A software requirements specification (SRS) is a comprehensive description of the intended purpose and environment for software under development. The SRS fully describes what the software will do and how it is expected to perform.
An SRS minimizes developers' time and effort required to achieve desired goals. It also minimizes development costs. A good SRS defines how an application will interact with system hardware, other programs and human users in a variety of real-world situations. Parameters such as operating speed, response time, availability, portability, maintainability, footprint, security and speed of recovery from adverse events are evaluated. Methods of defining an SRS are described by the IEEE specification 830-1998.
Key components of a software requirements specification
The main sections of a software requirements specification are the following:
- Business drivers. This section describes the reasons the customer is looking to build the system, including problems with the current system and opportunities they'd like the new system to provide.
- Business model. This section describes the business model the system will support, including the organizational structure, business context, main business functions and process flow diagrams.
- Business, functional and system requirements. This section consists of requirements organized in a hierarchical structure. The functional and business requirements are at the top level, and the detailed system requirements are listed as subsections or child items.
- Business and system use cases. This section consists of a Unified Modeling Language use case diagram depicting the key external entities that will interact with the system and the different functions they'll provide.
- Technical requirements. This section lists the nonfunctional requirements that make up the technical environment and technical restrictions where the software will operate.
- System qualities. This section is used to describe the nonfunctional requirements that define the quality attributes of the system, such as reliability, serviceability, security, scalability, availability and maintainability.
- Constraints and assumptions. This section includes any constraints the customer has imposed on the system design and any assumptions the requirements engineering team has about what's expected to happen during the project.
- Acceptance criteria. This section details the conditions that must be met for the customer to accept the final system.
Purpose of an SRS
An SRS forms the basis of an organization's entire project. It sets out the framework that the development team follows and provides critical information to all the teams involved, including development, operations, quality assurance and maintenance. This approach ensures the teams are in agreement.
Enterprises use an SRS to confirm that the requirements are fulfilled and to help business leaders make decisions about product lifecycles, such as when to retire a feature or technology.
In addition, writing an SRS can help developers reduce the time and effort necessary to meet their goals as well as save money on development costs.
Alternatives to an SRS
In Agile methodologies, companies usually favor a more lightweight documentation of the requirements. These include using acceptance tests and user stories as part of the process.
For this approach to work, the customer must be easily accessible during development to provide any necessary clarification on the requirements. It also assumes that the developers writing the user stories with the customer will be the developers building the system.
The rapid application development, another alternative software engineering methodology, favors speed and flexibility over upfront planning. This approach has a short development time span. Typically, a project developed with this model has a delivery time of 60 to 90 days.
Features of an SRS
An SRS should have the following characteristics:
- Correct. Should accurately reflect product functionality and specification at any point of time.
- Unambiguous. There shouldn't be any confusion regarding interpretation of the requirements.
- Complete. Should contain all the features the client requests.
- Consistent. Same abbreviations and conventions must be followed throughout the document.
- Ranked for importance and stability. Every requirement is important. But some are urgent and must be fulfilled before other requirements, and some can be delayed. Each requirement should be classified according to its importance and stability.
- Verifiable. An SRS is verifiable only if every stated requirement can be verified. A requirement is verifiable if there's some method to quantifiably measure whether the final software meets that requirement.
- Modifiable. An SRS must clearly identify each and every requirement in a systematic manner. If there are any changes, the specific requirements and the dependent ones can be modified accordingly without affecting the others.
- Traceable. An SRS is traceable if the origin of each of its requirements is clear and if it makes it easy to reference each requirement in future development.
The goals of an SRS
Some of the goals an SRS should achieve include the following:
- Provide feedback to the customer, ensuring that the IT company understands the issues the software system should solve and how to address those issues.
- Help to break a problem down into smaller components just by writing down the requirements.
- Speed up the testing and validation processes.
- Facilitate reviews.
How to write a successful SRS document
Creating a clear and comprehensive software requirements specification document is critical to the success of most software projects. The SRS serves as a blueprint for all stakeholders, including project managers, business analysts, developers and testers. The following 10 steps are key to writing a successful SRS:
- Stakeholders and their needs. Gather project managers, business analysts, end users and any other stakeholders. Once these people are identified, SRS authors must work with them to assess their needs, business goals and project dependencies.
- Project scope. Define the project's scope, including its functional and nonfunctional requirements. The project's objectives must be clearly stated, along with a high-level overview of what the software will achieve.
- System features and requirements. Provide a detailed description of system features, user requirements, external user interface requirements, and ensure the software requirements are precise and measurable.
- Functional vs. nonfunctional requirements. Functional requirements and nonfunctional requirements should be separated and clearly iterated. Functional requirements describe what the system should do, and nonfunctional requirements include equally important considerations such as security and performance.
- Inputs and outputs. Define what inputs the system will receive, and what outputs it will generate. This can help testers and developers understand how the system will behave under various conditions.
- Roadmap and development lifecycle. Define the software development process, including key milestones and phases in the development lifecycle, from requirements gathering to testing and deployment. A roadmap should be included showing how the project planning process will be managed and tracked.
- Usability and accessibility. Include details about usability requirements, focusing on how end users will interact with the system. Accessibility standards should be considered to ensure the software meets the needs of all users.
- Team members and their responsibilities. Identify team members and what tasks they will be responsible for throughout the development process.
- Intended audience. Identify the intended audience for the SRS and make sure that it's tailored to their level of technical understanding.
- Requirements management. Set up a process for managing changes to the requirements throughout the development project. Ensure traceability so that each requirement can be tracked from its origin to its implementation.
A sample of the table of contents for a successful SRS template might look like this:
1. Introduction
1.1 Purpose of this document
1.2 Scope of this document
1.3 Overview
1.4 Business Context
2. General Description
2.1 Product Functions
2.2 Similar System Information
2.3 User Characteristics
2.4 User Problem Statement
2.5 User Objectives
2.6 General Constraints
3. Functional Requirements
4. Interface Requirements
4.1 User Interfaces
4.2 Hardware Interfaces
4.3 Communications Interfaces
4.4 Software Interfaces
5. Performance Requirements
6. Other Nonfunctional Attributes
6.1 Security
6.3 Reliability
6.4 Maintainability
6.5 Portability
6.6 Extensibility
6.7 Reusability
6.8 Application Affinity and Compatibility
7. Operational Scenarios
8. Preliminary Use Case Models and Sequence Diagrams
8.1 Use Case Model
8.2 Sequence Diagrams
9. Updated Schedule
10. Appendices
10.1 Definitions, Acronyms, Abbreviations
10.2 References
Mistakes to avoid when building an SRS
There are several common mistakes organizations make when building an SRS. The most important mistakes an organization should not make include the following:
- Lack of clarity. An SRS document must be written in clear, unambiguous language and avoid vague terms. This also includes making the SRS too technical and inaccessible to team members who may not be technically versed.
- Overlooking nonfunctional requirements. Many projects focus solely on functional requirements and neglect nonfunctional requirements such as usability, performance and security. These are just as important for the success of the software product.
- Incomplete stakeholder input. If an organization fails to include all stakeholders, especially end users and business analysts, critical gaps can occur in the SRS.
- Not managing changes. The development process of an SRS is dynamic, meaning requirements will often evolve. Failing to establish a strong requirements management process can lead to confusion, rework and project delays.
- Neglecting usability and accessibility. The SRS needs to make sure its product is easy to use. Its development should account for how real users will interact with the system and should include usability and accessibility requirements.
What are nonfunctional requirements?
Nonfunctional requirements include quality attributes of a software product, focusing on how the system performs rather than what it does. Unlike functional requirements, which describe what a system should do, nonfunctional requirements describe characteristics that affect the user experience and system performance. Some common types of nonfunctional requirements include the following:
- Performance.
- Usability.
- Security.
- Scalability.
- Maintainability.
- Reliability.
Difference between SRS and a functional specification document
Both an SRS and a functional specification document (FSD) are important for software development, but they serve different purposes and include different categories of information. An SRS is a high-level document that outlines all requirements and constraints of the system, providing a comprehensive overview of what the software must accomplish. An FSD focuses on functional requirements and breaks out each feature and function in detail.
Similarly, while an SRS addresses both functional requirements and nonfunctional requirements, including user needs, system features and external interface requirements, the FSD concentrates solely on the system's function. This might include wireframes, mockups and detailed use cases and case studies. The FSD also provides the technical team with a clear guide on how to build each feature.
Lastly, an SRS is intended for a broad audience that includes stakeholders, business analysts and project managers. Its purpose is to align everyone on the overall goals and requirements. Compared to an SRS, the FSD is more technical and is aimed primarily at developers and testers who need detailed instructions to build and test the system.
Artificial intelligence is becoming a prominent part of software development and the software requirements specification process. Learn the risks and benefits of AI in software development.