RST Software
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Ross Krawczyk
Reviewed by a tech expert

That's how to write a software requirements specification (SRS document)

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Developing software solutions requires careful planning and documentation to translate concepts into tangible products. One of the most indispensable documents in this process is the software requirements specification (SRS).

What is a software requirements specification (SRS) document?

At the heart of software development lies the Software Requirements Specification (SRS) document, a detailed account that captures the core intent, distinct features, systematic architecture, interfaces, and performance criteria of a software application or component. It stands as a pivotal cornerstone, providing clear guidance and direction for every phase, from conceptualization and design, to construction, validation, rollout, and ongoing upkeep throughout the software's lifespan.

In concrete terms, an SRS for an e-commerce mobile app would detail required functionality like user account registration, product catalogs, shopping carts, payment processing, order management, and so on. It would outline how the app should handle issues like security, reliability, and scalability. And it would define exact performance metrics like transaction response times etc.

SRS equips the developers to construct an app that meets business objectives. Software requirements document would also provide quality assurance engineers with criteria to effectively test the software. And it would give the operations team a blueprint for deploying and supporting the application.

Software requirements specification (SRS) vs. system requirements specification (SyRS)

While a software requirements specification (SRS) focuses on an individual software component, a system requirements specification (SyRS) details requirements for an entire system encompassing software, hardware, data assets, manuals, and more.

For example, consider a self-driving vehicle system. Its SyRS would describe required mechanical sensors and automation processors, onboard software capabilities, maintenance procedures, operational parameters, and other elements that comprise this complex system. The SRS would then drill down into the exact requirements just for the vehicle's software components, like the autonomous driving algorithm.

So in summary, an SRS provides granular requirements for software only, while a SyRS addresses an entire system solution. An SRS delivers detailed guidance for software developers, while a SyRS gives system architects a comprehensive specification.

Why do you need an SRS document?

In the business landscape, it is not uncommon to perceive software requirements documentation, such as an SRS, as mere procedural red tape, eagerly prioritizing the software development phase. Nonetheless, channeling resources into crafting a precise and comprehensive SRS not only epitomizes foresight but also reaps exponential benefits throughout the software application's lifecycle.

Here are four major benefits a software specification provides:

  • A shared source of truth – The SRS provides a single touchpoint where every stakeholder such as customers, users, project managers, developers, and support engineers can reference to obtain a complete understanding of the software's goals and parameters. This aligns perspectives and prevents misinterpretations that lead to wasted effort.
  • Early detection of issues – Meticulously detailing every requirement makes invalid assumptions, conflicting objectives, insufficient needs, and gaps apparent right away before they derail development. Issues caught early in requirements planning are far less costly to address.
  • Development roadmap – The SRS gives developers, architects, designers, and testers the comprehensive blueprint they need to craft the right software solution. It transforms nebulous concepts into actionable specifications that set clear direction.
  • Future-proofing – Even long after deployment, the SRS preserves institutional knowledge and reasoning behind why the software was created, so enhancements can adhere to original intent. And it facilitates reuse of components in future software.

In short, diligently writing software requirements results in better software that fully and accurately addresses end-user needs. The upfront time investment in an SRS returns manifold over the software's lifespan.

Key components of an SRS document

While the structure of a software requirements document varies based on the complexity of a software application, most SRS documents contain these core sections.

Document's purpose and scope

The SRS should begin with an introduction that explicates the document's aims and defines the boundaries of the software system covered. It may also provide a high-level background on the business drivers and context.

This overview helps readers frame the SRS's intents and understand how the software fits into larger goals.

For example, a software requirements document could define the scope for just an Android version of a mobile app rather than all mobile platforms.

High-level view of the system's architecture and components

A short section with a diagram provides a conceptual overview of the system structure and essential software components. It demonstrates relationships and interactions between components.

In a software requirements document for a web app, this view may show a web server providing pages to browsers while integrating with a database for storage. For complex projects, cite or attach more elaborate architecture documents.

Functional requirements

Functional requirements, the heart of the SRS, detail the software's capabilities, behaviors, and processes it must execute. Effective requirements are precise, unambiguous, and constrain implementation.

For instance, functional requirements for financial software may require supporting Debit/Credit transactions, maintaining ledgers, tracking 1099 tax forms, and importing bank statements. Requirements use declarative language like The system shall…”

External interface requirements

These specifications describe how the software system connects with other entities like users, hardware, databases, operating systems, and external applications.

For example, an SRS may define support for retina display resolutions, integration with a document management API, or a web-based user interface accessible from Chrome and Firefox browsers.

Non-functional requirements

Non-functional requirements listed in a software requirements document detail the quality attributes and performance the system should possess, like usability, accuracy, security, availability, speed, recoverability, and more.

A non-functional requirement could require user logins occur in under 2 seconds at peak load or that monthly uptime percentage exceeds 99.95%. Non-functional requirements complement functional ones.

Data models

The data model outlines the structure and relationships of the information that is stored, consumed, and transmitted by the software system. This includes data entities, attributes, relationships, and formatting.

For example, an e-commerce SRS may define customer, product, order, and payment data entities and their connections that represent information processed during transactions.

Constraints and assumptions

This section of a software requirements document lists factors like regulatory policies, technology standards, hardware limitations, and support for legacy software that may constrain the solution. It also calls out any assumptions that influence the requirements.

Industries like medical, automotive, aerospace, and financial services often must adhere to strict compliance regulations that heavily influence requirements definition. Hence, if regulatory compliance applies, mandate standards mappings, auditable requirements traceability, design control, and other applicable procedures right in the software requirements document.

For instance, e-commarce platform may need to integrate payment processing gateways that are PCI-compliant.

Glossary and appendices

Glossary and appendices provide helpful clarification and additional technical details without cluttering the core specification.

The glossary provides specific definitions of important terms used throughout the software requirements document. It eliminates ambiguity and misinterpretations caused by assumptions readers make based on their own unique experiences. For example, it may define a transaction” as a complete payment processing event from a customer order submission to fulfillment.

While including a glossary in an SRS document improves precise understanding of terminology that may have special meaning within the context of the project, attaching appendices provides supplementary reference materials that support the main content of the SRS. Examples include screen mockups, design systems, database schema diagrams, API documentation, mathematical models, calculations, simulation results, standards mappings, and compliance checklists.

Best practices for writing a software requirements specification (SRS)

When you approach your project's software requirements document with a keen sense of purpose and adhere to tried-and-true best practices, you position yourself to create a standout specification. Let's delve into some premier advice for SRS composition.

Utilize industry-standard templates like IEEE 830 to maintain consistency

The IEEE 830 Systems and Software Requirements Specification standard defines proven SRS section headings and content guidelines developed through decades of industry consensus.

Leveraging IEEE 830 provides consistency, ensures you cover all necessary specification areas, and allows you to focus on your project's unique requirements rather than SRS structure. The IEEE 830 table of contents serves as an excellent SRS template.

Enrich your SRS with visuals

IEEE 830 specifies that SRS must contain full labels and references to all figures and diagrams it mentions. Diagrams, mockups, UI screenshots, decision tables, flowcharts, data models, state transition diagrams, and other visual artifacts enrich any software requirements document. Simply speaking, they can convey complex ideas and requirements far better than dense text.

Visual models synthesize details into easily digestible graphical formats. They demonstrate relationships, illustrate workflows, and provide context. Images speak a thousand words so take advantage of visualization, but ensure diagrams are legible and clearly referenced.

Use the MoSCoW method for prioritization

The MoSCoW method, developed in the early 1990s as part of the Dynamic Systems Development Method (DSDM), provides an elegant technique for designating requirement priority categories.

MoSCoW stands for Must have, Should have, Could have, Won't have. Categorizing each requirement using MoSCoW tags identifies critical functions the software must deliver versus nice-to-haves. This focuses development efforts on truly essential features.

For example, if you’re creating an online marketplace, the following requirements are categorized using the MoSCoW method:

  • Must have – The system must allow customers to browse, search, and purchase products using a secure payment method. The system must also provide order confirmation and tracking information to the customers via email.
  • Should have – The system should offer recommendations based on the customers' browsing and purchase history. The system should also allow customers to rate and review the products they bought.
  • Could have – The system could integrate with social media platforms to enable customers to share their purchases and reviews with their friends. The system could also provide loyalty rewards and discounts to encourage repeat purchases.
  • Won't have – The system won't support multiple languages and currencies. The system won't allow individuals to sell their own products on the platform.

Keep it clear and concise

A software requirements document must be clear and concise. It reduces the risk of misunderstandings, errors, and rework. For clarity’s sake:

  • Use simple and direct language – avoid jargon, acronyms, and technical terms that may confuse the reader.
  • Use bullet points and tables to present information in an organized and easy-to-read way.
  • Use consistent terminology and formatting throughout the document. Define any terms that may have multiple meanings or interpretations.
  • Use active voice and present tense to describe the current state and desired outcomes of the software.
  • Use examples, scenarios, and use cases to illustrate the functionality and behavior of the software.
  • Review and edit the document for clarity, accuracy, completeness, and consistency.

Equally important for clarity and conciseness of the software requirements document is making it easy to update.

Implement version control mechanisms

Requirements management tools like Jama or code versioning systems like Git allow maintaining SRS revisions and tracing requirements into downstream development artifacts like designs, code, and tests.

Controlling versions and tracking linkages are crucial for reviewing how requirements evolve and validating that software deliverables meet specifications. Traceability becomes mandatory for compliance purposes like medical device development.

Involve all relevant stakeholders

Actively elicit SRS feedback from any groups invested in the software, like project sponsors, customers, end-users, and the development team. These stakeholders should review and approve the SRS to secure their buy-in.

Broad collaboration and consensus ensure the SRS addresses all aspects important to stakeholders. Proactively resolving conflicting perspectives leads to better requirements.

Need help with writing your SRS document?

Producing a polished, easy-to-comprehend, and thorough software requirements specification is challenging. If you need some help with drafting an SRS that leads to software development success, partnering with us is an excellent option.

Our cross-domain experts will guide you through structured requirement elicitation workshops. We’llhelp uncover every nuanced functionality and quality need related to your software goals, and then synthesize this learning into a meticulous SRS document conforming to industry best practices.

The professionally-crafted SRS will give your software project a tremendous head start by providing the development team an unambiguous roadmap for delivery. It also reduces business risks by confirming the software, once built, will fully meet your needs and compliance obligations.

Reach out to us via our contact form if you need any guidance on writing SRS documentation for your next big project.

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