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The requirements phase is one of the most critical phases in software engineering. Studies show that the top problems in the software industry stem from poor requirements elicitation, inadequate requirements specification, and ineffective management of changes to requirements. Derived from customer, user, and other stakeholder needs, as well as design and development constraints, requirements are crucial for successful software delivery.
Importance of the Requirements Phase
Requirements provide the foundation for the entire software life cycle, influencing the software product’s quality, reliability, and maintainability. They also serve as a basis for planning, estimating, and monitoring project progress. Requirements are derived from the needs and constraints of customers, users, and other stakeholders, shaping the design and development process.
Developing comprehensive requirements involves elicitation, analysis, documentation, verification, and validation. Continuous customer validation ensures that the end products meet customer needs and is an integral part of the lifecycle process. This can be achieved through rapid prototyping and customer-involved reviews of iterative and final software requirements.
The Software Requirement Specification (SRS) document serves as the foundation for any successful software development project. It’s a comprehensive guide that outlines the functionalities, features, and overall behavior of the software being built. Think of it as the architectural blueprint for your software, meticulously detailing everything from the foundation (core functionalities) to the finishing touches (user interface). A well-crafted SRS is essential for ensuring clear communication, managing expectations, and ultimately, delivering high-quality software.
The Software Requirements Specification (SRS) document must address the needs of two primary audiences. The first is the user or client, who may not be technically inclined. User requirements must be expressed in the user’s language, ensuring clarity and alignment with their expectations. The second audience is the development team, who require detailed specifications to understand precisely what the system should and shouldn’t do, thus bridging the gap between user expectations and technical implementation. This includes the system specifications, which outline how the system will fulfill user requirements, providing a clear roadmap for software design and development. It outlines what the system should and shouldn’t do
Functional and Non-Functional Requirements
Functional requirements describe the specific behaviors or functions of the system, such as data handling logic, system workflows, and transaction processing. These requirements ensure that the system performs as intended and includes aspects like input validation, error handling, and response to abnormal situations.
Non-functional requirements, on the other hand, describe how the system performs its functions rather than what it does. These requirements are categorized into product, organizational, and external requirements. Product requirements include security, performance, and usability attributes. Organizational requirements encompass company standards and development processes. External requirements address compliance with regulatory standards and interactions with external systems.
Product requirements include aspects like protocol standards, encoding, and encryption requirements, directly impacting the software’s behavior and quality attributes such as security, performance, and usability. Organizational requirements are defined by the company’s internal standards, including coding style, development processes like Scrum, and tools like Microsoft Project or Jira for project management and bug tracking. External constraints are especially significant in regulated industries, where adherence to specific development processes or testing metrics is mandated by regulatory bodies such as the FAA.
WRSPM Reference Model: Understanding Requirements and Specifications
The WRSPM (World, Requirements, Specification, Program, Machine) model is a reference framework for understanding the difference between requirements and specifications.
WRSPM Model is a reference model of how we understand problems in the real world. It helps to understand the difference between the requirement and the specification. Requirements are always in the problem domain. It is all about what the users want to do in order to solve some problems that they have.
W — is the world assumptions, that we know are true. They have an impact on our system and on our problem domain. These things everyone takes for granted, and they are one of the most difficult parts to capture.
R — is the requirements. This is the user’s language understanding of what the user wants from the solution. For example, users want to withdraw money. The ATM is the solution.
S—is the specification. It is the interface between how the system will meet those requirements. It is written in system language that says in plain English what the system will do. The specification is how the system meets the requirements. For example, in order to withdraw money from the ATM, you have to insert your card, insert PIN-number, etc. Those are the things user doesn’t care about. The user wants to get money.
P—is the program. It is what software developments will write. The program will meet the specifications to provide the user goal for requirements. The program has all the code, underlying frameworks, etc.
M—the machine. It is the hardware specification. For example, it includes the roller for distributing money, the lock box etc.
Eh — are the elements of the environment that are hidden from the system. These are parts of the environment that the user wants. For example, the card.
Ev—are the elements of the environment that are visible to the system. For example, the data generated, when you read a mag strip on the card and the entered PIN number.
Sv—are the elements of the system that are visible in the environment. For example, the button, the information on the screen.
Sh—are the elements of the system that are hidden from the environment. For example, the roller inside the machinery that the user can’t actually see, making sure that the machinery gets the approval number from the bank before distributing money.
Understanding this model helps in capturing and translating user requirements into technical specifications effectively.
Components of an SRS Document
A well-structured SRS document typically includes the following sections:
1. Introduction
Product Scope:
The product scope should align with the overall business goals and strategic vision of the product. This is particularly important when multiple teams or contractors will access the document. Clearly list the benefits, objectives, and goals intended for the product, providing a comprehensive overview of its intended impact and purpose.
Product Value:
Explain why your product is important. How will it help your intended audience? What problem will it solve or what function will it serve? Describe how your audience will derive value from the product, ensuring they understand its significance and potential impact.
Intended Audience:
Describe your ideal audience in detail. The characteristics of your audience will influence the look, feel, and functionality of your product. Identify the different user groups and tailor your descriptions to their specific needs and expectations.
Intended Use:
Illustrate how your audience will use your product. List the primary functions and all possible ways the product can be utilized based on user roles. Including use cases can provide a clear vision of the product’s applications and benefits in real-world scenarios.
Definitions and Acronyms:
Every industry or business has its own unique jargon and acronyms. Define these terms to ensure all stakeholders have a clear understanding of the document. This ensures clarity and prevents misunderstandings.
Table of Contents:
A thorough SRS document can be extensive. Include a detailed table of contents to help all participants quickly find the information they need. This enhances the document’s usability and accessibility.
2. System Requirements and Functional Requirements
Functional Requirements:
Functional requirements specify the features and functions that enable your system to perform as intended. This includes:
- If/Then Behaviors: Define conditional operations based on specific inputs.
- Data Handling Logic: Detail how the system manages, processes, and stores data.
- System Workflows: Describe the flow of operations and processes within the system.
- Transaction Handling: Specify how transactions are processed and managed.
- Administrative Functions: Outline the functions available to system administrators.
- Regulatory and Compliance Needs: Ensure adherence to industry regulations and standards.
- Performance Requirements: Define the expected performance metrics and criteria.
- Details of Operations: Describe the specific operations for each user interface screen.
Considerations for Capturing Functional Requirements (NASA):
- Validity checks on inputs
- Exact sequence of operations
- Responses to abnormal situations (e.g., overflow)
- Communication facilities
- Error handling and recovery
- Effect of parameters
- Relationship of outputs to inputs (e.g., input/output sequences, conversion formulas)
- Relevant operational modes (e.g., nominal, critical, contingency)
3. Required States and Modes
Identify and define each state and mode in which the software is required to operate, especially if these have distinct requirements. Examples include idle, ready, active, post-use analysis, training, degraded, emergency, backup, launch, testing, and deployment. Correlate each requirement or group of requirements to the relevant states and modes, which can be indicated through tables, appendices, or annotations.
4. External Interface Requirements
External interface requirements encompass all inputs and outputs for the software system and expand on the general interfaces described in the system overview. Consider the following:
- User Interfaces: Key components for application usability, including content presentation, navigation, and user assistance.
- Hardware Interfaces: Characteristics of each interface between software and hardware components (e.g., supported device types, communication protocols).
- Software Interfaces: Connections between your product and other software components (e.g., databases, libraries, operating systems).
- Communication Interfaces: Requirements for communication functions your product will use (e.g., emails, embedded forms).
For embedded systems, include screen layouts, button functions, and descriptions of dependencies on other systems. If interface specifications are captured in a separate document, reference that document in the SRS.
5. Internal Interface Requirements
Internal interface requirements address interfaces internal to the software (e.g., interfaces between functions), unless left to the design phase. These should include relevant information similar to external interface requirements and reference the Interface Design Description as needed.
6. Internal Data Requirements
Internal data requirements define the data and data structures integral to the software, including:
- Data types
- Modes of access (e.g., random, sequential)
- Size and format
- Units of measure
For databases, consider including:
- Types of information used by various functions
- Frequency of use
- Accessing capabilities
- Data entities and their relationships
- Integrity constraints
- Data retention requirements
7. Non-Functional Requirements (NFRs)
Common types of NFRs, often referred to as the ‘Itys,’ include:
- Security: Measures to protect sensitive information.
- Capacity: Current and future storage needs and scalability plans.
- Compatibility: Minimum hardware requirements (e.g., supported operating systems and versions).
- Reliability and Availability: Expected usage patterns and critical failure time.
- Scalability: System performance under high workloads.
- Maintainability: Use of continuous integration for quick deployment of features and bug fixes.
- Usability: Ease of use for the end-users.
Other NFRs include performance, regulatory, and environmental requirements.
8. Safety Requirements
Safety requirements must be included in the SRS and designated for traceability. These requirements:
- Carry a unique identification or tag for traceability purposes.
- Must be traceable throughout development and operational phases to assess impacts and changes.
- Are derived from system safety requirements, standards, program specifications, vehicle or facility requirements, and interface requirements.
A method of identification, such as a special section in the requirements document, a flag beside the requirement, or a database entry, is essential for traceability and assessment.
In summary, the SRS document should be a comprehensive blueprint that guides the development process, ensuring that the final product meets all stakeholder needs while adhering to regulatory and organizational standards. Properly managed requirements mitigate risks, streamline development, and lead to the successful delivery of high-quality software systems.
Best Practices for Creating SRS Documents
Creating an effective SRS document involves thorough requirement gathering, clear and precise articulation of requirements, and continuous validation with stakeholders. Here are some best practices:
- Gather Requirements Thoroughly: Use interviews, surveys, and workshops to gather comprehensive requirements from all stakeholders.
- Ensure Clarity and Precision: Use clear and concise language to avoid ambiguity.
- Prioritize Requirements: Distinguish between mandatory and optional requirements to manage stakeholder expectations effectively.
- Review and Validate Requirements: Regularly review and validate requirements with stakeholders to ensure accuracy.
- Maintain Traceability: Establish traceability between requirements and other artifacts to facilitate change management.
- Iterate and Evolve: Recognize that requirements evolve and embrace an iterative approach to requirements management.
Example 1; What’s Included in an SRS for a Satellite’s Embedded System?
An SRS document for a satellite’s embedded system typically encompasses the following key sections, with specific examples tailored to this unique environment:
- Introduction:
- Purpose: “To develop the embedded software for the XYZ Satellite, a scientific research satellite tasked with collecting data on Earth’s atmosphere.”
- Target Audience: “Satellite engineers, mission controllers, and the scientific team responsible for the atmospheric research.”
- Project Scope: “The initial phase will focus on core functionalities such as attitude control, telemetry (data transmission to Earth), and command reception and execution from ground control.”
- System Requirements:
- Performance: “The system must respond to attitude control adjustments within 10 milliseconds to maintain pointing accuracy.”
- Real-time Constraints: “The system must prioritize real-time tasks such as attitude control and data acquisition over non-critical tasks.”
- Radiation Tolerance: “The software must be resistant to potential radiation effects experienced in the space environment.”
- Functional Requirements:
- Use Case: The satellite detects a slight shift in its orientation due to micrometeoroid impact. The attitude control software activates thrusters to realign the satellite within the designated tolerance.
- Functional Requirement: “The software shall continuously monitor the satellite’s orientation using gyroscopes and star trackers.”
- Non-Functional Requirements:
- Reliability: “The software must be exceptionally reliable with minimal risk of errors or malfunctions. A single error could jeopardize the entire mission.”
- Maintainability: “The software code should be modular and well-documented to facilitate troubleshooting and potential future updates in orbit.”
- External Interfaces:
- The software will communicate with onboard sensors (gyroscopes, star trackers) to receive real-time attitude data.
- The software will interface with the satellite’s communication subsystem to transmit telemetry data and receive commands from ground control.
- Appendices:
- State diagrams: Illustrating the different operational states of the satellite (e.g., idle, data collection, thruster activation) and the transitions between them triggered by various events.
- Error handling procedures: Detailing how the software will respond to potential errors or unexpected situations (e.g., sensor failure, loss of communication with ground control).
Here are some key considerations for creating an effective SRS for a satellite’s embedded system:
- Focus on Real-Time Performance: Prioritize real-time tasks critical for satellite operation (attitude control, data acquisition).
- Rigorous Testing: Facilitate the development of robust test cases that simulate various real-world scenarios the satellite might encounter in orbit.
- Error Handling and Recovery: Outline how the software will respond to potential errors and attempt recovery or transmit diagnostic data for troubleshooting from Earth.
- Version Control: Implement a meticulous version control system to track changes to the SRS throughout the development process.
Benefits of a Well-Defined SRS for a Satellite’s Embedded System:
- Enhanced Communication: A clear SRS ensures everyone involved in the mission, from engineers on Earth to the scientists awaiting data, has a clear understanding of the satellite’s capabilities and limitations.
- Reduced Risk of Mission Failure: By clearly defining the system requirements and real-time constraints, the SRS helps mitigate potential risks that could lead to mission failure.
- Efficient Testing: A well-defined SRS provides a roadmap for developing comprehensive test cases, simulating various scenarios the satellite might encounter in space.
- Effective In-Orbit Maintenance: A clear SRS allows engineers on Earth to understand the satellite’s software functionalities and troubleshoot any issues that might arise during the mission.
Conclusion
The SRS document is crucial for the successful delivery of software projects. By clearly defining project requirements, the SRS document ensures that all stakeholders are aligned and that the development team has a precise blueprint to follow. Following best practices and fostering collaboration among stakeholders, organizations can create and maintain effective SRS documents that serve as the cornerstone of successful software development initiatives.
References and Resources also include:
https://asana.com/resources/software-requirement-document-template
