Software Requirements Specification: functional requirements, non functional requirements and other constituents

Writing a Software Requirement Specification (SRS) is the first and crucial step in any product’s software development process (Mobile App Development or Web App Development) using any methodology, whether it is Waterfall or Agile.


What are the requirements you need to know about to build great software?


Read this article and get to know about:

    • what is the software requirements specification
    • functional requirements
    • non functional requirements
    • how to write specifications when you know all the necessary software requirements
    • get a simple software requirements document template that you can use in your work right away


Let’s go!


Definition of a requirement for software

To understand what is a software requirement, you need to start with the requirement definition.


Here it is.

Software requirement is a condition or characteristic of software (or a system component) that can solve problems such as:

  • system automation
  • correction of shortcomings of the existing system
  • device control, etc.


Software requirements describe such aspects of the system as:

  • operating activities
  • general form
  • main features 


There are functional and non-functional requirements, which we will talk about next.


Functional vs Non Functional Requirements

When developing a new information system or implementing an existing one, you need to define your system’s functional and non functional requirements.

Let's figure out you should consider functional vs non functional requirements.


Functional requirements

Functional requirements define the function to be performed by a system or system component. In other words, a functional requirement describes what a software system should do.


Functional requirements are detailed, specified in the system design, and documented in various ways, particularly through written descriptions in documents and use cases.


Functional requirements examples

As functional requirements are a list of conditions that the user expects from the system, these can be:

  • Calculations
  • Data manipulation
  • Data processing
  • Technical details
  • Other specific functionality


Functional requirements example:
Let's say you are developing a payroll system. In this case, the functional requirements example for the system will be as following:

  • generation of electronic invoices
  • funds transactions
  • calculation of the commission amount
  • calculation of payroll taxes, etc.


You got the point, right?


Non Functional Requirements

Since we figured out that functional requirements describe what a software system should do, then non-functional requirements impose restrictions on how the system will do it.


Thus, comparing functional vs non functional requirements, we can conclude that a functional requirement refers to the system’s behaviour, and non-functional requirements determine the performance and other technical aspects of the same system.


Non functional requirements refer to a general property of the system as a whole or an individual aspect of it, rather than a specific function. The system’s available properties usually mark the difference between whether the development project was successful or unsuccessful.


Non functional requirements examples

Here is a list of basic non functional requirements that should be included in a software requirements document:

  • Performance requirements
  • Safety requirements
  • Interface requirements
  • Quality requirements
  • Operational requirements
  • Verification requirements
  • Reliability requirements
  • Documentation requirements
  • Compatibility requirements
  • Scalability requirements
  • Resource requirements
  • Maintainability requirements


Non functional requirements example: 

Returning to our example for the payroll application, the non functional requirements for it are:

  • system performance during bulk computing
  • time for data retrieving
  • security of user data
  • the ability of the system to process all necessary transactions, etc.


Requirements analysis

Since we have already figured out that software requirements can be functional and non-functional, and both types are of great importance for compiling a software specification, it is time to talk about how and who should collect and analyse them.


Let's start by defining the requirements analysis.


Requirement analysis is an essential phase in the software development process which consists of three crucial steps.


Requirements analysis contains:

  1. Collection and systematisation of software requirements.
    This stage involves communicating with customers and users to determine their needs.

  2. Identification of relationships and software requirements analysis.
    At this stage, you should determine whether the collected requirements are clear and unambiguous or contradict each other and resolve these problems.

  3. Software requirements documentation.
    Software requirements can be written as a simple description or use cases, user stories, and process specifications.


Important! In the requirements gathering process, you should consider possible conflicting requirements from all parties involved in the development - customers, developers or users.


Requirement gathering tools

Analysts use a variety of requirement gathering tools, e.g.:

  • interviewing
  • use of focus groups
  • prototyping
  • application of use cases


Often, analysts combine these methods to identify stakeholders’ exact requirements to create a system that meets all software requirements.


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General Form of Software Requirement Specification

Since we have already figured out what software requirements are and how and who should analyze them, it's time to move on to the documenting step.

In the software requirement doc, the solid and monotonous text is complicated for the reader. It is always great when the document’s structure is logical and supplemented with visual materials. Feel free to use:

  • numbered or bulleted lists
  • pictures and diagrams
  • spreadsheets
  • “air” (empty gaps in the text that make reading easier)

And, of course, screenshots! They are probably the most useful in software specifications. As the saying goes, “instead of a thousand words”.


Example of software requirement doc

Writing an SRS is a responsible task. And certainly not the easiest one. Therefore, authors often use already proven software requirements specification template in their work.


We offer our vision of the SRS main sections that we use in our work. Use it, create software requirement specification and get what you want!


Software Requirements Specification Example

INTRODUCTION AND CONCEPT of Software Requirements Specification

Overview document content.



1.1 Objectives

It contains detailed information about the goals that can be achieved with the product being developed.


1.2 Users characteristics and target audience

A description of the future target audience and how it affects software requirements. Here you can describe the types of users: visitor, user, client, admin.


1.3 Scope 

A description of what the application should or should not do, a functionality description.


1.4 Timeline/Lifecycle

Description of post release, changes in the product / its functionality later on.


1.5 General restrictions

It contains information about frameworks and standards that limit the software developer’s options when creating a system:

  • programming language
  • coding standards
  • data exchange standards


1.5 Definitions and abbreviations

This chapter provides explanations for all specific terms to make the document more understandable.



Software Requirements 

The main section of the document. It answers the question about what we want to receive. Contains the information necessary for the software development process.


2.1 Functional requirements

This subsection tells you what the software product should do.


  • input data and its transformation
  • necessary operations
  • output results
  • arguing the need for certain requirements



2.2 Non-functional requirements

This subsection defines the criteria for evaluating important system parameters, such as:

  • performance
  • data integrity 
  • security


2.3 Risks and difficulties

About the relationships within the project and the influence of external factors. 


What to consider:

  • terms and quality requirements
  • monitoring, logs, transactions
  • lack of competence in the use of tools and technologies, lack of knowledge of the subject area
  • human relationships (team-management-team)
  • interaction with subcontractors and partners
  • political situation



2.4 Other requirements

This subsection usually contains a list of applications with common diagrams, for example:

  • information flow diagram (input-output data, their sources, storage and destination points)
  • user scenarios (interaction of objects with a software product, taking into account the main functionality)


Final words about  software requirement specification

If you aim to write useful software requirement specification you need to think carefully about requirements analysis. Keep in mind that writing a software requirements document for each specific software product is individual, and in the end, the SRS document really solves a lot of problems at all stages of software development as it serves as a basis for:

  • further planning 
  • design 
  • coding
  • testing user documentation

Start with making a software requirements document, send it over to your team, and get the software product that meets all your needs!

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