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Systems Analysis II Use Cases

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  1. Systems Analysis IIUse Cases INFO 355 Glenn Booker Week #2

  2. Use Cases • As part of the activity to define functional requirements, we can capture those requirements in “use cases” • “A use case is an activity the system performs, usually in response to a request by the user” (text, 69) Week #2

  3. Use Cases and OO • Though use cases are often associated with object oriented analysis, use cases are not object oriented • They could be used to help capture functional requirements for any type of system Week #2

  4. Finding use cases • Two methods (at least) for finding use cases for a system • User goal technique • Event decomposition technique • To validate a draft list of use cases, the CRUD technique can be used Week #2

  5. User goal technique • The main idea behind this technique is to identify the types of users of a system, then determine what goals or assigned tasks each type of user has when using the system • Those goals often correspond to use cases Week #2

  6. User goal technique • The user goal technique is: • Identify all potential users for a system • (Optional) Classify users by functional role (shipping, marketing, sales) and operational level (operational, management, executive) • Interview each user and determine what goals they have when using the system Week #2

  7. User goal technique • Make a preliminary list of use cases for each type of user • Look for duplicates and inconsistencies across users • Identify when multiple users need the same use case • Review completed list with users and other stakeholders for validation Week #2

  8. Event Decomposition • This approach looks for all events that would lead to the information system being used • Each event typically leads to a use case • Simplify events to ones that have a clearly defined start and end, and achieve a clear business purpose • Those are Elementary Business Processes (EBPs) = use cases Week #2

  9. Event Decomposition • Focusing on events keeps attention on the macro scale purpose of the system, not internal details • Events can be • External – caused by an actor • Temporal – done at fixed time intervals • State – triggered by an internal condition, e.g. low inventory Week #2

  10. Event Decomposition • Focus on events that directly cause the system to be used • Not prior conditions that are invisible to the system • Many important business processes do not involve the system directly! • Avoid trivial use cases (logging on) but DO include system controls (admin functions such as backup) The last point differs from the text! Week #2

  11. Event Decomposition • The event decomposition process is • Identify relevant external events • For each, name a use case • Identify relevant temporal events • For each, name a use case and define when it occurs • Identify relevant state events • For each, name a use case • Omit trivial use cases, but keep system controls Week #2

  12. CRUD technique • Use this technique for verifying an existing list of use cases • Recall CRUD – create, read, update, or delete data • The goal of this technique is to verify at least one use case has been identified to perform all relevant aspects of CRUD • Relevant aspects? Week #2

  13. CRUD technique steps • Identify major data entities for your system • For each, verify that at least one use case does each of CRUD, as appropriate to your system • Add use cases if needed • Make sure data ownership is clear if more than one application interacts Week #2

  14. Naming use cases • Give use cases a short name (2-4 words), starting with an action verb • Track shipment • Create new user • Produce monthly sales report • The reason for brevity is so we can put them on a use case diagram Week #2

  15. Use Case Diagram • A use case diagram summarizes all the major use cases for a system • To define a use case diagram, need: • List of Use Cases • Actors • External Systems (if any) • System Boundary (automation boundary) Week #2

  16. Actors • Actors are types of users of the system – the role of someone who uses the system • Actors must interact directly with the system • Interaction could be through any mechanism – keyboard, mouse, touch screen, card reader, voice, biometric,… Week #2

  17. Actors • Examples of actors include • Customer/Client/Patient/Patron/Donor/Subscriber (if they interact directly) • Manager (should be more specific) • System Administrator • Clerk • Foreman Week #2

  18. External Systems • External systems are any non-human (generally computerized) actor which your system needs to perform one or more use cases • Can be a Timer, to initiate automatically repeating use cases • Are systems you don’t control, and are outside the scope of your system development Week #2

  19. External Systems • Could include systems owned by vendors • E.g. a service to maintain your online catalog • Could be a custom legacy system which isn’t being replaced • E.g. an existing human resource system, or accounting system Week #2

  20. System Boundary • The use case diagram includes a box to show the boundaries of your system • Actors are not within the boundary • External systems are not within the boundary • Box is labeled with your system’s name (not just “System”) • The use case diagram acts like a context diagram Week #2

  21. Naming Use Cases • Each use case should have a brief name, typically 2-4 words • Start with a verb, and end with a noun • Cancel Customer Order • Place Order • Validate New Customer • Number use cases sequentially  Week #2

  22. Use Case Diagram Notation • Actors are represented by stick people, with their role below them • Use cases are represented by ovals • External systems are represented by rectangles, with “<<actor>>” before the system name • “<<actor>>” is a stereotype • The “<< >>” should be guillemets Week #2

  23. Sample Use Case Diagram Tip: Straight lines are easier to use! Week #2

  24. Use Case Diagram Notation • Lines connect actors to the use cases they can perform • Hence a major purpose of the use case diagram is to show what functions the system can perform, and who can use them • Notice there is no indication of when use cases are performed, or any of the logic behind them Week #2

  25. Generalization • A common concept for the use case diagram is when one actor has some special use cases, but also can do everything some other actors can • A manager or supervisor can do everything their staff can do, plus additional functions • Show this using generalization Week #2

  26. Generalization • Notice the triangle at the top of the line between Manager and Staff • This means that Manager inherits all use cases which Staff can do • (Also can use generalization in Class diagrams) • Helps keep use case diagram simpler and easier to read Week #2

  27. “Included” Use Cases • When documenting use cases, might find a clear set of activities that appears in two or more use cases • Can pull those activities out and make that an included use case • In Visio, lines to an included use case have the stereotype “<<uses>>” • In other applications, stereotype is “<<includes>>” Week #2

  28. “Included” Use Cases • The included use case is documented separately from the use cases which use it Use case numbering and system boundary omitted Week #2

  29. Subsystem use case diagram • Ideally all actors and use cases should fit on one diagram • If not, it’s ok to have a separate use case diagram for each major subsystem • Specify whether the diagram includes all users (preferred) or a limited subset of them Week #2

  30. Documenting Use Cases • Use cases can be documented to varying levels of detail • We’ll define two of them • Casual use case documentation • Detailed use case documentation • These are local terms for the level of documentation, not Cockburn’s • (that’s pronounced CO-burn) • The text uses ‘Brief’ and ‘Fully developed’ Week #2

  31. Casual Use Case Documentation • Consists of: • Use case number and name • Objective – A sentence to elaborate on the main purpose of the use case • Primary Actor – what actor initiates the use case • Main Success Scenario – a step-by-step description of the events which should occur during this use case Week #2

  32. Detailed Use Case Documentation • Consists of: • Use case number and name • Objective • Primary Actor • Secondary Actor(s) – other actors who play a significant role in this use case • Trigger – what event forces the start of this use case? • Main Success Scenario (MSS) Week #2

  33. Detailed Use Case Documentation • Extensions – when performing the MSS, what other events could occur? • Extensions often include alternate methods of processing, different ways to do the same thing, and error conditions • Performance time – how long it should typically take to perform this use case • Frequency – how often will this use case be performed? • Open Issues – for scope issues, if any Week #2

  34. Beyond Detailed • The MSS and/or extensions should cite included use cases, where appropriate • Additional documentation is possible beyond the detailed template just given – see Cockburn’s site Week #2

  35. Main Success Scenario • The Main Success Scenario describes the interaction between actors and the system in order to perform a use case • They are critical to write well, since later documentation depends upon them (e.g. sequence diagrams) • See Summary of UML Diagrams handout for details on MSS writing Week #2

  36. Main Success Scenario • Where to begin? • For most use cases, you can assume the actor has logged into the system (if needed) and are starting at the application’s Main Menu or its equivalent • A MSS describes a use case in more detail than you would typically consider necessary Week #2

  37. Main Success Scenario • The MSS describes the most common way a use case will be performed successfully • If there are multiple processing options, pick the most frequent one for the MSS, and describe the rest in the Extensions • In the MSS, assume all actions will be successful; describe how to handle unsuccessful outcomes in the Extensions • The MSS is the Disney version of the use case • Extensions are the Tim Burton version Week #2

  38. Main Success Scenario • Typical actor activities in a MSS are: • Navigate through the interface to a defined objective • “Shipping clerk navigates to the New Shipment Screen.” • Notice we didn’t say HOW they navigate, just that it’s accomplished somehow • Enter data onto an interface • “Shipping clerk enters the data for a new shipment.” • Notice every field entered is not specified Week #2

  39. Main Success Scenario • Describe when an actor selects something on an interface • “Dispatch manager selects the number of drivers needed.” • Indicate when an actor completes an activity, such as by submitting data • “Shipping clerk submits the new shipment data.” • This prompts the system to do something with the data Week #2

  40. Main Success Scenario • The only remaining type of actor action is to exit the application, which often isn’t part of a MSS (assume you can always cancel or exit from the system) • So there are few types of things an actor might do during a MSS • System actions can be much more complex Week #2

  41. Main Success Scenario • Typical system actions include: • Create a new interface screen • “System displays Define Shipment screen.” • Change or update the contents of an existing screen • “System updates the screen to show the list of artifacts.” Week #2

  42. Main Success Scenario • Communicate with an external system • “System gets current stock price from NYSE.” • “System emails drivers with shipment info.” • Perform an included use case • “System performs Validate Credit Card use case.” • Perform background processing • “System validates new customer data.” • “System computes the sales tax.” Week #2

  43. Main Success Scenario • Notice that the MSS includes steps which aren’t visible to the actors! • Other background processing might include • “System prioritizes the list of drivers.” • “System produces weekly sales summary.” • Background processing can include any activity needed to prepare data for presenting the results to the actor Week #2

  44. Main Success Scenario • Finally, make sure a MSS achieves the overall objective of the use case! • “System saves new customer data.” • “System updates order status.” • “System deletes completed orders.” • These steps are typically performing one of the CRUD functions Week #2

  45. Main Success Scenario • Writing a MSS might involve making assumptions about where or how data is stored • Can assume there is a place that stores Customer Data, or Shipment Data, etc. • External systems should be labeled consistently throughout the design • Can name interfaces (New Customer Screen), but don’t design them (‘click on the submit button’ = no!) Week #2

  46. Main Success Scenario • Throughout a MSS, look for actions which need to be repeated • Specify in generic terms how many times steps need to be repeated • For each book to be checked out, … • For each driver assigned to a shipment, • Repeat five times, or until login is successful Week #2

  47. Extensions • Extensions, or alternate scenarios, handle when something doesn’t go normally during a use case • Extensions are numbered, starting with the MSS step from which they depart • If an extension starts from step 5, the first extension is a condition called 5.a • Then the steps to respond to that condition are 5.a.1, 5.a.2, etc. • A second extension from step 5 is condition 5.b and has steps 5.b.1, 5.b.2, 5.b.3, etc. Week #2

  48. Extensions Handle • So the lettered step (5.a) describes the condition under which you perform that extension • And the steps under it (5.a.1, 5.a.2, …) are the actor and system actions that follow • Optional processing steps can be an extension; the rest of the MSS isn’t affected if they aren’t used • Adding sales tax to an order • Adding gift wrapping to an order Week #2

  49. Extensions Handle • Failure conditions – when a MSS step can’t be performed successfully • If a search yields no results • If payment is insufficient • Can’t connect to an external system • Alternate processing – when a MSS step can be processed differently • Handle domestic versus international orders • Handle different forms of payment Week #2

  50. Extensions • Assume that interfaces and data transactions inside your system are successful, and won’t result in an extension, e.g. • Saving or reading data locally • Creating or updating interfaces • Otherwise every system step could be an extension! Week #2