October 05, 2022

How to Write a Software Requirements Specification (SRS) Document

Yulya Glamazdina

Marketing Manager

10 minutes

There are plenty of horror stories about outsource software developers and clients clashing on project requirements, and one of the most common causes of such quarrels is ambiguous language. Vague terms like "urgent," "pending," "ASAP," and so on might mean something different to the client and to the contractor. Or, perhaps, the involved parties have a different impression of the product's prioritized functionalities. It's neither party's fault, per se — they are just speaking different languages, in a sense. The customer has an idea for the product, its functions, and what the user interface might look like. But the developers are thinking about the "behind-the-scenes" of the project, how to write a scalable and readable code, ensure security, make a solution performant, minimize bugs, etc. When there is a disconnect between both viewpoints, it can lead to extra work, wasted money, stress, and even a sub-par final product. But, thankfully, there's a way to avoid such miscommunications from the get-go: Software Requirements Specifications.

Content
What Are SRS and Why Are They Needed?
Basic Elements of an SRS
Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples
How to Write Documentation
Needs Document
Architecture Document
SRS Document
SRS Design Guidelines
Summing Up

What Are SRS and Why Are They Needed?

good srs characterics.png

An SRS is a document that describes a software system that will be developed. It's accessible to all parties, including the stakeholders, investors, a project manager, programmers, designers, and so on. There are three types of requirements that the SRS contains:

  1. Top-tier: These are the high-level business requirements. They outline the business' measurable goals, define the purpose behind the project, and align the project goals with stakeholder goals.
  2. Middle-tier: These are the user requirements. They reflect specific user needs and expectations, describe who is using the software, and highlight user interactions.
  3. Bottom-tier: These specify the product's functionality in tech terms. They identify functions, features, use cases, and non-functional requirements, as well as describe the project as functional modules + non-functional attributes.

By combining specifications from all three tiers, you're giving all involved parties a clear understanding of the project and measurable deliverables. This is especially important if you're outsourcing the project; a thorough SRS document enables an outsourcing agency to communicate your project's requirements clearly and accurately to their own team. For a deeper illustration of how SRS improve communication and collaboration between parties, check out the table below:

PartyBenefit of SRS
Software DevelopersThey can accurately estimate the scope of the project.
Project ManagersThey can use the documentation to plan their work.
DesignersThrough an SRS document, they gain insight into a project and adapt the design to the use case.
TestersThey receive guidance from the SRS on how to create test cases that meet business needs.
StakeholdersThey use the SRS to understand the software — for instance, how their idea is translated into architecture, functionality, and tasks, which are then all evaluated.
InvestorsThe SRS provides them with an overview of the system's functions, enabling them to make more informed investment decisions.

Summed up, an SRS document is important because it provides stakeholders, PMS, programmers, and so on with a single source of information and expectations. By compiling all necessary information in one place, you can greatly reduce the frequency of misunderstandings.

Basic Elements of an SRS

In order for an SRS to be useful to all involved parties, it must have several basic elements, as outlined below:

  • Business Drivers: Include the reasons why the customer wants you to build the software. It's important to include the rationale or the driving force behind the new system; this information guides the decisions that the business analysts, system architects, and developers will make.
  • Business Model: Which basic customer business model will the system need to support? Include organizational and business context, key business functions, current and future state diagrams, and process flow diagrams.
  • Business and System Use Cases: Typically, this section will contain a UML - Use Case Diagram that shows which main external objects the system will interact with.
  • Technical Requirements: In this section, the document will list any "non-functional" requirements that compose the technical environment.
  • System Qualities: You will list any non-functional requirements that determine the system's quality.
  • Constraints and Assumptions: This section includes software constraints that the customer requires, which can be used to exclude options from development consideration.
  • Acceptance Criteria: Lastly, this element illustrates how the customer will sign off on the final system.

Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples

Functional vs. Non-functional Requirements

If you’re ordering custom software, you and your development team need to hash out its requirements — otherwise, you risk getting a final product that is unreliable, poor performing, and non-secure. In the software development world, there are two types of requirements: functional and non-functional. Read on to learn the distinction between the two and why they’re crucial to your project’s success.

Within the SRS, you’ll need to address functional and non-functional requirements. What do these terms imply?

  • Functional Requirements — The functions a software must perform.
  • Non-Functional Requirements — Standards used to judge the system’s operating quality And why are both requirements important?

In essence, functional requirements help software engineers capture the intended behavior of the application — and said behavior may be expressed as services, tasks, or functions that the system must perform. On the other hand, a non-functional requirement ensures that the entire system is usable and effective. If all functional requirements are met but non-functional requirements are lacking, the system would fail to meet the user's needs.

Some examples of functional requirements include:

  • If/then behaviors
  • A description of the system's workflow
  • Data inputs and their corresponding outputs
  • The logic behind data handling

Non-functional requirements might look like:

  • Performance and scalability
  • Compatibility and portability
  • Maintainability, availability, and reliability
  • Security
  • Usability
  • Localization

What Are Functional Requirements.jpg

What Are Functional Requirements?

Functional requirements specify which functions your system (or system component) must have. In other words, they define what your custom software needs to do. If these requirements are not met, the software fails; it’s non-negotiable. The most common functional requirements examples are (1) use cases and (2) document descriptions.

Use Case

Use cases can be numbered lists with text or diagrams that describe user actions. Each use case illustrates behavioral scenarios through one or more functional requirements. First, a set of use cases is selected — and then an analyst can derive the functional requirements that need to be implemented in order for each use case to be fulfilled. In the diagram below, a navigational use-case model for a conference management system is proposed. Each main functionality is associated with user roles: navigational use-case model for a conference management system is proposed.png And here’s an example of a use case numbered list for a subway ticket machine: example of a use case numbered list for a subway ticket machine.png

Written Document Descriptions

A typical written functional requirement will contain a unique name and number, a brief summary, and a rationale. This information is used to help the reader understand why the requirement is necessary and develop a system to track it. Some examples of written functional requirements include:

  • Interface requirements
  • Data manipulation and processing requirements
  • Verification requirements
  • Documentation requirements
  • Security requirements
  • Acceptance criteria

During the development stage, functional requirements are used to describe whether or not the system is working as intended. That means that there might be many more requirement types for you to write down until you create a complete picture of a correctly working system.

Who Is Responsible for Preparing Functional Requirements?

Functional requirements are typically collaborated on by a business analyst, system analysts, and an implementation team. The project manager is responsible for making sure the functional requirements are delivered on time and they’re signed off by the System Owner and QA. What Are NFRs or Non-Functional Requirements.jpg

What Are NFRs or Non-Functional Requirements?

Non-functional requirements define how the software fulfills its functional goals - not what the system will do but how the system will do it. These requirements define criteria that are used to evaluate the system’s performance, reliability, and maintainability. They define and impose external constraints on the software being developed, as well as on the development process. Non-functional requirements are presented as a specific aspect or general property of the system as a whole — as opposed to an exact function. Aspect = a feature linked to many parts of the system but not related to its primary function Properties = characteristics of software, including integrity, reliability, accuracy, etc. Non-functional requirements can be divided into two main categories: performance and evolutionary qualities.

Performance QualitiesEvolutionary Qualities
ReliabilityTestability
SurvivabilityMaintainability
EfficiencyExtensibility
IntegrityScalability

Performance qualities are observed at runtime, whereas evolutionary qualities are embodied in the system’s static structure. Let’s take a closer look at some non-functional requirements examples presented in a paper published by the IEEE.

Performance

Efficiency: How well does it utilize resources?

Integrity/Security: How vulnerable is it to cyberattacks?

Reliability: What level of confidence can be placed in its functions?

Survivability: How well does the system perform under poor conditions?

Design

Correctness: How well does the system conform to requirements?

Verifiability: How easy is it to verify the system’s performance?

Maintainability: How easy is it to repair the software?

Adaptation

Expandability: How easy is it to upgrade or expand its capability/performance?

Interoperability: How easily does it interact with other systems?

Flexibility: How easy is it to change?

Portability: How easily transportable is the system?

Reusability: How easily can it be converted for use with another system?

Who Is Responsible for Preparing Non-Functional Requirements?

While functional requirements can be presented in a Functional Requirements Document (FRD), a Product Requirements Document (PRD) combines functional and non-functional ones. Non-functional requirements are usually defined by developers, software architects, and other technical experts, and the PRD is typically prepared by the product manager.

Comparison Table "Functional vs. Non-Functional Requirements"

To see the differences between functional and non-requirement functions at a glance, just check out the table below:

Functional RequirementsNon-Functional Requirements
Helps the reader understand what functions the system will haveHelps the reader understand how the system will perform
Explains the system’s necessary featuresExplains the way the system should behave
Identifies what the system must (and mustn’t) doIdentifies how the software should do it
The system won’t work if these requirements aren’t fulfilledThe system will still work if these requirements aren’t fulfilled
Ensures the software meets user requirementsEnsures the software will meet user expectations
Straightforward to defineHarder to define
Can be documented through a use caseCan be documented as a quality attribute

Conclusion about Functional vs. Non-Functional Requirements

Your software needs to fulfill both functional and non-functional requirements for optimal performance. The former enables it to run, while the latter enhances the user experience. It’s important to work with a custom software development team that meticulously documents both kinds of requirements. Otherwise, you could end up with a disappointing product or a decent one that exceeds budget and time constraints. At Brocoders, we strive to create successful software that streamlines workflows and an enhanced customer experience — and our deep requirements-gathering process makes this possible. Want to know how Brocoders can develop an outstanding product for your business? Get in touch, and we’d be happy to give you more info!

How to Write Documentation

You now know the main elements of an SRS, but how do you put them in order to create one cohesive document? Not to worry: we've created a template that addresses all requirements relevant to all parties. Requirements engineering is based on:

  • requirements elicitation (interview with stakeholders and decision makers)
  • requirements analysis (are they consistent)
  • requirements documentation
  • requirements verification and validation (am I building the system right and am I building the right system)

Requirements engineering in SDLC

Frame 1261 (1).png

To write SRS, you need a framework, and there are plenty of templates to streamline the process. There are also standards to adhere to when writing technical requirements. For instance, there is IEEE830 — which is outdated but still regularly used.

IEEE830.png

There is also an international standard called IEEE/ISO/IEC 29148-2018, which we have adapted for our needs. Below we'll show examples from Reqview tool that is used to organize requirements documentation in accordance with standard mentioned above.

Frame 10592.png

One more tool that is used in our company for requirements management is Confluence by Atlassian.

Product documentation can be separated into:

  1. Needs Document (It describes high-level user needs via user stories)
  2. SRS (Software Requirements Specification)
  3. Tests (This document describes test cases verifying functionality described in Requirements)
  4. Architecture (Describes the architecture of the application)

In the examples below, we'll show you how to create Needs, Architecture, and SRS documents. At the very least, we suggest preparing a Needs document so it can be sent to potential vendors.

Needs Document

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1. Introduction

This document describes the high-level functional requirements of software.

1.1 Purpose. This is informational section that is used to describe the purpose of this document. Usually we can put there "The purpose of this document is description of software that is going to be build for..."
1.2 Project Scope. Briefly describe the scope of the product.
For example for development software that is used for creating SRS (such as Reqview tool) it can be described in a such way
Users should be able to:

  • Record requirements specifications
  • Use custom attributes to manage requirements
  • Set up requirements traceability & browse the matrix
  • Review and comment upon requirements
  • Filter and search for requirements
  • Import requirements from Excel and MS Word
  • Export requirements to PDF, HTML, DOCX, XLSX, or CSV
  • Print requirements specifications
  • Analyze requirements coverage and change impact

1.3 User Roles. Identify user roles for the product; for each role, describe the characteristics of a typical user. Consider creating personals with a real name — each one should be described sufficiently, so all team members feel like they know the persona.

This is an example of user roles definition from Reqview User stories example in Reqview tool

Below is example from our product documentation

User typeApplicationSub-roles
AdminAdmin panelModerator. Admin invites Moderator in the system (admin panel) via email. Moderator can access only the sections, which are assigned to them via Admin.
DistributorWeb app-
InvestorWeb app-
Customer--
Energy CompanyWeb appSales Agents. Sales Agents select prefferable EC at the registration and then are managed by dedicated EC. Customers. Customers are assigned to EC by Admin or automatically linked to EC if dedicated SA creates customer’s record in the system.
Sales AgentMobile appSub Agents. Sales Agents can be assigned to other Sales Agents within and by dedicated Energy Company. This distinguishment is implemented for futher deal calculations flow refactoring.

2. Product Functions

2.1 User stories. We use user stories to describe a product and its functionality, and for easier navigation, they can be grouped into "epics." To write a user story, set the following attributes:

  • Heading: A short story name
  • As a(n): The type of user
  • I Want: A goal
  • So That: A reason
  • Acceptance Criteria: Details on how the story should be verified; basic criteria to determine if the story is fully implemented
  • Priority: "High" for fundamental features, "Medium" for important features with a short-term workaround, and "Low" for features that will be needed in a later release or that can be omitted if time runs out.
  • Risk: "High," Medium," or "Low"
  • Estimation: Story points that offer relative estimates of the complexity, effort, or duration of a story
  • Status: Workflow information that classifies stories as "New," "Planned," "Implemented," or "Verified."

User Stories example from Reqview:
user stories example.png

User Stories example from Confluence. Source: Atlassian Documentation
requirements-confluence.png

2.2 User statuses flow. Sometimes, you may need to show flow using diagrams — like, for example, in this User Status flow.

user-status-flow.png

As another example, here's a Deal/Order Creation and Status flow:

Frame 10593.png

3. Constraints

In this section we should describe constraints that are effectively global requirements, such as speed of application, limited development resources.
Example from Reqview

constraints reqview.png

Architecture Document

1. Introduction

2. Architecture

It can be described this way.
Overall architecture
Operates with 6 types of users, which are allowed to access the system via 3 interfaces:

  • Web application
  • Admin panel – web application
  • Mobile application – both Android and iOS platforms
InterfaceUser types
Admin panelAdmin Moderator (sub-admin)
Web applicationDistributor Investor Company Customer (currently - all the activities within the UI are blocked)
Mobile applicationSales Agents Sub Agents

Staging Environment
Is hosted at Brocoders AWS resources and can be handled by the local team. The overall infrastructure includes Redis instance for delayed jobs handling, Amazon s3 bucket storage for system files, and general instances for the admin panel and web app.
Production Environment
Is hosted on the client's AWS resources. Overall infrastructure is set up in the same manner.

3.References

Lists the SRS, as well as any documents or websites that the document refers to.

SRS Document

1. Introduction

1.1 Purpose. The main purpose of this document is to provide a working example of a Software Requirements Specification (SRS) based on the ISO/IEC/IEEE 29148:2018 standard.
1.2 Scope. As with the Needs Document, you should describe what problem the software will solve and how it will be used.
Users should be able to:

  • Record requirements specifications
  • Use custom attributes to manage requirements
  • Set up requirements traceability & browse the matrix
  • Review and comment upon requirements
  • Filter and search for requirements
  • Import requirements from Excel and MS Word
  • Export requirements to PDF, HTML, DOCX, XLSX, or CSV
  • Print requirements specifications
  • Analyze requirements coverage and change impact

1.3 Product Perspective. In this section you describe how this product relates to the others elements of the system, for example if it is an element of the larger system. You can put here requirements for User interfaces, System interfaces, Hardware interfaces, Software interfaces, Communications interfaces, Memory constraints etc.
1.3.1 System Interfaces. List each system interface and identify the functionality of the software to accomplish the system requirement. For example: The application runs on the latest version of Chrome or Firefox browser on Windows, Linux, and Mac.
1.3.2 User Interfaces. List the logical characteristics of all interfaces between the users and the software product — for instance, GUI standards and sample screen images
1.4 Product Functions. Link this section to Section 2 of the Needs document (User Stories).

2. Requirements

Product functions are summarized in Section 2 of the Needs document (User Stories).

2.1 Functions. This section describes functional requirements. Every function should be linked to Needs, Tests, and Architecture documentation and sometimes it is convenient to linked it with the task in Jira.
This is how it looks like in Reqview tool

functions_reqview

2.2 Usability Requirements. Define usability and quality in use requirements and objectives for the software system, which can include measurable effectiveness, efficiency, satisfaction criteria, and avoidance of harm that could arise from specific contexts of use.

An interface design's usability requirements should support the following for its primary users:

  • Efficiency: Little to no setbacks for user and quickly-accomplishable goals.
  • Intuitiveness: Easy-to-learn interface with simple navigation. All headings, buttons, and error messages are easy to understand.
  • Low perceived workload: The interface appears simple to use and isn't intimidating, frustrating, or demanding.

2.3 Performance Requirements. Performance requirements define how well the software system accomplishes certain functions under specific conditions. Examples include the software's speed of response, throughput, execution time, and storage capacity. The service levels comprising performance requirements are often based on supporting end-user tasks. Like most quality attributes, performance requirements are key elements in the design and testing of a software product. For example, some performance requirements include:

  • Database requirements
  • Design constraints
  • Standards compliance
  • Software system attributes

3. Verification

Verification tests are specified in the [TESTS] document.

4. Supporting Information

Provide any information and define any terms necessary to understand the document. Define abbreviations and acronyms, provide diagrams and models, and create a numbered list of any TBD occurrences.

5. References

Include the following information regarding references:

  • Provide a complete list of all documents referenced elsewhere
  • Identify each document by title, report number (if applicable), date, and publishing information
  • Specify the sources from which the references can be obtained

Once you've completed all sections of the SRS, you'll need to get it approved by the project's key stakeholders. Make sure everybody is reviewing the document's most recent version!

SRS Design Guidelines

Now that you know the general structure of an SRS document, you're well on your way! We'd just like to give you a few design guidelines, so you can be sure your SRS is as clear as possible.

  • Include plenty of diagrams and models, as these contribute to a better understanding of processes.
  • Avoid ambiguity. The SRS should be clear, so the involved parties can avoid endless discussion and doubt. Pro tip: to identify ambiguity, carry out a formal peer review.
  • Pay attention to sequence. One section of the SRS shouldn't conflict with another. Make sure to use the same terminology and ensure the document is in a consistent format.
  • Use accurate, precise language. Avoid saying things like "the app needs to be released as soon as possible." Use specific dates, figures, and goals.
  • Create a glossary, especially if you're using "internal" definitions.
  • The following five tips will help you to work with SRS efficiently:
  • Save the history of changes, so anybody can trace changes back to the first version.
  • Assign each requirement an identifier so it can be easily traced in the documentation.
  • Use critical thinking to prioritize the product's main functions.
  • Focus on the customer. There should be a portrait of the average end-user.
  • The SRS document should be flexible enough to be updated.

Summing Up

SRS documents can be lengthy, and they might seem complicated at first. But by following the template above, you'll see that creating the document really isn't that bad. The benefits that come from SRS (improved communication and aligned goals) greatly outweigh the effort of creating and approving the form. At Brocoders, we create SRS for all of our projects, and our clients are big fans! Reach out today if you want to learn more about our software development process.

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