How to Write a Software Requirements Specification (SRS) Document

Despite the importance of software requirements specification (SRS) documents in software engineering, many stakeholders still hope to get a high-quality product without investing resources in SRS documentation.

At ITRex, we believe an SRS is a crucial part of software product development. We sat down with our Lead Business Analyst, Vladimir Sechko, to clarify what a software requirements specification document is, which benefits it brings about, who needs it the most, and how to approach SRS creation. Vladimir has 14 years of experience in business analysis, and in this article, he shares some insightful tips and ideas.

So, let’s get started.

What is SRS, and How to Approach It?

SRS Definition and Concept Clarification

An SRS document describes what the resulting solution will do, how it is expected to perform, and how the team should approach development.

SRS specifies the final product’s functional and non-functional requirements, implementation constraints, and use cases (i.e., how users will interact with the system).

A software specification provides everyone involved with a roadmap of the project.

Functional vs. Non-functional Requirements of a Software Product

Functional requirements describe the product’s functionality. They define what the solution will do and how it will interact with and respond to different user inputs. You can view functional requirements as a detailed description of the solution’s features and fundamental behavior. If they are not met, the system will not work.

If you register and sign in to an application, the system will send a welcome email. This is one example of a functional requirement.

Non-functional requirements specify how the system will perform the aspects covered in the functional requirements section. They define the general characteristics impacting user experience. Non-functional requirements relate to:

• Performance
• Security
• User interface/experience
• The cultural context of the product

Coming back to the registration confirmation email example above, this system’s non-functional requirement would be to send the welcome email within five seconds after signing in.

Who Should Write the Software Requirements Document?

Business analysts should develop this document after completing the requirements elicitation process. But in the absence of a business analyst, the following team members can take over the SRS document:

• Project managers
• Solution architects
• Business consultants

Main Elements of a Software Requirements Document

Software requirements documentation should offer enough information for developers to build the solution in question. Keep in mind, this specification focuses on requirements and doesn’t outline design choices or technology stack.

Depending on the selected methodology (agile or waterfall), the level of details in SRS will vary.

Software requirements specification document in waterfall methodology

Waterfall allocates more time for SRS document creation, and it is perfect for projects where requirements are unlikely to change soon. However, if the change is inevitable, updating the SRS document will be rather expensive.

Software requirements document in agile methodology

This approach implies that business analysts gather feedback from the client iteratively and are likely to update the SRS several times during the course of the project. Even though the requirements remain flexible, and changes can occur relatively often, composing SRS is crucial in agile methodology. It defines the scope, explains the business logic, and makes expectations transparent to all the participating teams.

Generally, a requirements specification document should contain the following information:

• Product value: specifies which value the potential product will deliver. Will it solve a particular problem? Or will it provide an innovative way to increase market value?
• Business model: explains the customer’s business model that the prospective solution will need to support. This section includes organizational context, process flow diagrams, business functions, etc.
• Motivation: presents a rationale for why the client wants to build this solution in the first place. This information will be crucial to keep the project on track if the client organization undergoes shifts in key personnel. Business analysts and developers will refer to this section when making decisions about the project.
• Functional and non-functional requirements: provides hierarchical documentation of different requirements.
• System qualities: includes items that are typically known as “-ilities”, such as, scalability, reliability, security, and availability, among others.
• Users (personas): describes up to three target user segments, specifying their characteristics, such as age, gender, profession, education, etc.
• Business use cases: one of the most popular forms is Unified
  Modeling Language (UML) use case diagrams, which showcase different users interacting with the system to accomplish various tasks. This section formalizes the steps that need to be carried out for each use case, together with any rules or conditions.
• Assumptions and constraints: highlights design constraints that will influence developers’ choices removing specific options from their consideration.
• Acceptance criteria: presents the criteria to be satisfied for the customer to accept the resulting product. In the case of waterfall methodology, these are evaluated at the end of the project. With agile, this is done after every iteration.

Why Write Software Requirements Specifications?

When you have a vision of a system you want other people to build, you need to make sure that programmers and engineers understand what you want them to build exactly and not make their own decisions at liberty. For this purpose, you compose an SRS document specifying every detail of the desired solution in a language programmers understand. This presents a written agreement between you and your vendor. It will not only help developers select the most suitable tech stack and meet their expectations but also aid inaccurate cost estimates.

Here is what the requirements specification document does for you:

• Serves as a reference in case of disputes between designers, developers, project managers, and any other team members
• Allows to precisely estimate time and finances needed for a project
• Guarantees consistency. After some time into the project, the participating parties might lose track of what the final system needs to accomplish. They can get back to the software requirements specification to gain clarity and focus
• Facilitates solution scaling in the future if needed
• Defines and outlines complex business logic
• Helps investors with decision making as they can have a clear overview of all system’s features and interactions with users.

How to Write an SRS Document?

Here is how Vladimir recommends approaching SRS document creation:

1. Always clarify. What is obvious to you is not necessarily apparent to others, so write everything in detail and don’t make assumptions that others will know something just because you do. This document needs to be comprehensive and understandable to everyone involved.
2. Descriptions must be brief and divided into sections. Even though understandability is an important factor, software requirements specification documents must be as concise as possible. Five sections with one paragraph each are more favorable than one section with five paragraphs.
3. Draw and include diagrams to clarify and support your text. If you can't draw it, you don't understand it. There are different types of diagrams that you can use to enhance your SRS, such as Business Process Modeling Notation (BPMN), Unified Modeling Language (UML), use cases, Entity Relationship models (ER), user journey, and user flow diagrams among others. Make sure your diagrams always have legends.
4. Hyperlink different sections, terms, and clarifications to facilitate navigation. The resulting software requirements specification document will be large, even if it describes a relatively small project. Hyperlinking will help you find what you need fast.
5. Don’t repeat the same thing twice. Whether it’s a definition, functionality description, or anything else. If you described a button’s functionality once, it is best to link back to the original description when mentioning the same information again.
6. If it's not an established truth — it's a lie. Business analysts shouldn’t make assumptions about the product. They only include what they know to be true. If a business analyst is not sure about something, they need to clarify instead of assuming.
7. Base everything you include on solid facts and sources. Every functionality needs to have a source (i.e., stakeholders’ statements, technical and business needs) and a goal.
8. Invent solutions, not requirements. Only include the client’s requirements, be careful not to write anything from your own imagination.
9. Eat an elephant bit by bit. Complex functionality needs to be adequately explained and broken down into smaller pieces.
10. Capitalize important terms, such as objects and process names. This will make it stand out among the rest of the text. This approach will improve readability and make it easier for the reader to find what they are searching for.
11. Make sure the resulting SRS is verified and approved by all stakeholders involved.

Main Challenges of SRS in Software Engineering

Writing an SRS document is a necessary, albeit complicated, task. Vladimir highlights the main challenges that business analysts are likely to face during this process and that stakeholders need to be aware of:

• It requires extensive manual labor. It’s an exhausting task to write and maintain such a large document. So, do not expect it to be done in two days.
• Overcomplicated requirements can make the writing process even more tedious. This is especially true for large projects with many stakeholders.
• The involved parties might have different opinions on the system. Building on the previous point, having multiple stakeholders from different departments can result in conflicting requirements as every shareholder has their view on the system/product.
• Many functional areas are to be tracked. A software requirements specification document describes several functional areas, such as user management, product management, etc., and they all need to be consistent. Any changes in one area must be reflected across all the affected areas. Such changes include coming up with new requirements or testing a different tech approach.
• Constant updates and maintenance. All in all – a software requirements specification is a large document, which needs to be maintained and kept up to date. Any changes in the system’s behavior need to be reflected. Otherwise, the SRS loses its validity.

Downloadable Software Requirements Specification Template

Most experienced business analysts have their own SRS template that they enriched throughout the years of their practice. However, if you are looking for a solid publicly available software requirements documentation template, Vladimir suggests using the classic template composed by Karl E. Wiegers. You can download it here.

Things to Consider if You are Still not Convinced in Software Requirements Documentation

Composing an SRS document is a complex task. It might be tempting to move on through the project, skipping this step. However, if you do that, be prepared for the following challenges:

• Incorrect price and effort estimations. One of the main benefits of a software requirements specification document is that it allows you to foresee the time and effort required to build the product. If you don’t have such a detailed description, your vendor can’t make a precise estimation.
• Unpredictable development results. If you don’t specify what you want exactly, designers and developers might start making assumptions, delivering a product that is far from what you originally wanted.
• Unreliable delivery schedule. No one can confidently set dates for intermediate and final deliverables

Which Projects Can Benefit from Software Requirements Specification the Most?

Certainly, avoid skipping the SRS document if your project corresponds to the following criteria:

• Large and complex
• Depends on investors’ funding
• The development team is distributed over different vendors

On a Final Note…

According to studies, only about 46% of delivered projects
are deemed satisfactory by stakeholders. We firmly believe that a carefully crafted and approved software requirements document will increase your vendor’s chances of delivering on your expectations.

Here at ITRex, we can draft your software requirements specification document either as a part of a full-cycle project or as a part of a stand-alone discovery phase. In the latter case, you can use the resulting documentation to initiate a procurement process where several vendors compete for a contract.

This is how the process of writing SRS goes: after hearing your business idea, our business analyst(s) will start eliciting requirements and writing the document. SRS is the final product of business analysis. Hence, the time needed for its composition depends on the complexity of your product. We can’t give you a time estimate for delivering the software specifications document without seeing the scope of work. Still, we understand how crucial this document is for the development process, and we can promise to deliver an accurate and comprehensive SRS in the shortest time possible.

Do you see SRS as an indispensable part of software engineering? Get in touch! Vladimir and other experts will craft a high-quality SRS document that will ensure a seamless development of the final product.