Software Requirements Analysis

Software Requirements is a field within software engineering that deals with establishing the needs of stakeholders that are to be solved by software. The IEEE Standard Glossary of Software Engineering Technology defines a software requirement as:

1. A condition or capability needed by a user to solve a problem or achieve an objective.
2. A condition or capability that must be met or possessed by a system or system component to satisfy a contract, standard, specification, or other formally imposed document.
3. A documented representation of a condition or capability as in 1 or 2.

Requirements Analysis is a process of studying and analyzing the customer and the user needs to arrive at a definition of software requirements.

Requirements Specification is a document that clearly and precisely describes each of the essential requirements (functions, performance, design constraint, and quality attributes) of the software and the external interfaces.

Each requirement being defined in such a way that, its achievement is capable of being objectively verified by a prescribed method. For example inspection, demonstration, analysis, or test.

Requirements Engineering
Requirements engineering is the process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed

Firstly we should know what exactly requirement is:

• It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification
• This is inevitable as requirements may serve a dual function
  • May be the basis for a bid for a contract - therefore must be open to interpretation
  • May be the basis for the contract itself - therefore must be defined in detail
  • Both these statements may be called requirements

Requirements Documents
“If a company wishes to let a contract for a large software development project it must define its needs in a sufficiently abstract way that a solution is not predefined. The requirements must be written so that several contractors can bid for the contract, offering, perhaps, different ways of meeting the client organization’s needs. Once a contract has been awarded, the contractor must write a system definition for the client in more detail so that the client understands and can validate what the software will do. Both of these documents may be called the requirements document for the system.”- [A.M. Davis “Software Requirements: Objects, Functions and States”, Englewood Cliffs, 1993]

Types of requirement

• User requirements: Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers
• System requirements: A structured document setting out detailed descriptions of the system services. Written as a contract between client and contractor
• Software specification: A detailed software description which can serve as a basis for a design or implementation. Written for developers

Activities of Software requirement:
The activities of software requirements can broadly be broken up into:

• Elicitation
• Analysis
• Specification and
• Management

Functional, non-functional and domain requirements

• Functional requirements: Statements of services the system should provide how the system should react to particular inputs and how the system should behave in particular situations.
• Non-functional requirements: constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc.
• Domain requirements: Requirements that come from the application domain of the system and that reflect characteristics of that domain

Functional Requirements
Describe functionality or system services

• Depend on the type of software, expected users and the type of system where the software is used
• Functional user requirements may be high-level statements of what the system should do BUT functional system requirements should describe the system services in detail

Examples of Functional requirements:

• The user shall be able to search either all of the initial set of databases or select a subset from it.
• The system shall provide appropriate viewers for the user to read documents in the document store.
• Every order shall be allocated a unique identifier (ORDER_ID) which the user shall be able to copy to the account’s permanent storage area.

Requirements imprecision

• Problems arise when requirements are not precisely stated
• Ambiguous requirements may be interpreted in different ways by developers and users
• Consider the term ‘appropriate viewers’
  • User intention - Special purpose viewer for each different document type
  • Developer interpretation- Provide a text viewer that shows the contents of the document

Requirements completeness and consistency

• In principle requirements should be both complete and consistent

Complete: They should include descriptions of all facilities required
Consistent: There should be no conflicts or contradictions in the descriptions of the system facilities

• In practice, it is very difficult or impossible to produce a complete and consistent requirements document

Non-functional requirements
Define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc.

• Process requirements may also be specified mandating a particular CASE system, programming language or development method
• Non-functional requirements may be more critical than functional requirements. If these are not met, the system is useless

Product requirements: Requirements which specify that the delivered product must behave in a particular way, e.g. execution speed, reliability etc.

Organizational requirements: Requirements which are a consequence of organizational policies and procedures, e.g. process standards used implementation requirements etc.

External requirements: Requirements which arise from factors which are external to the system and its development process, e.g. interoperability requirements, legislative requirements etc.

Goals and requirements

• Non-functional requirements may be very difficult to state precisely and imprecise requirements may be difficult to verify.
• Goal: A general intention of the user such as ease of use
• Verifiable non-functional requirement: A statement using some measure that can be objectively tested
• Goals are helpful to developers as they convey the intentions of the system users

Examples

• A system goal: The system should be easy to use by experienced controllers and should be organised in such a way that user errors are minimised.
• A verifiable non-functional requirement: Experienced controllers shall be able to use all the system functions after a total of two hours training. After this training, the average number of errors made by experienced users shall not exceed two per day.

Requirements measures
Speed: Processed transactions/s, User/Event response time/ Screen refresh time
Size: Kbytes Number of RAM chips
Ease of Use: Training time & Number of help frames
Reliability: Mean time to failure, Probability of unavailability, Rate of failure occurrence & Availability
Robustness: % events causing failure, Time to restart after failure Probability of data corruption on failure
Portability: % target system dependent statements & Number of target systems

Requirements interaction

• Conflicts between different non-functional requirements are common in complex systems
• Spacecraft system
  • To minimise weight, the number of separate chips in the system should be minimised
  • To minimise power consumption, lower power chips should be used
  • However, using low power chips may mean that more chips have to be used.

Which is the most critical requirement?
Domain requirements

• Derived from the application domain and describe system characteristics and features that reflect the domain
• May be new functional requirements, constraints on existing requirements or define specific computations
• If domain requirements are not satisfied, the system may be unworkable

Domain requirements problems

• Understandability
  • Requirements are expressed in the language of the application domain
  • This is often not understood by software engineers developing the system
• Implicitness: Domain specialists understand the area so well that they do not think of making the domain requirements explicit

User requirements

• Should describe functional and non-functional requirements so that they are understandable by system users who don’t have detailed technical knowledge
• User requirements are defined using natural language, tables and diagrams

Problems with natural language

• Lack of clarity: Precision is difficult without making the document difficult to read
• Requirements confusion: Functional and non-functional requirements tend to be mixed-up
• Requirements amalgamation: Several different requirements may be expressed together

Guidelines for writing requirements

• Invent a standard format and use it for all requirements
• Use language in a consistent way. Use:
  shall for mandatory requirements,
  should for desirable requirements
  
• Use text highlighting to identify key parts of the requirement

Alternatives to NL specification
Structured natural language: Depends on defining standard forms or templates to express requirements specification
Design description languages: Similar to programming languages but with additional, more abstract features
Graphical notations: A graphical language, supplemented by text annotations, is used to define functional requirements (e.g. use-case diagrams)
Mathematical/formal specifications: Based on mathematical concepts such as finite-state machines or sets; unambiguous specifications reduce arguments between customers and contractors but most customers don’t understand formal specifications

Published date : 17 Mar 2015 01:09PM