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Non-functional Requirements in Software Engineering

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Non-functional requirements in software engineering refer to the characteristics of a software system that are not related to specific functionality or behavior. They describe how the system should perform, rather than what it should do. Examples of non-functional requirements include:

  1. Performance: This includes requirements related to the speed, scalability, and responsiveness of the system. For example, a requirement that the system should be able to handle a certain number of concurrent users or process a certain amount of data within a specific time frame.
  2. Security: This includes requirements related to the protection of the system and its data from unauthorized access, as well as the ability to detect and recover from security breaches.
  3. Usability: This includes requirements related to the ease of use and understandability of the system for the end-users.
  4. Reliability: This includes requirements related to the system’s ability to function correctly and consistently under normal and abnormal conditions.
  5. Maintainability: This includes requirements related to the ease of maintaining the system, including testing, debugging, and modifying the system.
  6. Portability: This includes requirements related to the ability of the system to be easily transferred to different hardware or software environments.
  7. Compliance: This includes requirements related to adherence to laws, regulations, industry standards, or company policies.

Non-functional requirements are important because they can have a significant impact on the overall quality and success of a software system. They are often closely related to the system’s performance, security, and usability. They also help to ensure that the system is maintainable, portable, and compliant with relevant laws and regulations.

Non-Functional Requirements are the constraints or the requirements imposed on the system. They specify the quality attribute of the software. Non-Functional Requirements deal with issues like scalability, maintainability, performance, portability, security, reliability, and many more. Non-Functional Requirements address vital issues of quality for software systems. If NFRs not addressed properly, the results can include:

  • Users, clients, and developers are unsatisfied.
  • Inconsistent software.
  • Time and cost overrun to fix the software which was prepared without keeping NFRs in mind.

Types of Non-functional Requirement :

1. Performance Requirements

  • Response Time: It indicates the longest period of time that the system will allow to react to a user’s request.
  • Throughput: The quantity of transactions or processes that the system should be able to process in a specific amount of time is known as throughput.
  • Scalability: It defines how well the system can add resources to accommodate a growing workload or user base.

2. Reliability and Availability Requirements

  • Availability: It indicates the proportion of time that users should be able to access and use the system.
  • Reliability: It defines the system’s capacity to carry out its operations consistently over a predetermined time frame.
  • Fault Tolerance: This describes the system’s capacity to keep working even when there are hardware or software errors.

3. Security Requirements

  • Authentication: The process by which users are validated and given access to the system is known as authentication.
  • Authorization: It establishes the access levels and permissions for various user roles.
  • Data Encryption: It refers to the techniques and formulas used to protect private information both during transmission and storage.

4. Maintainability Requirements

  • Modifiability: It explains the ease with which a system can be upgraded or changed to accommodate shifting needs.
  • Testability: It defines how simple it is to test a system to make sure it is reliable and accurate.
  • Portability: It refers to a software’s capacity to function across many contexts or systems.

5. Performance Efficiency Requirements

  • Resource Utilization: It explains how to use resources like CPU, memory and network traffic in an effective manner.
  • Optimization: It indicates the requirement for performance optimization in particular use cases or scenarios.

Non-functional requirements (NFRs) are an essential part of software engineering. They describe the characteristics and qualities of the software system that are not related to its functional requirements. These qualities include performance, reliability, maintainability, usability, security, and many others. Non-functional requirements are critical to the success of a software system as they ensure that it meets the expectations of its users and stakeholders.

Some common examples of non-functional requirements include:

  1. Performance: Performance requirements describe the expected performance of the software system, such as its response time, throughput, and resource utilization. The software system should perform efficiently under different conditions and loads.
  2. Reliability: Reliability requirements describe the ability of the software system to perform its functions consistently and accurately over time. The system should be available and responsive when needed, and should not experience frequent failures or crashes.
  3. Usability: Usability requirements describe the ease of use and user-friendliness of the software system. The system should be easy to navigate and understand, and should provide clear and concise feedback to the user.
  4. Security: Security requirements describe the measures taken to protect the software system from unauthorized access, attacks, and data breaches. The system should be secure and protect the privacy and confidentiality of its users.
  5. Maintainability: Maintainability requirements describe the ease with which the software system can be modified, updated, and maintained over time. The system should be designed with modularity and extensibility in mind, and should be easy to test and debug.

It is important to note that non-functional requirements are often interdependent and can affect each other. For example, a highly secure software system may have lower performance due to the additional security measures in place. Therefore, it is essential to carefully consider and balance the non-functional requirements when designing and developing a software system.

In summary, non-functional requirements are a critical aspect of software engineering that describe the qualities and characteristics of the software system beyond its functional requirements. These requirements ensure that the software system meets the expectations of its users and stakeholders and performs efficiently and reliably under different conditions and loads.

These can be classified as :

  1. Performance constraints – Reliability, security, response time, etc.
  2. Operating constraints – These include physical constraints (size, weight), personnel availability, skill level considerations, system accessibility for maintenance, etc.
  3. Interface constraints – These describe how the system is to interface with its environment, users, and other systems. For example, user interfaces and their qualities (e.g., user-friendliness).
  4. Economic constraints – Immediate and/or long-term costs.
  5. Lifecycle requirements – Quality of the design: These measured in terms such as maintainability, enhance ability, portability.

Advantages of Non-Functional Requirement

  • They ensure the software system follows legal and adherence rules.
  • They specify the quality attribute of the software.
  • They ensure the reliability, availability, performance, and scalability of the software system
  • They help in constructing the security policy of the software system.
  • They ensure good user experience, ease of operating the software, and minimize the cost factor.
  • Improved system quality: By specifying non-functional requirements, engineers can ensure that the system will have desired characteristics such as performance, security, usability, reliability, maintainability, portability and compliance, which can improve the overall quality of the system.
  • Increased user satisfaction: By ensuring that the system has good usability and meets the user’s performance and security expectations, engineers can increase user satisfaction with the system.
  • Better alignment with business goals: By aligning non-functional requirements with the business goals, engineers can ensure that the system supports the overall objectives of the organization and meets the expectations of the stakeholders.
  • Reduced rework: By identifying and addressing non-functional requirements early on in the development process, engineers can reduce the likelihood of costly rework later on.
  • Improved system scalability and adaptability: By specifying non-functional requirements related to scalability and adaptability, engineers can ensure that the system can handle a growing number of users or changing business needs.
  • Improved compliance: By specifying non-functional requirements related to compliance, engineers can ensure that the system adheres to relevant laws, regulations, industry standards or company policies.
  • Better system maintenance: By specifying non-functional requirements related to maintainability, engineers can ensure that the system is easy to maintain, test and debug, which can save time and cost in the long run.

Disadvantages of Non-functional requirement

  • The nonfunctional requirement may affect the various high-level software subsystem.
  • They generally increase the cost as they require special consideration during the software architecture/high-level design phase.
  • It is difficult to change or alter non-functional requirements once you pass them to the architecture phase.
  • Difficulty in identifying all the requirements: It can be difficult to identify all the non-functional requirements, especially when dealing with complex systems or new technologies.
  • Difficulty in predicting future requirements: It can be difficult to predict future non-functional requirements, which can result in the software becoming outdated or needing to be redesigned.
  • Difficulty in measuring and testing: Non-functional requirements can be difficult to measure and test, which can make it challenging to ensure that they have been met.
  • Time-consuming: Gathering and specifying non-functional requirements can be time-consuming, which can slow down the development process.
  • Costly: Identifying, specifying and implementing non-functional requirements can be costly, especially when involving multiple stakeholders.
  • Risk of changing requirements: Non-functional requirements may change over time, which can lead to confusion and require additional work to keep the project aligned with the updated requirements.
  • Conflicting requirements: Non-functional requirements may be in conflict with each other and it can be difficult to balance them and prioritize which one to implement.
  • Unforeseen requirements: Non-functional requirements may not be fully identified during the requirement gathering phase, and some requirements may only be discovered after the system has been deployed.

Last Updated : 23 Jan, 2024
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