Use Case

Methodology for Use case development The king’s Ship Wasa - 1628        No Architecture description Changes done on the fly, often under market/customer pressure Testing ignored Didn’t know how to tell the clients No The system last longer than was ever imagined Maintenance costs far exceed ordinary development No Specification ! Agenda Vocabulary  Why Use Cases?  Why should we care?  The challenges of UC modeling in large projects  The Methodology  Summary  Vocabulary  Actor – Role(s) external parties that interact with the system Use Case – A sequence of actions that the system performs that yields an observable result of value to an actor. [Booch 1999] Use Case Model - Bag that contains – Actors list, packages, diagrams, use cases, views   Use Cases benefits   Promote customer involvement Help manage complexity – Layers – Focus on real user needs  Groundwork for user manual, test cases  Help us work in iterations Use cases aren’t everything  Non-behavioral requirements – Performance – Design constrains – Etc.  Sometimes – an overkill Use cases & Architects ?!     Requirements drive the design !!! Help force designers focus on concrete issues Help identifying technical and business risks Can be used to help validate the architecture Use cases & Architects ?! (cont.)  Architects should be involved in (if not responsible for) - UC prioritization ! Architectural design workflow (Kruchten 2003): – – – – – – – – Select scenarios : criticality and risk Identify main classes/components and their responsibility Distribute behavior Structure into subsystems, layers and define interfaces Define distribution and concurrency Implement architectural prototype Derive tests from use cases Evaluate architecture  Overview Use case modeling for large projects is problematic  Most literature is lacking (too simplistic / Diagram Vision not practical) PDO M  A practical UC prioritie s reasonable Verify process is needed! Refactor Validate  Tea m Naïve approach    Find Actors Find Use Cases Describe Use Cases Challenges  Model – Duplicates – Explosion – Making sure the requirements are good  Team – Efficiency – Fragmentation  Process – Details too early – Quitting Time – Waterfall The Methodology  To resolve the challenges we need a process that is: – – – – – Ordered Controlled Not too complicated Not too demanding Flexible Methodology – Initialization Steps Define System Boundary  Organize the Team  Build a Problem Domain Object Model  Methodology - Process Find Actors  Find Use Cases  Organize the Model  Prioritize Use Cases  Describe Use Cases  Refactor the Model  Methodology – Supporting Steps Verify and Validate  Add Future Requierments  Methodology – End Game  Knowing when to stop ! Step 1: Define System Boundary  Vision and Scope – – – – – – What problems are solved Who are the stakeholders Client’s Organization main goals System main goals Boundaries of the solution Future Directions Step 2: Organize the Team Small teams  Heterogeneous  Multi-tier reviews  Requirements manager  Step 3: Build a PDOM Terms and relations  Iterative development  Police HQ Commands Commands Watch Commander Commands District Has an Emergency Center Is made of Is a Sector Is made of Policeman Beat Work in Are Watch Beat Team Allocated to Allocated to Drive Is a Police Car Is a Allocated to Rapid Response Car Beat Car Step 4: Find Actors  Identify – Ask the End-Users – Documentation  Issues – Roles Vs. Job Titles – The Clock Actor Hierarchy User Cop Emergency Center Operator Watch Commander User (from Actors) Log in Emergency Center Supervisor HQ Watch Commander Step 5: Find Use Cases  Scenario Driven – – – – – Find measurable value Business events Services actor needs / supplies Information needed Recurring     Actor/Responsibility Unstructured aggregation Mission decomposition Misuse cases Step 5: Find Use Cases ../2  Initial Description – – – – – Unique ID Scope Pre conditions Success Guarantee Trigger Example : Initial description Use Case: Run Special Op. ID: UC4 Scope: The Watch Commander chooses a Special operation to manage. The task team chosen for the operation is briefed The watch commander then monitors the operation as it unfolds (sending out orders as needed) The task team is debriefed for the results and a final report is made. Primary Actor: Watch Commander Preconditions: A Special Op. Plan is saved in the system. Success Guarantees:  The Special Op. recordings (Forces movement, Voice recordings etc.) are saved in the system.  The operation's statistics are saved in the system.  Operation Final Report is saved and printed. Trigger: The Watch Commander chooses a Special Op. Step 6: Organize the Model Ever Unfolding story  Category sets  – Status, scope, stakeholders, sub-systems Subject Category hierarchy  Views  – Architectural view (i.e. SAD - Use Case View) Step 7: Prioritize Use Cases  Risk Classes – Business Risks – Architectural Risks – Logistical Risks  Iterative development – Small vs. Large projects Step 8: Describe Use Cases  Template – – – – – – – – – – Main success Scenario Variations Exception Assumptions Status Priority Stakeholders and concerns Issues Non-behavioral reqs. Extension points. Step 8 : Describe Use Cases ../2    Focus Technology neutral Activity diagrams Step 9: Refactor the Model  Relations – – – – Trace (decomposition) Include (common sub-behavior) Extend (promoted alternatives) Generalize  Merge droplets Step 10: Verify & Validate   Verification – Making sure we build the product right Validation – Making sure we build the right product Traceability Inspection Reviews Walkthroughs Prototypes      Step 10 : V&V ../2  Actors – Are all the actors abstractions of specific roles? – Are all the actors clearly described, and do you agree with the descriptions? – Is it clear which actors are involved in which use cases, and can this be clearly seen from the use case diagram and textual descriptions Step 10: V&V ../3  Use Cases – Does the use case make sense? – For each iteration: Are all the use cases described at the same level of detail? – Are there any superfluous use cases, that is, use cases that are outside the boundary of the system, do not lead to the fulfillment of a goal for an actor or duplicate functionality described in other use cases? – Do all the use cases lead to the fulfillment of exactly one goal for an actor, and is it clear from the use case name what is the goal Step 10: V&V ../4  The Scenarios – Are there any variants to the normal flow of events that have not been identified in the use cases, that is, are there any missing variations? (“happy days scenarios”, exceptions, variation, “soup-opera scenarios”) – Are the triggers, starting conditions, for each use case described at the correct level of detail? – Does the behavior of a use case conflict with the behavior of other use cases? – Is the number of steps in the complex scenarios excessive (12 to 15 is getting borderline)? Step 10: V&V ../5  Organization & Prioritization – Are all the use cases organized in an appropriate manner (e.g. by functional area, by dependency, by actor etc)? – Are all the use cases within a package consistent with the theme of the package? – Is the priority mechanism documented? – Are the use cases prioritized correctly? Step 11: Add Future Requirements  Capture Change cases – Preparing for change – Impact analysis Example: Future Requierments Step 12: Knowing When to Stop  Project Level – Complete list of actors and goals – Customer approval – Design ready  Iteration Level – Covered all currently prioritized use cases – Level of detail Summary What we have seen…   Additional Issues – Project Management – Requirements Management – Configuration Management Further Reading… Writing Effective Use Cases (Cockburn)  Patterns for Effective Use Cases (Adolph & Bramble)  Advanced Use Case Modeling (Armour & Miller)  The End… CHAOS Chronicles III - Jan. 2003 Success Factors  Executive-management support “CHAOS research is  User involvement dedicated to solving  Clear business objectives the mystery of project  Minimizing scope success and failure” – Time is the enemy of all projects – Scope equals time  Firm basic requirements – Balance between "Paralysis through Analysis" and what happens if requirements are not specified Example: Finding Use Cases  What measurable value is needed by the actor? – – – Plan Special Op. Monitor Special Op. Analyze Crime Patterns. Handle Emergency Call Call Car for Service  What business event might this actor initiate (based on her role)? – –  What services does the actor need from the system? – – – Find Navigation Route Get Unit Status Map Incidents Dispatch Units Issue Tickets Get Car Registration History List Duties Get Updated Situation Awareness Map Generate Emergency Center Statistics Report Generate Crime Trends Report.  What services does the actor provide? – –  What information does the actor need from the system? – –  What are the activities that are recurring and triggered by time? – – – Example : Mis-Use Cases Tap Communications User (from Actors) <> Log in <> Tap Communications Hacker <> <> (from Mis-Actors) Obtain Password Enforce Password Regime Hacker (from Mis-Actors) <> <> User (from Actors) Log in Obtain Password Sys Admin (from Actors) <> Monitor System Example : Use Case Use Case: Handle Emergency Call ID: UC24 Scope: The Operator accepts an incoming call, enters the incident information and dispatch a unit to the location of the incident Stakeholders and Concerns:  Victim - wants the police to arrive as soon as possible  Beat Team – don't want to be dispatched to handle false incidents. Primary Actor: Emergency Center Operator Preconditions: Operator logged in. Success Guarantees:  The Call has been recorded  A unit has been dispatch to investigate the incident  The incident details are saved in the system Trigger: A Citizen's incoming call has been directed by the Call Center system to an Operator. Example : Use Case ../2 Main Success Scenario: 1.The system begins recording the call. 2.The system traces the caller address. 3.The Operator takes the incidents location 4.The system calculates available police units. 5.The Operator takes the incidents detail 6.The system presents a list of available teams and their distance from the incidents estimated location. 7.The Operator chooses a unit to handle the incident 8.The system dispatches the incident details to the chosen team. 9.The Operator takes the caller details 10.The system saves the incidents details including call statistics 11.The system ends recording. Variations: 1.step 2 - when the caller uses a mobile phone a. Locate the callers current location 2.step 2 - when the caller is on the black list (known to call for no reason) a.The Operator is presented with additional questions to ask the caller b.The system marks the incident as low-priority on count of possible false alarm. 3.step 7 - when the incident does not require police intervention. a.The Operator closes the incident b.The system saves the termination reasons and continues from step 10 4.step 7 - if the incident requires a fire truck/ambulance a. The Operator chooses which authority to notify (fire / ambulance etc) b.The system dispatches the incident details to the appropriate authority's system Example: Use Case ../3 Main Success Scenario: 1. The system begins recording the call. 2. The system traces the caller address. 3. The Operator takes the incidents location 4. The system calculates available police units. 5. The Operator takes the incidents detail 6. The system presents a list of available teams and their distance from the incidents estimated location. 7. The Operator chooses a unit to handle the incident 8. The system dispatches the incident details to the chosen team. 9. The Operator takes the caller details 10. The system saves the incidents details including call statistics 11. The system ends recording. Exceptions: 1.step 2 - when the call cannot be traced a.The system suggests lowering the priority of the call on the count of an unknown caller b.The operator decides what priority to allocate for the incident. 2.step 6 – when there is no available free force a.The system presents the operator with low-priority incidents (along with the reason for low-priority 3.step 8 – communication problem with the unit dispatched a.The system performs step 6 and 7 again. 4.step 8 – communication problem with all the units. a.The system presents the operator the incidents details to allow dispatching by radio/mobile phone. Non Behavioral Requirements: The system should present as few screen as possible to the operator Locating a free unit should take less than 30seconds Communications to and from the unit should be secure (encrypted) to prevent eavesdropping by offenders/media Example: Use Case Levels Maintain Police Cars (from Servi ce/Mai ntenance management) <> <> <> Service Cars (from Servi ce/Mai ntenance management) Fix Car After Accident Track Police Cars Usage (from Servi ce/Mai ntenance management) (from Servi ce/Mai ntenance management) Example : Refactoring Common Sub-behavior <> Respond to Incident (from Incidents Response) Find Navigation Route Beat Cop (from Actors) <> (from Navigation) Perform Assignment (from Special Ops support) Use Case View  Concerns – What’s the conceptual framework in which the system operates – What are the key processes and events that must be presented in the system – Why the architecture is the way it is  Stakeholders – Users – Designers & Developers  Integrate the other views