Chapter 4 – Requirements Engineering -L4

 Topics covered

  •   Functional and non-functional requirements
  •   The software requirements document 
  • Requirements specification 
  • Requirements engineering processes 
  • Requirements elicitation and analysis 
  •  Requirements validation 
  • Requirements management

Establishing what the customer requires from a software system 
what is it?

  • Requirements describe “What is to be ..” 
  • Produces one large document written in natural language contains a description of what the system will do without describing how it will do it. 
  • Without well written document 
  1. Developers do not know what to build
  2.  Customers do not know what to expect 
  3. What to validate
The process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed

 Requirements may be functional or non-functional
  •   Functional requirements describe system services or functions 
  • Non-functional requirements is a constraint on the system or on the development process
Crucial Steps of RE


What is a requirement?
  •  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
  1.   May be the basis for a bid for a contract - therefore must be open to interpretation; 
  2.  May be the basis for the contract itself - therefore must be defined in detail;
  3.   Both these statements may be called requirements.

Requirements abstraction (Davis)

“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 pre-defined. 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."

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’s functions, services and operational constraints. Defines what should be implemented so may be part of a contract between client and contractor. 

User and system requirements


Readers of different types of requirements specification


Types of Requirements

  • Known Requirements
  • Unknown Requirements
  • Undreamed  Requirements

Functional and non-functional 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.
  •   May state what the system should not do. 

Non-functional requirements 

  •  Constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc
  • .  Often apply to the system as a whole rather than individual features or services. 

 Domain requirements
  •   Constraints on the system from the domain of operation


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. 
  •  Functional system requirements should describe the system services in detail.

Functional requirements for the Mental Healthcare - Patient Management System.

  • A user shall be able to search the appointments lists for all clinics.
  •  The system shall generate each day, for each clinic, a list of patients who are expected to attend appointments that day. 
  • Each staff member using the system shall be uniquely identified by his or her 8-digit employee number.
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 ‘search’ in a requirement
  1.  User intention – search for a patient name across all appointments in all clinics; 
  2.  Developer interpretation – search for a patient name in an individual clinic. User chooses clinic then search.
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 impossible to produce a complete and consistent requirements document.

Non-functional requirements

  • These 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 IDE, programming language or development method.
  •  Non-functional requirements may be more critical than functional requirements. If these are not met, the system may be useless.





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