands more sophisticated approaches and may involve simulating various conditions or loads.
3. Impact on User Experience
While both types of requirements affect user experience, they do so in different ways. Functional requirements directly influence what users can do with the system, shaping its features and capabilities. They determine the system’s functionality and are often more visible to end-users.
Non-functional requirements, however, impact how users perceive the system’s quality and efficiency. They affect aspects like system responsiveness, reliability, and ease of use. While users might not explicitly request these qualities, they significantly influence overall satisfaction and usability.
4. Relationship to System Architecture
Functional requirements primarily influence the system’s logical architecture, determining what components or modules are needed to deliver the required functionality. They guide developers in structuring the system’s features and data flow.
Non-functional requirements, on the other hand, have a more profound impact on the physical architecture and technical design choices. They often dictate decisions about hardware, software frameworks, and architectural patterns. For instance, scalability requirements might lead to a distributed system design, while security requirements could necessitate specific encryption methods.
5. Flexibility in Implementation
Functional requirements generally offer more flexibility in how they are implemented. Developers can often choose from various approaches to achieve the desired functionality, as long as the end result meets the specified behavior.
Non-functional requirements tend to be more restrictive, often setting specific constraints or standards that must be adhered to. They may limit technology choices, dictate certain design patterns, or require specific methodologies to ensure compliance with performance, security, or other quality attributes.
By understanding these key differences, you can more effectively manage and prioritize requirements throughout the software development lifecycle, ensuring a balance between functionality and system quality that leads to successful project outcomes.
Functional vs Non-Functional Testing
Testing is a critical phase in the software development lifecycle, ensuring that a system meets both its functional and non-functional requirements. Functional and non-functional testing serve different purposes and employ distinct methodologies.
Functional Testing
Functional testing focuses on verifying that the system behaves according to its specified functional requirements. This type of testing examines the system’s features and functionality, ensuring they work as intended. When conducting functional tests, you’ll typically:
- Validate input handling and output generation
- Verify data processing and storage mechanisms
- Test user interface elements and navigation
- Ensure proper integration with external systems or APIs
Functional testing often employs techniques such as unit testing, integration testing, and user acceptance testing. These methods help identify bugs, logic errors, and deviations from expected behavior.
Non-Functional Testing
Non-functional testing assesses the system’s performance characteristics and quality attributes. It examines how well the system operates rather than what specific functions it performs. When conducting non-functional tests, you’ll focus on aspects like:
- Performance under various load conditions
- Scalability to handle increased user numbers or data volumes
- Security against potential vulnerabilities or unauthorized access
- Usability and user experience factors
Non-functional testing often requires specialized tools and environments to simulate real-world conditions. It may involve stress testing, security audits, or usability studies to evaluate the system’s overall quality and reliability.
By conducting both functional and non-functional testing, you ensure a comprehensive evaluation of your system.
Functional vs Non Functional Requirements Examples
Here are some examples of functional and non-functional requirements:
Examples of Functional Requirements
Functional requirements define specific system behaviors and features. They outline what the system should do to meet user needs and business objectives. Here are some examples:
- User Authentication: The system shall allow users to log in using their email and password.
- Data Processing: The application must calculate and display monthly sales totals for each product category.
- Reporting: Users should be able to generate PDF reports of customer orders within a specified date range.
- Integration: The system must interface with the company’s existing CRM software to retrieve customer data.
- Workflow: When a new order is placed, the system shall automatically send an email confirmation to the customer.
- Search Functionality: Users must be able to search for products using keywords, filters, and categories.
- Data Validation: The system should validate all input fields to ensure data integrity before submission.
These examples illustrate how functional requirements directly relate to the system’s features and capabilities, forming the basis of user stories and guiding development efforts.
Examples of Non-Functional Requirements
Non-functional requirements focus on the system’s quality attributes and performance characteristics. They describe how well the system performs its functions. Here are some examples:
- Performance: The system shall respond to user queries within 2 seconds under normal load conditions.
- Scalability: The application must support up to 10,000 concurrent users without degradation in performance.
- Security: All user passwords must be encrypted using industry-standard hashing algorithms.
- Usability: The user interface should be accessible to users with visual impairments, complying with WCAG 2.1 guidelines.
- Reliability: The system shall have an uptime of 99.9% during business hours.
- Compatibility: The web application must function correctly on the latest versions of Chrome, Firefox, Safari, and Edge browsers.
- Maintainability: The codebase should follow object-oriented design principles to facilitate future updates and modifications.
These examples demonstrate how non-functional requirements address the system’s overall quality, performance, and user experience, ensuring that the product not only works but works well under various conditions.
Final Thoughts
Functional and non-functional requirements are both vital for comprehensively defining software needs. While functional requirements specify capabilities and behaviors, non-functional requirements set quality standards and constraints.
To build high-quality systems that solve business problems, teams need to capture requirements that outline not just essential functionality but also critical quality attributes like performance, security, and reliability.
Carefully balancing functional and non-functional needs leads to solutions that reliably perform necessary functions while meeting user expectations.
