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Project Support : Actors and Use cases. Telematics systems and their design. Doc.Ing. Ond řej Přibyl , Ph.D. Department of applied mathematics Faculty of Transportation sciences, CTU. Project Task 3. Task #: Actors 1) Define graphically all actors in the system and relations among them
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Project Support:Actors and Use cases Telematics systems and their design Doc.Ing. Ondřej Přibyl, Ph.D. Department of applied mathematics Faculty of Transportation sciences, CTU
Project Task 3 Task #: Actors 1) Define graphically all actors in the system and relations among them Task #: Use case analysis 1) For the defined subsystems create structured UC. 2) Demonstrate traceability among requirements and UC
What is an actor? Definition • An actor in the UML "specifies a role played by a user or any other system that interacts with the subject." • An actor is a person, organization, or external system that plays a role in one or more interactions with your system. • Actors are drawn as stick figures
Why to talk about Actors? • Actor is everybody who interacts with the system • i.e. he is using some of the functions (use cases)
Relationships among actors UML 2 does not permit associations between Actors Yet, this constraint is often violated in practice since the generalization/specialization relationship between actors is useful in modeling overlapping behaviours between actors
Relationships among actors (2) • There are several types of relationships: • An association between an actor and a use case • An association between two use cases • A generalization between two actors • A generalization between two use cases
Example of actors Generalization
Writing effective use cases (Example ebay) - Actors • Define your use case actors • There are possibly over a dozen actors that interact with Ebay, from buyers and sellers, down to suppliers, wholesalers, auditors, and customer service. But we're going for grass-roots, so who are the basic users of Ebay? BUYERS and SELLERS. So lets put them down as our first actors. • Do you notice how the actors aren't John and Sue which would be people? While John may be a seller and Sue may be a buyer, an actor is a Role. And a role in this case would be that of a buyer and that of a seller. Now that things are clicking, lets throw some more actors on your paper just so we can try and identify more possible users. • Now we have a bunch of actors. Wait a minute? Paypal? That's not a person. An actor can be a system, because a system plays another role in the context of your new system and has goals and interacts with other actors as you will see later.
Identifying Actors Look for: • the users who directly use the system • also others who need services from the system To find actors that are people/roles ask: • Who will be a primary user of the system? (primary actor) • Who will need support from the system to do her daily tasks? • Who will maintain, administrate, keep the system working? (secondary actor) • Who or what has an interest in the results that the system produces ? To find actors that are external systems ask: • Which hardware devices does the system need? • With which other systems does the system need to interact with?
Sources • Actor in Wikipedia • Use case in Wikipedia • Use Case document in our course web-sites
Major types in Use Case diagrams • Use cases. A use case describes a sequence of actions that provide something of measurable value to an actor and is drawn as a horizontal ellipse. • Actors. An actor is a person, organization, or external system that plays a role in one or more interactions with your system. Actors are drawn as stick figures. • Associations. Associations between actors and use cases are indicated in use case diagrams by solid lines. An association exists whenever an actor is involved with an interaction described by a use case. Associations are modeled as lines connecting use cases and actors to one another, with an optional arrowhead on one end of the line. The arrowhead is often used to indicating the direction of the initial invocation of the relationship or to indicate the primary actor within the use case. The arrowheads are typically confused with data flow and as a result I avoid their use. • System boundary boxes (optional). You can draw a rectangle around the use cases, called the system boundary box, to indicates the scope of your system. Anything within the box represents functionality that is in scope and anything outside the box is not. System boundary boxes are rarely used, although on occasion I have used them to identify which use cases will be delivered in each major release of a system. Figure 2 shows how this could be done. • Packages (optional). Packages are UML constructs that enable you to organize model elements (such as use cases) into groups. Packages are depicted as file folders and can be used on any of the UML diagrams, including both use case diagrams and class diagrams. I use packages only when my diagrams become unwieldy, which generally implies they cannot be printed on a single page, to organize a large diagram into smaller ones. Figure 3 depicts how Figure 1 could be reorganized with packages.
Recommended Process Place Your Primary Actor(S) In The Top-Left Corner Of The Diagram Draw Actors To The Outside Of A Use Case Diagram Name Actors With Singular, Business-Relevant Nouns Associate Each Actor With One Or More Use Cases Actors Model Roles, Not Positions Actors Don’t Interact With One Another Introduce an Actor Called “Time” to Initiate Scheduled Events Recommended process: actors and use cases
Finding Use Cases For each actor, ask the following questions: • Which functions does the actor require from the system? • What does the actor need to do ? • Does the actor need to read, create, destroy, modify, or store some kinds of information in the system ? • Does the actor have to be notified about events in the system? • Does the actor need to notify the system about something? • What do those events require in terms of system functionality? • Could the actor’s daily work be simplified or made more efficient through new functions provided by the system?
A use case describes a actions that provide a measurable value to an actor. Use Case Names Begin With a Strong Verb Name Use Cases Using Domain Terminology Place Your Primary Use Cases In The Top-Left Corner Of The Diagram Imply Timing Considerations By Stacking Use Cases. As you see in Figure 1, the use cases that typically occur first are shown above those that appear later. Introduction
Generalization • Shows inheritance and specialization • One use case is simply a special kind of another
Includes • “Factor out” of a use case commonly used behavior • Allows reuse of functionality by multiple use cases
Extends • Indicates that one use case adds or replaces behavior of another • Must have a an associated extension point • May have a condition
You are "done" when: • You have named all the primary actors and all the user goals with respect to the system. • You can captured every trigger condition to the system either as a use case trigger or an extension condition. • You have written all the user-goal use cases, along with the summary and subfunction use cases needed to support them. • Each use case is clearly enough written that • the sponsors agree they will be able to tell whether or not it is actually delivered. • the users agree that is what they want or can accept as the system’s behavior. • the developers agree they can actually develop that functionality. • The sponsors agree that the use case set covers all they want (for now). • . . .
Sources • http://www.oracle.com/technetwork/testcontent/gettingstartedwithusecasemodeling-133857.pdf • http://alistair.cockburn.us/get/2465 • http://www.gatherspace.com/static/use_case_example.html • http://www.andrew.cmu.edu/course/90-754/umlucdfaq.html • http://www.objectmentor.com/resources/articles/usecases.pdf • http://www.math-cs.gordon.edu/courses/cs211/ATMExample/UseCases.html
