How to Write a Software Requirements Specification (SRS)
Let's pretend you want to get a custom-sewn jacket. Going to a tailor and saying "Make me a jacket, please" probably won't be enough to meet your expectations (unless you're ready for a surprise). You'll need to provide core information like your size, the fabric you want, the colors, design, intended seasonality, when you want it delivered, and, of course, your budget.
Planning development projects is quite similar, yet probably takes a lot more intricacy when it comes to formulating requirements. You need to lay down the facts that define the product, and a software requirements specification document can be indispensable for some scenarios.
What is SRS? And how do you use it? On this page, we'll go over all the must-knows, explaining how to write requirements for software with tips and mistakes to avoid.
What Is a Software Requirements Specification?
Let's start with the basics. According to the SRS meaning, such a specification is a technical document describing a software product's functional and non-functional requirements, characteristics, capabilities, features, performance expectations, limitations, and environment. It also gives a comprehensive description of its intended purpose, goals, and how it should work and behave.
The document's aim is to clearly outline what the software should do and how to build it, forming a blueprint for future development. It accurately reflects the requirements for the software under development and serves as a common reference for developers and other people involved in the project.
Apart from a description in written text, an SRS can also be fitted with visuals such as diagrams, schemes, models, or mindmaps to illustrate precise points more explicitly. Moreover, use cases are commonly added to it as well to give a better explanation of how the product should operate and what it's supposed to accomplish.
At what point of the product development life cycle is an SRS document usually written? An SRS is typically created early on during the project assessment and requirements engineering phase. Generally, it's one of the initial stages of the development process when information about the future product is gathered and analyzed.
Oftentimes, it's impossible to take all matters into account in advance, so the SRS might have roughly three-quarters complete before development work starts and then be supplemented later. The document is customarily created as a PDF or Google Doc, a page within a collaborative platform, via special SRS software, or in some other online format for simple access, sharing, and editing.
What Is an SRS Document Used For?
Since an SRS defines the requirements, it's a baseline for decision-making that will serve various purposes. As such, it can be handy for making project estimations, calculating costs, planning possible schedules, sprints, and milestones, and measuring execution success. It may also contribute to decisions regarding hiring developers and specialists for other product team roles, and be applied for coordinating development work.
Who can get the most value from an SRS? Here's a rundown of those involved in software development who often use it:
- Client or product owner (to line out the deliverables, expectations, and possibly even the deadlines, which is crucial if you're outsourcing product development to a dedicated team);
- Project managers (to cross-check whether the built product meets the requirements);
- Developers (to be certain about how to build the product);
- Designers (to make sure that the designs correlate with the given use cases);
- Quality assurance engineers (to ensure the built functionality works as intended when testing it);
- Maintenance developers (to get a clear understanding of how the system must be supported and updated).
Software Requirements Specification vs System Requirements Specification
How does an SRS differ from a system requirements specification (SyRS)? While the terms are at times used interchangeably, they aren't the same thing. Let's dot the i's to not confuse these two technical document types.
As follows from the name, a SyRS document focuses on system requirements. It can serve as the foundation for an SRS and usually covers both hardware and software components and such aspects as data assets and how to structure data, manuals, the overall system architecture, and more.
In turn, SRS engineering documentation is generally a lot more detailed than a SyRS from a business perspective. It places emphasis on how the product should be built and is expected to function, as well as the main business processes. It’s thus a guiding light for developers and their understanding of the to-be product's functional specifications and how to bring it to life and is a valuable asset for other product team roles as well. A specification document may include an overview of the user interfaces and other aspects.
Why Do You Need a Software Requirements Specification Document?
As you see, an SRS can be the single source of truth that everyone falls back on. It takes effort and time to put it together and get approval, yet the return can sometimes be very tangible in the long run. How exactly can you benefit from writing software requirements? Here are a few reasons to use an SRS and not neglect its creation.
- Lays the groundwork for other technical documentation (an SRS may become the go-to document for many years, becoming the foundation and backbone of other product documents);
- Quality delivery (with a helpful reference at hand, the standards are set, so possible scaffolding and duck-taping risks that directly affect the product's quality are omitted, which can assure a smoother tech audit in due time);
- Fewer cases of miscommunication (since lots of possibly vague spots are described, there's less room for confusion and less time wasted on clarification, as all parties are aligned and follow one document);
- Keeps everyone on track (with technical criteria and business objectives shaping the perspective "on paper", there's fewer miscommunication and debates on priorities and rules, as the team sticks to the outlined plan, and the opinions regarding product development are aligned);
- More concise planning (a clear document lets you make more to-the-point projections on the costs and other vital points essential for those who want to stick to a business or startup budget).
When Is an SRS Unnecessary?
While an SRS can be a valuable tool for development teams to fall back on, is it possible to spare SRS documentation in software engineering? Is it obligatory in all cases? Some may argue that documentation in excess isn't a good thing, and that truly is a valid point.
Fair enough, there are scenarios when teams don't have SRS specs and don't even need them that much. For example, small projects with crystal clear requirements might go about without a super detailed SRS report, especially if the development team is small. In this event, less formal yet comprehensive documentation can be enough.
It is also quite common to skip the creation of a requirement specification in software engineering that follows agile development methodologies. The thing is that putting together such documentation and then keeping it up-to-date over time is a rather lengthy and complicated process. When a project is prone to constant changes (say, when something is modified, added, or iterated on a weekly basis, which is a frequent scenario with agile MVPs and other flexibly developed products), an SRS might not bring as much value as expected. Its relevance will be simply tough to maintain.
This doesn't imply that such products are built without any documentation or requirements whatsoever. Similar work regarding requirement gathering is handled during the discovery phase when teams plan out the product's development, mark what's in and out of the project scope, and make vital decisions regarding third-party app integration, which technology stack to choose, and so on.
What Does an SRS Document Include? (Sample Structure of SRS)
Before we dive into the details of how to write business requirements for software, let's go over what to include in such specs. Here's a sample structure of an SRS document example.
Although there is no universal standard regarding the length or format of SRS that all teams should adhere to, a software requirement document basically has a few main sections:
- Introduction (presents a clear overview and high-level business requirements, the purpose of the document and who it's for, it also notes the product's purpose, who it's built for and the product's value, objectives, what's included in the project scope, and a table of contents);
- System and functional requirements (goes over the main features the product should have as well as the environment, system architecture, software and hardware components, data handling requirements, core workflows, and so on);
- Non-functional requirements (states the expectations regarding the product's security, performance, usability, compatibility, maintainability, etc.);
- Interfaces (specifies the main design requirements as well as external interfaces);
- Use cases (can optionally be included to explain how a user is supposed to interact with the product and which goals will be achieved);
- Appendix (can have a glossary with terminology, concept explanations, and other notes).
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How to Write a Software Requirements Specification
Now let's go over how to write software requirements the right way. We'll include the must-adds for each section, however, these descriptions are solely an SRS example, and you may omit some parts in yours.
Step 1. Make an Outline of Your SRS Document
In order to be sure that you're not leaving anything out, create an outline of the future document, noting the main sections and parts to include. You may use the sample structure we've overviewed in the previous section or find and customize an SRS template that'll suit your needs.
Step 2. Work on the Introduction Section
The introductory part of your SRS gives an overview of the major takeaways of everything covered in the document. This summary should be concise and also often features a table of contents for simpler navigation. This part of the document covers the business requirements at a high level as well. Here's what you should include in the SRS introduction:
- what this document is about, who it's created for, and when to use it;
- the purpose of the document;
- the solution's intent;
- project objectives;
- who the target end-users are, their types, and needs;
- how the users are expected to interact with the product;
- which problem the product solves, it's value, what makes it stand out, and why it matters;
- what's included in the scope (e.g., major functionality to develop).
Step 3. Line Out the Functional and System Requirements
This is one of the largest sections of the SRS devoted to describing the product. Basically, it's a guide for developers that unveils important information on its execution. Try to find concise answers to the following questions:
- What are you building (e.g., a mobile app vs. a web product, a new product vs. an add-on)?
- What are the main features?
- What makes it unique?
- How should the product function?
- What must it do?
This is also the place to go into the details regarding the functional requirements you have. It doesn't necessarily have to include a detailed overview of the tech stack, as it may change, however, the SRS can place some boundaries on technology-related decisions, forming the basis of the product and how it must operate. Here's what you need to add:
- a list of the core features and functionality;
- what the features should do;
- environment;
- constraints or limitations;
- system architecture specifics with software and hardware components;
- requirements on how to handle data (process it, store it, etc.);
- core workflows;
- criteria determining the product is ready to go live.
If you're not sure how to write an SRS document and which technology is optimal, it makes sense to get a CTO or tech lead onboard or get a consultation from those who know the ropes.
Step 4. Note the Non-Functional Requirements
Now you can move on to formulating the non-functional requirements. How should the product behave? How should it perform?
For example, you can jot down the basics on the product's security, safety, and accessibility. You may also take note of its performance, such as expected speed or scalability opportunities. Sometimes information about the product's compatibility, concurrency, capacity, and maintainability are added as well.
Step 5. Clarify the Information on Interfaces
The user interface is all about what the client-facing side of the product will be like. If you have requirements on product design, logic, layouts, navigation, sequence, and a vision of how the users interact with the product, this is the place to spell them out.
As an option, you can add use cases that'll serve as a story of how a user interacts with the product and what happens during the process to achieve a goal. If there are any potential issues or expectations to be met, this should be written too.
External interfaces regard not only what users see, but also software, hardware, and communication interfaces. It's also an important matter to bring up when writing requirements for software, as the product is likely to interact with various external systems, APIs, databases, and so on. You may thus unravel such details as the data formats, libraries, and communication protocols.
Step 6. Provide any Extra Details
At times, an SRS also has an appendix where various miscellaneous information is added. This could regard additions like extra research, instructions on how to prioritize product features and other development decisions, timelines, schedules, which areas of the project have room for changes, and so on.
Some include references here, make notes on possible dependencies, or use the space to share ideas, assumptions, and suggestions. Having a glossary with the terminology definitions may also be handy to ensure everyone understands the vocabulary, abbreviations, and acronyms in use, doesn't confuse startup jargon, and gets the major concepts.
Step 7. Submit the SRS for Approval
Finally, when the software requirements specification draft is proofread and ready, you should pass it to the stakeholders or decision-makers for review. If they have ideas on what else should be mentioned or suggest edits, the document gets modified and finalized. After that, the document will be updated and modified more than once to keep the version relevant, substantially, this may happen with every major change.
More Tips on Writing Software Requirements and Mistakes to Avoid
Now that you know more about how to prepare SRS documents, here are some best practices and tips on what to do, a few secrets to writing good software requirements, and common mistakes to avoid.
Stick to the Characteristics of a Good SRS
It's quite easy to come across a software requirements specification example that seems like it's some alien language. You read it and doubt that it's even English. That's not the right way to go. Therefore, one of the first things you should know about how to write a requirements document for software is that you must ensure that it's readable and understandable to both techie and non-technical users. If you have doubts, add a glossary.
It's a good idea to describe each function or feature one at a time. Use clear, simple language, and break down the content into sections for better readability. For an even more explicit result, add diagrams, charts, and other visual means and stick to a neat, consistent format throughout the entire document.
Plus, don't serve the SRS "undercooked". Though it might not be 100% complete before you begin a development project, it has to feature the most prominent information with accurate descriptions. It should be logical, realistic in terms of budget and the way it'll be brought to life, and must lack obvious gaps that'll only confuse those reading it.
Sure enough, you will make changes to the document, so ensure that it's flexible to be modified easily. Of course, an SRS also has to be accessible to those who the document is intended for.
Avoid Common Mistakes When Making an SRS
Once again, the reason why an SRS is created in the first place is to omit unnecessary questions and misunderstandings regarding the product and how to develop it. This is exactly why those putting together the document should be careful with the words they use and the messages they're trying to get across. Don't make the SRS complicated and do your best not to confuse people by all means, that's a goal.
Not including references or examples to fall back on, when you have them is another mistake. They may remove vagueness, so they should be present if possible. If you can make the SRS simpler and easier to comprehend, review the document to make the formatting and structure better, delete repetitive phrases, as well as clarify those parts that can be improved.
Here's another important point, although a software requirement specification in software engineering can be a pillar document, it shouldn't be too technically detailed. Otherwise, you might end up tying your developers' hands by forcing them to follow strict guidelines. What if some parts could be developed faster, better, cheaper, or more effectively? Hence, leave room for adjustment.
Apply Tools for SRS Creation
If you don't want to put your software requirements specification in a Google Doc, there are plenty of tools for SRS documentation creation out there that you can use and customize. For instance, you may look for a software requirement template that'll help you with the document's structure, so all you'll have to do is fill out the fields. Moreover, there are standalone tools for writing and then managing requirements that may also be of help.
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Final Say on Software Requirements Specification
And there you go. Hopefully, now you know how to write a software requirements document. It's hard work, but in essence, you need to strike a balance and not make the SRS too technical and yet ascertain that it's accurate enough to instruct various team members. The information stated in the document shouldn't be contradictive, as its aim is to simplify the interpretation of the product in focus and serve as guidance.
Surely, the document may come in various shapes and sizes, not to mention that many teams manage developing products without an SRS. In the latter case, it is replaced with other forms of documentation that are created at the early stages of product development.
If you need a hand planning your future project, Upsilon's experts provide discovery phase services and can lend a helping hand. We can evaluate the project, gather requirements, select a proper tech stack, line out the scope, and handle lots of other tasks to lay the groundwork for a smooth development process. So feel free to reach out to us to discuss your needs!
FAQ
1. What is a software requirements specification (SRS) document?
According to the SRS meaning in software engineering, it's a document describing the product's functionality, the users it's built for, the problem it should solve, how it's supposed to work, and how it may be built. It's created to keep various people involved in a product's development process on the same page, from developers to stakeholders.
2. What is the need of SRS in software engineering?
An SRS in software engineering may serve as the foundation for other product documentation and often becomes the go-to document that provides clarification and keeps the team on track during development.
3. Why use an SRS?
When created correctly, such a specification can omit guesswork and reduce cases of misinterpretation, leading to better-aligned development processes. It can be handy for various roles and can be used to confirm that the product is being built in accordance with the requirements and meets them.
4. Who prepares SRS documents?
It depends, but commonly it's handled by those who initiate the product development process. It could also be a Project Manager or Business Analyst's responsibility, although some of its parts could be done by a technical writer, systems architect, developer, and so on. Some create it with the help of a customizable SRS document template that serves as a clear example of SRS.
5. Is SRS documentation obligatory in agile software development?
It is common that development projects are carried out without an SRS, which is replaced by other documents that line out the product requirements.
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