Software Requirements Specifications

A software requirements specification is a detailed description of the intended purpose, features, functionality and other aspects of a software application. It serves as a guide and foundational reference for the development team to follow during the building of the software. A good software requirements specification will aid in reducing errors and rework later on in the development life cycle.

The software requirement specification document captures what the software should do from the user’s perspective. It should be written in clear language understandable to both technical and non-technical stakeholders. The document acts as a contract between the project sponsor or client and the development team regarding the conditions that the final software product must meet.

The specification typically contains both functional and nonfunctional requirements. Functional requirements define the core capabilities and behaviors of the system. Non-functional requirements constrain the solution or process for achieving the functional requirements. Common sections in a software requirements specification include:

• Introduction and Purpose
• Business Requirement
• User Personas or Roles
• Feature List
• User Story or Use Cases
• User Requirements
• Functional Requirements
• Nonfunctional Requirements
• Interface Requirements
• Performance Requirements
• Security Requirements
• Design and Implementation Constraints
• External System Requirements
• Quality Assurance Requirements
• Documentation Requirements

The requirements specification evolves over the course of the project as more details emerge and changes are introduced. However, it is important to establish a baseline set of requirements upfront to guide the fundamental architecture and design choices. The development team refers continuously to the srs document as the authoritative information source regarding what the system should do.

Read more about software requirements specifications in the Software Engineer Book of Knowledge (SWEBOK)

A software requirement is a capability or condition that must be met in order for software to solve a real-world problem. More formally, a software requirement can be defined as a feature or non-functional constraint that the software must provide to fulfill the needs of users and other stakeholders. Requirements represent the core functionality that software must deliver in order to be useful and meet business process or user goals.

Examples of common requirements in software systems include:

• Functional capabilities – Allow a user to search a database, process an order, send a message, edit a photo, authenticate a login, etc.
• Data and storage – Store at least 1 TB of data, support 100 concurrent users, complete a transaction in 1ms.
• Security – Encrypt stored data, authenticate users, track access logs, resist SQL injection attacks.
• Reliability and availability – Experience less than 1hr of downtime/year, failover to a backup data center.
• Performance – Return search results within 1 second for 95% of queries, process credit card payments in under 2 seconds.
• Scalability – Handle a 10x increase in users over 6 months with no performance impact.
• Usability – Complete common tasks in under 3 clicks/screens, learn with 1hr of training.

Requirements form the bridge between business objectives and software implementation. They serve as the foundation for all design, development process, testing, and documentation activities. Clear, testable requirements are crucial to building software that meets stakeholder needs.

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Software requirements fundamentals can come from a variety of sources, but generally fall into two categories – business stakeholders and technical stakeholders.

Business stakeholders are people who represent the business, organizational or user interests. This includes:

• Clients – The party paying for the software to be built. They define business objectives.
• Customers – The organizations that will use the software. They specify needs.
• Users – The people who will actually interact with the software system daily. They identify workflows and usability.
• Software engineer – Those with knowledge about the problem domain.
• Project managers – The project manager defines budgets, timelines, and business constraints.

Technical stakeholders are involved in the software development process itself. This includes:

• Software development team – Make high-level technical decisions and define software structure.
• Software developers – Implement features and translate requirements into code.
• Test engineers – Verify software meets specific requirements and identify defects.
• Security experts – Define security and compliance requirements.
• System administrators – Administer hardware/infrastructure requirements.

Aligning requirements from both stakeholder groups is key to effective requirements engineering in the software industry.

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Some common examples of software requirements across different application domains include:

E-commerce site

• Catalog browsing and search
• Shopping cart and order management
• Payment processing
• Order tracking and history
• Inventory management
• Responsive web design
• 99.95% uptime

Mobile banking app

• Login with biometric or PIN authentication
• Transfer funds between accounts
• Pay bills
• Deposit checks via phone camera
• Data encryption
• PCI compliance
• Intuitive touch interface

Embedded system

• Read sensor data
• Control electrical outputs
• Real-time performance
• Reliable operation in -20C to 50C
• Low power consumption
• Remote firmware updates
• Security against tampering

Game application

• 3D graphics rendering at 60fps
• Handle 20 entities simultaneously
• Fluid object movement
• Realistic physics simulation
• Save/resume game state
• Multiplayer network synchronization
• Prevent cheating and hacking

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Requirements analysis is the process of examining requirements to improve quality, uncover issues, and gain a deeper understanding of what stakeholders want and need. Why do we analyze requirements? It serves several important purposes:

• Validate completeness – Ensure all necessary capabilities are captured.
• Remove ambiguity – Refine vague, unclear requirements.
• Check consistency – Identify conflicting or contradictory requirements.
• Assess feasibility – Determine if requirements are realistic and achievable.
• Prioritize implementation – Understand relative importance and urgency of each requirement.
• Estimate effort – Gauge work needed to satisfy requirements.
• Guide design – Derive technical implications and decisions from requirements.

Analyzing requirements reduces risk by catching problems early when they are easiest to fix. It also sets the stage for developers to build the right solution.

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High quality requirements often exhibit the following properties:

• Unambiguous – Open to only one interpretation, clearly defined.
• Complete – Covers all necessary capabilities within scope.
• Traceable – Linked to origin and dependent artifacts.
• Consistent – No conflicts or contradictions with other requirements.
• Measurable – Testable with clear pass/fail criteria.
• Relevant – Directly related to business needs and goals.
• Current – Up-to-date with latest stakeholder feedback.
• Feasible – Can be realistically implemented within constraints.
• Acceptable – Satisfies needs of all stakeholders.
• Prioritized – Ranked based on business value and urgency.

Analyzing requirements involves assessing them against these desired properties and making improvements wherever gaps exist.

It is also important to balance cost versus effectiveness. Balancing cost versus effectiveness for quality of service requirements involves economic analysis. The goal is to find the point where further increasing a non-functional requirement provides diminishing returns on investment.

Each incremental improvement in performance comes with an associated cost. The value delivered also increases with higher performance, but at a decelerating rate. There comes a point where the cost of further enhancement outweighs the added value it would provide.

For example, improving uptime from 99.9% to 99.99% costs much more than going from 99% to 99.9%, while yielding little extra value in availability. Similarly, developers should determine the response time and scalability needed to satisfy users rather than over-engineering for unlikely scenarios.

By modeling cost and value curves for key quality attributes, engineers can optimize requirements to find the ideal economic balance between capable enough software and excessive capabilities that users won’t actually leverage.

It is also important to identify and address conflicts in requirements.

When requirements from different stakeholders’ conflict, resolution usually requires consultation and negotiation. Letting stakeholders air their perspectives and priorities helps discover win-win solutions.

For example, marketing wants broad platform support while engineers seek minimal target platforms. Conflict arises but aligning on core platforms and making others lower priority or future work may address both viewpoints.

In cases of irreconcilable disagreement between groups, weighted priority voting, ranking methods or cost-benefit analysis provides data to make informed trade-off decisions.

When conflicts involve too many requirements for resources, deferred requirements aren’t discarded but go into a backlog to prioritize for future releases. Focused development on tightly scoped versions avoids creating overly ambitious software.

Handling conflicts collaboratively, backed by data, helps navigate tricky trade-offs between stakeholders to craft successful requirements.

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Requirements specification is the process of documenting requirements in a structured way for review, communication, and implementation. Well-specified requirements exhibit qualities like:

• Unambiguous – Open to only one interpretation
• Complete – Nothing missing within project scope
• Verifiable – Ability to test or demonstrate satisfaction
• Consistent – No conflicts between requirements
• Ranked – Clear relative importance and stability

Proper specification transforms requirements into an authoritative reference for design, development, and testing activities. It provides clarity, reduces confusion, and prevents misinterpretation of stakeholders’ needs.

A critical step is determining how the requirements for a project should be determined.

Determining requirements for a software project involves several key steps:

• Identify stakeholders – Determine all groups with an interest in the system.
• Elicit needs – Collect raw requirements from stakeholders via interviews, workshops, surveys, documents, usage scenarios, prototypes, and domain analysis.
• Analyze requirements – Clean up vagueness, resolve conflicts, remove duplication.
• Prioritize features – Rank requirements by value to guide implementation order.
• Allocate to releases – Group requirements into releases based on priority, dependencies, and resources.
• Validate with stakeholders – Confirm requirements fully meet needs and gain sign-off.
• Manage changes – Update specifications as new needs emerge through change control processes.

Different development approaches execute these steps at different points in the project lifecycle, but all are necessary to gather well-analyzed, validated requirements as the foundation for what software gets built and how.

Sometimes you might ask yourself if you should document your requirements. This often seems clear cut, but whether and how to document requirements depends on many contextual factors:

• Domain complexity – More documentation needed in sophisticated domains.
• System criticality – Higher risks require greater formality.
• Staff turnover – Documentation benefits new team members.
• Stakeholder location – Onsite customers need less documentation than offsite.
• Development approach – Formal docs in waterfall, just-in-time specs in Agile.
• Maintenance lifetime – Longer support requires more durable information.
• Outsourcing – External partners need comprehensible specifications.
• Contracts – Some projects mandate srs documentation standards.
• Company culture – Startups often favor working software over documentation.

Even on Agile projects, high-level user stories provide lightweight but useful specifications. Capturing the rationale behind requirements has long-term value even if details evolve further. The right documentation discipline for each project provides enough specification without over-engineering documentation.

Another topic area to consider are the various approaches to acceptance criteria-based requirements. Two common approaches to acceptance criteria-based requirements are:

• Acceptance Test-Driven Development (ATDD)

ATDD involves collaboratively defining test cases upfront that validate customer requirements before coding starts. Tests form the requirements specification. Code then gets written to pass the defined test cases. ATDD ensures developed features demonstrably meet stakeholder needs.

• Behavior-Driven Development (BDD)

BDD uses a format of User Story – “As a [type of user], I want [some goal] so that [benefit].” Detailed scenarios define acceptance criteria. For example: Given [preconditions], when [action occurs], then [expected outcome]. BDD focuses on expected behaviors from a user perspective. The language also promotes shared understanding between technical and non-technical stakeholders.

Acceptance criteria approaches like ATDD and BDD lead to well-defined, unambiguous requirements that closely align with business objectives. Automated testing verifies software meets specifications, preventing defects.

Read more about software requirements specifications in the Software Engineer Book of Knowledge (SWEBOK)

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When developing requirements for a software system, practical considerations such as available resources, capabilities of target platforms, compatibility with existing systems, organizational policies, and regulatory standards are vital. A realistic assessment of these factors will help shape requirements that are feasible to implement within project constraints. Requirements that ignore practical limitations may prove impossible to deliver.

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Requirements for real-world software systems tend to exhibit some key characteristics:

• Breadth – Requirements encompass a wide scope covering many different capabilities.
• Depth – Each requirement may imply deeper levels of additional detail.
• Interdependency – Requirements often relate to or depend on other requirements.
• Variability – Stakeholders have diverse needs leading to varied requirements.
• Dynamism – Requirements frequently evolve over the project lifetime.
• Ambiguity – Stakeholders have difficulty articulating needs clearly.
• Tacitness – Some requirements are unstated and implied.
• Volatility – Requirements are prone to change as knowledge increases.
• Conflict – Different stakeholders’ needs often clash.

These factors make requirements engineering quite challenging. No single pass through requirements activities will be sufficient for most projects. Instead, expect progressive elaboration of an evolving, complex requirements landscape.

Inevitably, any project has real world constraints. How do you choose what requirements to prioritize?

Choosing which requirements to prioritize involves assessing each requirement against criteria such as:

• Business value – How much value does this bring to users or organization?
• Penalty of exclusion – What harm if this isn’t included?
• Dependencies – Does another requirement rely on this one?
• Time sensitivity – Is there a deadline or external event driving this?
• Cost/effort – What resources are needed to implement this?
• Risk – What risks are entailed with this requirement?

Based on these factors, requirements can be ranked as high, medium or low priority. High priority requirements then get addressed first in implementation. Assessing both benefit and cost is key to maximizing return on investment.

One approach to requirements that is commonly used is the Kano Model. The Kano Model recognizes that user satisfaction depends on fulfilling two types of requirements:

• Basic Requirements – Essential functions users expect. Their absence causes dissatisfaction but their presence goes unnoticed. For example, brakes on a car.
• Delighter Requirements – Unexpected features that delight users. Their absence causes no dissatisfaction but their presence increases satisfaction. For example, heated car seats.

Understanding this distinction helps prioritize requirements that eliminate pain points over those that simply raise satisfaction higher. Eliminating dissatisfaction provides greater benefit than achieving high satisfaction. The Kano Model leads to more thoughtful prioritization by considering satisfaction and dissatisfaction separately.

For software products aimed at end users, social media can provide a valuable window into user needs and desires. Monitoring popular requests, complaints, and discussions on social platforms can reveal requirements that may be tacit or difficult for users to articulate directly. Social media analytics can complement traditional requirements gathering by uncovering what users want but may not think to ask for. However, care must be taken to distinguish widespread needs from niche requests. Trending topics on social media may distort perceptions of what capabilities matter most to the broader user base.

Tracing requirements can also be useful to a project. The prices of requirements tracing maintains links between related requirements and other development artifacts like design, code and test cases. Tracing supports essential development activities:

• Change impact analysis – Identify effects of changing a requirement across the system
• Coverage verification – Ensure all features link back to requirements
• Compliance validation – Prove system meets mandated requirements
• Root cause analysis – Discover sources of defects found during testing
• Progress tracking – Monitor implementation status against plans
• Reuse identification – Understand rationale behind existing capabilities

Requirements tracing provides key insights needed to control complexity as software systems evolve. The return on investment is highest for mission-critical systems where safety, security and accountability matter most.

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Software requirements are a crucial foundation of any successful development project. They represent the needs and desires of end users, and guide developers in building the right product. However, requirements management is filled with challenges. Stakeholders often have difficulty articulating their needs clearly. Requirements exhibit interdependency, ambiguity, volatility and conflict. Meanwhile, real projects involve many types of complex, shifting requirements.

To address these difficulties, requirements must be systematically elicited, analyzed, specified, validated and managed across the project lifecycle. Key techniques include stakeholder analysis, interviews, prototyping, and acceptance criteria definition. Prioritization based on business value and development cost focuses efforts on high return requirements. Tracing links requirements to downstream artifacts, enabling impact analysis. Careful specification and documentation provide a touchstone as needs evolve. Following proven processes helps deliver software that meets stakeholder needs within the typical constraints of budget and schedule.

Read more about software requirements specifications in the Software Engineer Book of Knowledge (SWEBOK)

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