Scenarios and Use Cases in Software Engineering, Lecture notes of Software Engineering

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 Computing and Information Science CS 5150 Software Engineering 7. Scenarios and Use Cases William Y. Arms Scenarios Scenario A scenario is a scene that illustrates some interaction with a proposed system. A scenario is a tool used during requirements analysis to describe a specific use of a proposed system. Scenarios capture the system, as viewed from the outside, e.g., by a user, using specific examples. Note on terminology Some authors restrict the word "scenario" to refer to a user's total interaction with the system. Other authors use the word "scenario" to refer to parts of the interaction. In this course, the term is used with both meanings. Developing a Scenario with a Client:
 a Typical Student Purpose: Scenario that describes the use of an online Exam system by a representative student Individual: [Who is a typical student?] Student A, senior at Cornell, major in computer science. [Where can the student be located? Do other universities differ?] Equipment: Any computer with a supported browser. [Is there a list of supported browsers? Are there any network restrictions?] Scenario: 1. Student A authenticates. [How does a Cornell student authenticate?] 2. Student A starts browser and types URL of Exam system. [How does the student know the URL?] 3. Exam system displays list of options. [Is the list tailored to the individual user?] Developing a Scenario with a Client (continued) 4. Student A selects CS 1234 Exam 1. 5. A list of questions is displayed, each marked to indicate whether completed or not. [Can the questions be answered in any order?] 6. Student A selects a question and chooses whether to submit a new answer or edit a previous answer. [Is it always possible to edit a previous answer? Are there other options?] 7. [What types of question are there: text, multiple choice, etc.?] The first question requires a written answer. Student A is submitting a new answer. The student has a choice whether to type the solution into the browser or to attach a separate file. Student A decides to attach a file. [What types of file are accepted?] Developing a Scenario with a Client (continued) 8. For the second question, the student chooses to edit a previous answer. Student A chooses to delete a solution previously typed into the browser, and to replace it with an attached file. [Can the student edit a previous answer, or must it always be replaced with a new answer?] 9. As an alternative to completing the entire exam in a single session, Student A decides to saves the completed questions to continue later. [Is this always permitted?] 10..Student A logs off. 11. Later Student A log in, finishes the exam, submits the answers, and logs out. [Is this process any different from the initial work on this exam?] 12. The Student A has now completed the exam. The student selects an option that submits the exam to the grading system. [What if the student has not attempted every question? Is the grader notified?] Scenarios for Analyzing Special Requirements Scenarios are very useful for analyzing special requirements. Examples • Reversals. In a financial system, a transaction is credited to the wrong account. What sequence of steps are used to reverse the transaction? • Errors. A mail order company has several copies of its inventory database. What happens if they become inconsistent? • Malfeasance. In a voting system, a voter has houses in two cities. What happens if he attempts to vote in both of them? Scenarios for error recovery Murphy's Law: "If anything can go wrong, it will". Create a scenario for everything that can go wrong and how the system is expected to handle it. Modeling Scenarios as Use Cases Models Scenarios are useful in discussing a proposed system with a client, but requirements need to be made more precise before a system is fully understood. This is the purpose of requirements modeling. A use case provides such a model. There is a good discussion of use cases in Wikipedia. The approach used in this course is less complex than the Wikipedia article. Two Simple Use Cases 12 Borrow Book BookBorrower Record Pressure PressureSensor Use Cases for Exam System Take Exam ExamTaker Check Grades Request RegradeThree separate use cases Describing a Use Case Some organizations have complex documentation standards for describing a use case. At the very least, the description should include: • The name of the use case, which should summarize its purpose • The actor or actors • The flow of events • Assumptions about entry conditions Outline of Take Exam Use Case Name of Use Case: Take Exam Actor(s): ExamTaker Flow of events: 1. ExamTaker connects to the Exam server. 2. Exam server checks whether ExamTaker is already authenticated and runs authentication process if necessary. 3. ExamTaker selects a exam from a list of options. 4. ExamTaker repeatedly selects a question and either types in a solution, attaches a file with a solution, edits a solution or attaches a replacement file. Relationships Between Use Cases: <<includes>> ExamTaker Authenticate Take Exam <<includes>> <<includes>> Check Grades The Authenticate use case may be used in other contexts Relationships Between Use Cases: <<extends>> Take ExamExamTaker Connection Fails<<extends>> <<includes>> is used for use cases that are in the flow of events of the main use case. <<extends>> is used for exceptional conditions, especially those that can occur at any time. Scenarios and Use Cases in the Development Cycle Scenarios and use cases are both intuitive -- easy to discuss with clients Scenarios are a tool for requirements analysis. • They are useful to validate use cases and in checking the design of a system. • They can be used as test cases for acceptance testing. Use cases are a tool for modeling requirements. • A set of use cases can provide a framework for the requirements specification. • Use cases are the basis for system and program design, but are often hard to translate into class models. An Old Examination Question The Pizza Ordering System The Pizza Ordering System allows the user of a web browser to order pizza for home delivery. To place an order, a shopper searches to find items to purchase, adds items one at a time to a shopping cart, and possibly searches again for more items. When all items have been chosen, the shopper provides a delivery address. If not paying with cash, the shopper also provides credit card information. The system has an option for shoppers to register with the pizza shop. They can then save their name and address information, so that they do not have to enter this information every time that they place an order. Develop a use case diagram, for a use case for placing an order, PlaceOrder. The use case should show a relationship to two previously specified use cases, IdentifyCustomer, which allows a user to register and log in, and PaybyCredit, which models credit card payments. An Old Examination Question Shopper PlaceOrder <<includes>> IdentifyCustomer PaybyCredit <<includes>> Correct solution An Old Examination Question Wrong Solution SearchMenu PaybyCredit <<includes>> IdentifyCustomer <<includes>> AddtoCart Pay Shopper