Use Case Modeling Example
Contents
1. CS 451 Software Engineering Yuanfang Cai Room 104 University Crossings 215 895 0298 yfcai cs drexel edu Drexel University Use Case rH A sequence of transactions performed by a system that yields a measurable result of values for a particular actor m What are the tasks of each actor Will any actor create store change remove or read information in the system m What use cases will create store change remove or read this information Will any actor need to inform the system about sudden external changes m Does any actor need to be informed about certain occurrences in the system m What use cases will support and maintain the system m Can all functional requirements be preformed by the use cases o A use case typically represents a major piece of functionality that is complete from beginning to end A use case must deliver something of value to an actor Drexel University Another Example ATM System Cy oe l Operator Mt P l f 3 Session Invalid NM PIN include N ae tend withdrawal Y Deposit Y Transfer y f Inquiry Drexel University Observed problems e Mixing up a use case with its detailed subflows Trying to show sequential relations among use cases e Trying to show conditions in use case diagrams Not using lt lt include gt gt or lt lt extend gt gt stereotypes correctly e Not using generalizatio2. ccident If an accident is blocking the intersection the car will cause a traffic problem by rubbernecking as it slowly drives around it 2 Flow of Events for the Check Status Use Case 2 1 Preconditions None 2 2 Main Flow Traffic controller gets the status of the queues S 1 and reports the status S 2 2 3 Subflows 1 Obtain Status Traffic controller sends a message asking for the how many cars are in each queue via the Heport Status subflow of the Car Queue use case 2 Report Status Traffic controller reports the number of cars in each queue 2 4 Alternative Flows 3 Flow of Events for the Change Light Use Case 3 1 Preconditions None 3 2 Main Flow Traffic controller initializes the traffic lights 1 Traffic controller changes the lights change 2 3 3 Subflows 1 Initiate Lights Traffic controller initializes all lights to red except NL and SL NL and SL are green 2 Change Light Traffic controller changes the lights in the following order and send a message to the Update Status subflow of the Light Status use case When the system is initiated the traffic signal allows traffic to flow from NL and SL Next it allows traffic to flow from N and S Then it allows traffic flow from EL and WL Lastly it allows traffic to flow from E and W then starts again with NL and SL and so forth 3 4 Alternative Flows 4 Flow of Events for the Light Status Use Case 4 1 Preconditions None 4 2 Main Flow The system upda
3. e light is green via the Report Status subflow of the Light Status use case 3 Check Queue The car checks whether it is at the front of the queue via the Report Status subflow of the Car Queue use case 4 Go The car clears the intersection 5 Join a Queue The car joins the queue via the Acc fo Queue subflow of the Car Queue use case 1 4 Alternative Flows Scenario Car approaches intersection with green light and no queue 1 1 Preconditions Traffic light has been initialized 1 2 Main Flow This use case begins when a car enters the intersection 1 The car checks status 2 3 If the light is green and the queue is empty the car clears the intersection 4 Otherwise it joins a queue 5 1 3 Subflows 1 Enter Intersection The car enters the intersection 2 Check Light The car checks whether the light is green via the Report Status subflow of the Light Status use case 3 Check Queue The car checks whether it is at the front of the queue via the Report Status subflow of the Car Queue use case 4 Go The car clears the intersection 5 Join a Queue The car joins the queue via the Add to Queue subflow of the Car Queue use case 1 4 Alternative Flows Car approaches intersection with red light or the queue is not empty 1 1 Preconditions Traffic light has been initialized 1 2 Main Flow This use case begins when a car enters the intersection 1 The car checks status 2 3 If the light is green and the queue is e
4. ection Drexel University Intersection Problem Statement ri When a car enters the intersection if the queue there is empty and the light is green they can clear the intersection Else they join the appropriate queue ri When the system is initiated the traffic signal allows traffic to flow from NL and SL Next it allows traffic to flow from N and S Then it allows traffic flow from EL and WL Lastly it allows traffic to flow from E and W then starts again with NL and SL and so forth ri When a signal light changes it can allow up to five cars to pass through the intersection get out of the queue Drexel University Clear Intersection Example o User wants to drive through an intersection o The user can only clear through the intersection if the traffic light is green and there are no cars in the intersection Otherwise the car needs to join a queue Drexel University Clear Intersection Use Case Diagram Traffic Controller Change Light Drexel University 1 Flow of Events for the Clear Intersection Use Case 1 1 Preconditions Traffic light has been initialized 1 2 Main Flow This use case begins when a car enters the intersection 1 The car checks status 2 3 If the light is green and the queue is empty the car clears the intersection 4 Otherwise it joins a queue 5 1 3 Subflows 1 Enter Intersection The car enters the intersection 2 Check Light The car checks whether th
5. mpty the car clears the intersection 4 Otherwise it joins a queue 5 1 3 Subflows 1 Enter Intersection The car enters the intersection 2 Check Light The car checks whether the light is green via the Report Status subflow of the Light Status use case 3 Check Queue The car checks whether it is at the front of the queue via the Report Status subflow of the Car Queue use case 4 Go The car clears the intersection 5 Join a Queue The car joins the queue via the Add to Queue subflow of the Car Queue use case 1 4 Alternative Flows 1 Flow of Events for the Clear Intersection Use Case 1 1 Preconditions 1 2 Main Flow This use case begins when a car enters the intersection 1 The car checks status 2 3 If the light is green and the queue is empty the car clears the intersection 4 Otherwise it joins a queue 5 1 3 Subflows 1 Enter Intersection The car enters the intersection 2 Check Light The car checks whether the light is green via the Report Status subflow of the Light Status use case 3 Check Queue The car checks whether it is at the front of the queue via the Report Status subflow of the Car Queue use case 4 Go The car clears the intersection 5 Join a Queue The car joins the queue via the Add to Queue subflow of the Car Queue use case 1 4 Alternative Flows E 1 Light Out If S 2 determines that the light is not red yellow or green the car must wait for a clear intersection and proceed with caution E 2 A
6. n relation correctly The implicit actor registrar is missing Notall requirements are covered Relations among use cases are not labeled Too much details Drexel University Template for Flow of Events X Flow of Events for the lt name gt Use Case X 1 Preconditions What needs to happen in another use case before this use case can start X 2 Main Flow X 3 Subflows Break normal flow into pieces X 4 Alternative Flows Things that happen outside of the normal flow Drexel University Flow of Events vs Scenario Flow of events enumerates all subflows and exception flows e Scenario is one path through your flow of events choose one set of subflows and maybe an exception When youre testing make sure you cover a reasonable 80 set of your scenarios Drexel University Intersection Problem Statement You will be simulating automobile traffic flow traffic signals at a typical intersection Traffic flows in both directions on each of the cross streets Cars form into eight different queues at the intersection N from the north headed straight south NL from the north headed east left at intersection E from the east headed straight west EL from the east headed south left at intersection S from the south headed straight north SL from the south headed west left at intersection W from the west headed straight east WL from the west headednorth left at inters
7. tes S 1 and reports S 2 the status of a traffic light color 4 3 Subflows 1 Update Status The system changes the color of the lights and sends a message to the Check Status subflow of the Clear Intersection use case 2 Report Status The system returns a report indicating the status of the light for each queue 4 4 Alternative Flows Problems observed e Not understand the requirements Incorrect initialization Repeat the content of other use cases e Not showing the relation to other use cases Show program constructs Drexel University Why Scenarios and Use Cases Utterly comprehensible by the user Use cases model a system from the users point of view functional requirements Define every possible event flow through the system Description of interaction between objects Great tools to manage a project Use cases can form basis for whole development process User manual System design and object design Implementation Test specification Client acceptance test An excellent basis for incremental amp iterative development Drexel University
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