Software Engineering - Lecture 3 - Requirements Engineering PDF

Software Engineering Lecture 3: Requirements Engineering Software Engineering Dr. Sayed AbdelGaber Professor Faculty of Computers and Information...

Software Engineering Lecture 3: Requirements Engineering Software Engineering Dr. Sayed AbdelGaber Professor Faculty of Computers and Information Helwan University Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering The process of establishing the services that the customer requires from a system. The constraints under which the system operates and is developed. Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering  “… to cover all of the activities involved in discovering, documenting, and maintaining a set of requirements for a computer-based system.” [Source: Kotonya and Sommerville, 2001]  Importance of requirements illustrated by need for rigor, structure, repeatable techniques, etc.  Key concepts: ◦ Discovery (capture, elicitation, identification) Requirements ◦ Documenting (specifying, modeling) analysis ◦ Maintenance (changes in environment of system) Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering  They are descriptions of how the system should ✓ behave, ✓ application domain information, ✓ constraints on the system’s operation, or ✓ specifications of a system property or attribute. [Source: Kotonya and Sommerville, 2001]  A requirement is a statement of need, something that some class of user or other stakeholder wants” [Source: Alexander and Stevens, 2002] Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering Requirements definition is a careful assessment of the needs that a system is to fulfill. It Says Why a system is needed? What system features will serve and satisfy? How the system is to be constructed? Requirements represent a specification for the new system. Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering Identifies stakeholder needs to ensure that you are building the right system Help you manage the development process to ensure a quality system Identify defects early, reducing costs Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering Consequences of poor or incorrect requirements are: System delivered later than planned. System cost more than planned. Customers and end-users unsatisfied (not used or even scrapped) Maintenance related costs higher. System is unreliable or does not work. important! Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering  Not well understood (domain complexity).  Requirements do not reflect the real needs of the customer.  Misunderstanding between various stakeholders: customers, requirements engineers and developers.  Expensive to make changes to requirements after they have been agreed upon (contract).  Requirements evolution: existing system enhancement not clear.  Requirements could be incomplete, ambiguous, inconsistent, overlapping, not implementable,... Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering problem correct wrong requirements requirements correct wrong design based on design design wrong requirements correct wrong code based on code based on code code wrong designs wrong requirements Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering  "Once your software hits the field, removing a requirements defect costs at least a hundred times as much, assuming you can fix it at all.“ [Source: Lawrence, Wiegers, and Ebert, IEEE Software, 2001]  Poor or incorrect requirements affects later stages of SDLC through exponentially increasing costs: ◦ investment far greater during design and coding.  Investing time in effective requirements analysis early saves time, effort, and money. Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering Usually classified into two categories: Functional (Behavioral) requirements Specify The function that the system should provide. How the system should react to particular inputs. How the system should behave in particular situations. (affordance, capability) of the system Non Functional (Non-Behavioral) requirements Describe Constraints on the services or functions offered by the system such as timing constraints. Constraints on the development process, standards, etc. (performance, reliability, usability, portability,...) Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering General requirements sets out in broad terms what the system should do For Example: The system shall maintain records of all library materials including books, serials, newspapers and magazines,...). Functional requirements define system’s functionality For Example: The system shall allow users to search for an item by title, author, or ISBN Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering Data requirements define the type of data the system shall operate upon or produce For Example: The ISBN is a 5-part item: the “ISBN tag” and a 4-part identifier. Implementation requirements states how the system must be implemented For Example: The system’s user interface shall be implemented using a WWW browser. Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering Performance requirements specify the minimum acceptable performance of the system For Example: The system shall support at least 20 transactions per second Usability requirements state user interface and system availability constraints For Example: Should use a hierarchical menu structure for navigation Operational requirements specify constraints that should be satisfied during system usage For Example: Reliability in terms of “mean-time to failure”. Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering  Understandable by users effectively contract and should be comprehendible.  Nonprescriptive describes what the system should do not how  Correct user judge of correctness  Complete nothing missing from set or individual requirement  Consistent does not contradict other requirements  Unambiguous only one interpretation  Precise, concise, feasible, testable, traceable,... Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering  The services and functions which the system should provide.  The constraints under which the system must operate.  Overall (emergent) properties of the system (reliability, maintainability, performance, usability, security, etc.).  System’s environment including related systems.  Application domain information.  Constraints on the development processes. Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering Definition of the function or entity. Description of inputs and where they come from. Description of outputs and where they go to. Indication of other entities required. Pre and post conditions (if appropriate). The side effects (if any) of the function. Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering 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. Avoid the use of computer jargon. Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering 1. Introduction 1.1Purpose of document 1.2 Scope of product 1.3 Definitions, acronyms, abbreviations 1.4 References 1.5 Overview of remainder of document 2. General description 2.1 Product perspective 2.2 Product functions 2.3 User characteristics 2.4 General constraints 2.5 Assumptions and dependencies Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering 3. Specific requirements Includes functional, non-functional, interface requirements (external interfaces, functionality, performance, logical database requirements,...) 4. Appendices 5. Index Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering  Requirements related problems contribute significantly to project failure.  Requirements engineering incorporates activities such as discovery, analysis, documenting, and management.  Many variations of requirement types.  Different kinds of stakeholders have say in requirements.  Requirements can be specified as documents, models, prototyping and formal methods. Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering 1. Why is it essential to capture good quality requirements? 2. Describe ways in which requirements can be substandard? How do we improve the chances of capturing good quality requirements? 3. Identify the stakeholders that can claim an interest in requirements capture and for each, explain why. 4. What characteristics are useful for analysts to possess during requirements determination? Which is the most important and why? Dr. Sayed AbdelGaber Software Engineering Lecture 3: Requirements Engineering Dr. Sayed AbdelGaber