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CS 501: Software Engineering

CS 501: Software Engineering. Lecture 8 Requirements II. Administration. Quiz 1 If you did not pick it up on Tuesday, you can collect your quiz from reception at 301 College Avenue Assignment 1: Feasibility study Due Friday at 5:00 p.m. Remember to send a copy to your client

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CS 501: Software Engineering

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  1. CS 501: Software Engineering Lecture 8 Requirements II

  2. Administration Quiz 1 If you did not pick it up on Tuesday, you can collect your quiz from reception at 301 College Avenue Assignment 1: Feasibility study Due Friday at 5:00 p.m. Remember to send a copy to your client The survey has not yet been posted.

  3. Administration GForge A new page has been added to the Web site with information about GForge.

  4. Requirements Analysis v. System Design Dilemma. • Requirements analysis should make minimal assumptions about the system design. • But the requirements definition must be consistent with computing technology and the resources available. In practice, analysis and design are interwoven. However: 1. Do not to allow the requirements analysis to prejudge the system design. 2. Do not allow assumptions about the design to influence the requirements analysis.

  5. Models: Useful Texts Grady Booch, James Rumbaugh, Ivar Jacobson, The Unified Modeling Language. Addison-Wesley 1999. Grady Booch, Object-Oriented Analysis and Design with Applications, second edition. Benjamin/Cummings 1994. Rob Pooley, Perdita Stevens, Using UML Software Engineering with Objects and Components. Addison-Wesley 1999.

  6. Models A model is a simplification of reality. • We build models so that we can better understand the system we are developing. • We build models of complex system because we cannot comprehend such a system in its entirety. Models can be informal or formal. The more complex the project the more valuable a formal model becomes. BRJ

  7. Principles of Modeling • The choice of what models to create has a profound influence on how a problem is attacked and how a solution is shaped. • Every model can be expressed at different levels of precision. • The best models are connected to reality. • No single model is sufficient. Every nontrivial system is best approached through a small set of nearly independent models. BRJ

  8. The Unified Modeling Language UML is a standard language for modeling software systems • Serves as a bridge between the requirements specification and the implementation. • Provides a means to specify and document the design of a software system. • Is process and programming language independent. • Is particularly suited to object-oriented program development.

  9. Rational Rose Rational Rose is an IBM-owned system for creating and managing UML diagrams. It is available for Computer Science Department computers.

  10. Diagrams and Specification in UML A diagram is the graphical representation of a set of elements, usually rendered as a connected graph of vertices (things) and arcs (relationships). Each diagram is supported by technical documentation that specifies in more detail the model represented by the diagram. A diagram without documentation is of little value.

  11. Actor and Use Case Diagram • An actor is a user of a system in a particular role. An actor can be human or an external system. • A use case is a a task that an actor needs to perform with the help of the system. BookBorrower Borrow book Use cases make more precise the concept of viewpoint analysis.

  12. Use Cases and Actors • A scenariois an instance of ause case • Actor is role, not an individual (e.g., librarian can have many roles) • Actor must be a "beneficiary" of the use case (e.g., not librarian who processes book when borrowed) In UML, the system boundary is the set of use cases.

  13. Scenario A scenario is a tool used during requirements analysis to walk through a specific interaction with a proposed system. Example The requirements are being developed for a system that will enable university students to take quizzes online from their own rooms using a Web browser. Create a scenario for a typical student.

  14. Scenario: a Typical Student Individual: Philip Glass, senior at Cornell, major in computer science, location Risley Hall. Equipment: Dell laptop attached to Cornell dormitory network. Firefox 1.5 browser and Sidecar authentication system installed. Scenario: 1. PG powers up computer and authenticates using Sidecar. 2. PG starts browser and types URL of Quiz system. 3. Quiz system displays list of options.

  15. Scenario (continued) 4. PG selects CS 501 Quiz 1. 5. A list of questions is displayed, each marked to indicate whether completed or not. 6. PG selects a question and specifies whether he will submit a new answer or edit a previous answer. 7. For the first question, he is submitting a new answer. He has a choice whether to type the solution into the browser or to attach a separate file. He decides to attach a file. 8. For the second question, he is editing a previous answer. He chooses to delete a solution previously typed into the browser, and to replace it with an attached file.

  16. Scenario (continued) 9. PG has now completed the quiz. He selects an option that submits the quiz to the grading system. 10. PG now wishes to change a solution. The system does not permit changes once the solution has been submitted. 11. PG logs off.

  17. Modeling Scenarios as User Cases A scenario is useful in discussing a proposed system with a client, but needs to be generalized as part of the requirements modeling. A use case provides such a model.

  18. Use Cases for Quiz System TakeQuiz QuizTaker CheckGrades RequestRegrade

  19. Use Cases for Quiz System SetQuiz Instructor Grade Note that actor is a role. An individual can be a QuizTaker on one occasion and an Instructor at a different time. Regrade

  20. Relationships Between Use Cases: <<includes>> <<includes>> TakeQuiz Authenticate QuizTaker CheckGrades <<includes>>

  21. Relationships Between Use Cases: <<extends>> <<extends>> ConnectionFails TakeQuiz QuizTaker <<include>> is used for events that are in the flow of events of the source use case. <<extends>> is used for exceptional conditions, especially those that can occur at any time.

  22. Specification of Use Case Name: TakeQuiz Actor(s): QuizTaker Flow of events: 1. QuizTaker connects to the Quiz server. 2. Quiz server checks whether student is already authenticated and transfer to Sidecar for authentication if necessary. 3. QuizTaker selects a quiz from a list of options. 4. QuizTaker repeatedly selects a question and either types in a solution, attaches a file with a solution, edits a solution or attaches a replacement file.

  23. Specification of Use Case (continued) Flow of events (continued): 5. QuizTaker either submits completed quiz or saves current state. 6. If a completed quiz is submitted, Quiz server checks that all questions have been attempted and either sends acknowledgement to QuizTaker, or saves current state and notifies QuizTaker of incomplete submission. 7. QuizTaker logs out. Entry conditions: 1. QuizTaker must have Cornell NetID. 2. Computing requirements: CIT supported browser and Sidecar

  24. Use Cases in the Development Cycle • Use cases are a tool in requirements analysis • Intuitive -- easy to discuss with clients • Use cases are often hard to translate into class models • Scenarios are useful to validate use cases and the design of a system.

  25. Documentation: General Reasons for documentation: visibility (e.g., project plan, interim report) user support (e.g., user manual) team communication (e.g., interface specifications) maintenance and evolution (e.g., requirements) Characteristics of documentation: accurate and kept current appropriate for audience maintained online (usually) simple but professional in style and appearance Documentation is expensive --> Quality not volume

  26. Documentation of Requirements Specification: Purpose 1. Document that describes the requirements to the stakeholders in a precise manner • Expressed in the terms that the stakeholders understand • As precise and specific as possible • Comprehensible from many viewpoints • Reviewed by stakeholders so that they understand implications • Must be clear about assumptions (things left out)

  27. Requirements Specification: Purpose 2. It describes the requirements to the implementers • As precise and specific as possible • Expressed in terms that they understand • Comprehensible to new team members

  28. Requirements Specification: Purpose 3. It records the requirements for the future • An essential part of system evolution 4. It may be a contractual document

  29. Requirements Specification: Details Requirements must be specific Examples -- university admissions system Requests for information received by email must be answered within one business day. An admissions officer who is talking to an applicant by telephone must be able to retrieve the applicant's records within 10 seconds. No financial aid offer may exceed the maximum defined in Section 8.7.

  30. Requirements Specification: Process The client must understand the requirements specification. • Do not assume that anybody has read a document. • Do not assume that anybody understands a document. Go through the requirements specification with the client, line by line. It is usual for the client and developer to sign the requirements document when it is agreed. [Compare with the plans to build a house. This is the specification of the system that you are about to build.] *

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