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This lecture is part of lecture series for Software Engineering course. Prof. Prateek Aron delivered this lecture at Allahabad University. Its main points are: Software, Requirements, Development, Specifying, Designing, Implementing, Testing, Services, Constraints
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Descriptions and specifications of a system
To introduce the concepts of user and system requirements To describe functional / non-functional requirements To explain two techniques for describing system requirements To explain how software requirements may be organised in a requirements document
The descriptions of the system services and constraints that are generated during the requirements engineering process
To ensure a software solution correctly solves a particular problem, we must initially fully understand the problem that needs to be solved, discover why the problem needs to be solved and determine who should be involved. Poorly defined requirements can cause major problems to a project in both financial terms as well as added time. There are specific techniques we may use in the requirements engineering phase which we shall be considering during the next four lectures.
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
Client managers System end-users Client engineers Contractor managers System architects System end-users Client engineers System architects Software developers Client engineers (perhaps) System architects Software developers User requirements System requirements Software design specification
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
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.
Complete They should include descriptions of all facilities required Consistent There should be no conflicts or contradictions in the descriptions of the system facilities
Define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc. They are often emergent properties of the system. 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
Performance requirements Space requir ements Usability requirements Ef ficiency requir ements Reliability requir ements Portability requirements Interoperability requirements Ethical requirements Legislative requirements Implementation requir ements Standards requirements Delivery requirements Safety requirements Privacy requirements Product requir ements Or ganizational requir ements External requirements Non-functional requir ements
Product requirement 4.C.8 It shall be possible for all necessary communication between the APSE and the user to be expressed in the standard Ada character set Organisational requirement 9.3.2 The system development process and deliverable documents shall conform to the process and deliverables defined in XYZCo-SP-STAN- 95 External requirement 7.6.5 The system shall not disclose any personal information about customers apart from their name and reference number to the operators of the system Performance requirement The system should respond to a user’s request for information in less than 3 seconds during “peak-time” and 3 seconds during “normal time”.
Property Measure Speed Processed transactions/second User/Event response time Screen refresh time Size K Bytes 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 Time to restart after failure Percentage of events causing failure Probability of data corruption on failure Portability Percentage of target dependent statements Number of target systems
