SE04-for+printSoftware Engineering SEI 软件工程概论Software Engineering 1g gSoftware Engineering SEICH4. Requirements2Software Engineering SEI Content The Requirements Process Requirements Elicitation Type of RequirementsCharacteristics of Requirements Modelling Notations 3Modelling NotationsRequirements and Specification Languages Prototyping Requirements Requirements Documentation Requirements Validation Information System ExampleSoftware Engineering SEI软件需求曾经让我们如此狼狈4Software Engineering SEI 4.1 The Requirements ProcessWhen a customer requests that we build a new system, the customer has some notion (概念/想法) of what the system will do.Often, the new system replaces (替换) an existing system or way of doing things The new system is an 5system or way of doing things. The new system is an enhancement or extension of a current (manual or automated ) system.No matter whether its functionality is old or new, each software-based system has a purpose , usually expressed in what the system can do.Software Engineering SEI4.1 The Requirements ProcessA requirement is a feature(特性/特征) of the system or a description of something the system is capable of doing in order to fulfill(完成/实现) the systems purpose.6Software Engineering SEI 4.1 The Requirements ProcessA requirement is an expression of desired behaviour A requirement deals withobjects or entitiesthe state they can be infunctions that are performed to change states or object 7p g j characteristicsRequirements focus on the customer needs, not on the solution or implementationdesignate what behaviour, without saying how that behaviour will be realizedSoftware Engineering SEI4.1 The Requirements ProcessWhy are Requirements important? Top factors that caused project to failIncomplete requirements (13.1%)Lack of user involvement(12.4%)Lack of resources(10.6%)Unrealistic expectations(9.9%)8Lack of executive support(9.3%)Changing requirements and specifications(8.7%)Lack of planning(8.1%)System no longer needed(7.5%)Some part of the requirements process is involved in almost all of these causesRequirements error can be expensive if not detected earlySoftware EngineeringSEI 4.1 The Requirements ProcessProcess for determining RequirementsProblem analysisProblem descriptionPrototyping and testingDocumentationand lid tiREQUIREMENTS ELICITATIONAND ANALYSISREQUIREMENTS DEFINITIONAND SPECIFICATION9validation Have we captured all the user need ?Are we using the righttechniques or views?Is this function feasible?Have we captured what the user expects?The process of determining requirementsSoftware EngineeringSEI4.1 The Requirements ProcessProcess for capturing RequirementsPerformed by the req. analyst or system analyst The final outcome is a Software Requirements Specification (SRS) document10Software Engineering SEI 4.1 The Requirements ProcessTwo Kinds of Requirements DocumentsRequirements definition: complete listing of what the customer expects the system to do.Requirements specification (规格说明书): restates the definition in technical terms so that the designer can 11g start on the design.Configuration management: supports direct correspondence between the two documents.Software Engineering SEI4.1 The Requirements Process Purposeand goalsRequire-ments elicitation and analysisRequire-ments definition Design Imple-mentationVerification12Understanding of the application domain and the problem ProblemSpecifi-cationsDesigncomponents and use casesCodecomponents and unittestsT est dataand scriptsProblem under-standingSoftware Engineering SEI 4.1 The Requirements Process Configuration management Set of procedures that trackrequirements that define what the system should do design modules that are generated from requirements program code that implements the design 13p g p gtests that verify the functionality of the system documents that describe the systemSoftware Engineering SEI4.2 Requirements ElicitationCustomers do not always understand what their needs and problems areIt is important to discuss the requirements with everyone who has a stake in the system Come up with agreement on what the 14p g requirements areIf we can not agree on what the requirements are, then the project is doomed to failSoftware Engineering SEI 4.2 Requirements Elicitation StakeholderCustomers : buy the software after it is developed Users : use the systemDomain experts : familiar with the problem that the software must automate15Market Researchers : conduct surveys to determine future trends and potential customersLawyers or auditors : familiar with government, safety, or legal requirementsSoftware engineers or other technology expertsSoftware Engineering SEI4.2 Requirements ElicitationHelpful to all parties in understanding what is really needed.