Use Case Diagrams Use Case Descriptions
Use Case Book (Chapter 2)
Use Case Diagrams…
Withdraw Money
Transfer Money
Simply actors and use cases
Can, of course, be much more detailed. Often the
3
Extending the UML with Stereotyping
Know we have ‘Change’ in everything.
Since there are so few graphics in UML, do we need more to communicate special means?
– special classes
– special kinds of use cases…
– Extend UML for new ‘types’ ? – New types of model elements?
– We often need customization of models for some projects.
Extend UML? No! This is a built in feature!
Extending the UML with Stereotyping
Enter: Stereotyping.
– Allow us to ‘refine’ / ‘reclassify’ ‘re-specify’ all model elements into a more specialized form rather than create additional symbols!
– Might specify a Use Case as a <<mission critical>> or class name with the stereotype: <<boundary>> or <<control>> etc.
– Indicate that the symbol is still a Use Case – but a ‘special one’
perhaps in a ‘special context.’
Most common UML stereotyped element is the class.
Stereotyping makes these different model elements!!!
– (Incidentally, additional icons can be added, if wanted…)
5
Examples
Choices
<boundary>
(attributes) (methods)
Authenticate User
<included>
A ‘special’ kind of class with special behaviors
A ‘special’ kind of Association – indicates a use case that supports a more fundamental use case – one that is more significant.
Stereotypes in Modeling:
Built-ins and User-Defined
Stereotypes can be used to ‘increase relevance’ of model elements, such as use cases in requirements gathering.
– (Much controversy on ‘extends’ and ‘includes’ and use cases invoking use cases...(Later) <<includes>> and <<extends>> …
Use Cases are commonly stereotyped
– A <mission critical> use case ‘may’ be specified in a separate document addressing all stereotypes
– “Stereotyped element” implies that there are ‘special’ criteria.
– e.g. A use case that is “mission critical” => must be processed within five seconds.
Classes may also very often stereotyped:
– <boundary>, <control>, <entity> (as found in Analysis Modeling)
• A boundary class is one that interacts directly with an actor…
7
Use Case Template
One of many different kind of formats…
Will discuss others in next lecture.
Use Cases
Use Cases – a great tool that helps us to express
interactions between system (application) and actors.
We capture desired behaviors of the system.
Meaningful to customer: concerned with ‘whats’ of the system.
Successful development of Use Cases requires an
understanding of the goals of each of the use cases,
which, in turn, is developed from identified, required
functionality – namely Features.
9
Goals of Use Cases
Interactions must present a value to actor.
– actor may be an accounting system; general ledger system; person; customer; a relay; or thermostat…
• person, device, external system/database/file, …
Use Cases are black box representations
– Do not show implementation-specific language
– Do not want to imply how use case will be realized.
– Do not include language such as: people, interface
widgets (buttons, menus…), IFTHENELSE statements, pseudo-code, more…
– Are written in language of the application domain.
– Are ‘abstractions’ of detailed interactions.
Capture stories addressing functional requirements…
Goals: Use Cases capture ‘whats’
“
Context-free” use (that is, no design constraints) Excellent for requirements gathering / modeling; analyzing and capturing desired user interactions
Numbers of Use Cases?– 70-80 for very large system?
– Medium sized – 20 to 50;
– Small systems - fewer.
Smaller number forces abstraction. Desirable. There is no ‘one size / number’ fits all. But there are guidelines in their development! (Coming)
11
Use Case Diagrams and Relationships
Actors
Actors trigger the start of a use case.
– people, computer systems, date/time, relay, device...
– Actors must receive value from the system
– Systems that are actors generally supplying data or services to your system and vice versa.
• May be a legacy RDBMS or a legacy VSAM file…
• Pressure and temperature may be actors (triggers), as well as a device, such as a conveyer belt, given a signal to start…
Secondary actors (surrogate actors) (next slide)
– actor/role providing system inputs/outputs from another actor/role.
– First (outside) actor/role is ‘secondary actor’ and should be shown if its actions affect system reaction.
13
Secondary Actors
Register Guest
Desk Clerk
Customer
<communicates>
(inheritance arrow)
Associations: Generalization in Actors
(next slide)
Generalization: Abstract the commonalities.
– two sub-actors generalized into a super-actor – Have both behavior and attributes in common
– described under the super-actor.
– Super-actor should interact with use cases when ALL of its sub-actors interact the same way;
– Sub-actors should interact with use cases
when their individual interactions differ from
that of the super-actor.
15
Some use case
Different use case
Actors and Classes
Actors may influence how classes are built because they will be using the services of the objects of those classes.
Actors may themselves be classes –
Customer
– often an Actor – often a Class
• Probably: Customer exists in the domain model and is likely a business ‘entity.’
Makes sense to take care in creating the right actor to
interact with the use cases.”
17
Actors and Associations
Not customary to label association lines between actor & use case.
– Default is <<communicates>>
• But there are others…
Role names. Actors have ‘role names’ – useful if ‘association’
needs information beyond the fact that they simply interact.
– Actor: Professor. role: student; role:
assessing another’s teaching; …
Associations exist in many forms and among many model elements…
– between classes – between Use Cases
– between design entities (subsystems, packages, etc.) – between analysis classes, etc etc.
Use Case Associations Extend/Include Stereotypes
Stereotyped associations indicate a special kind of association.
(good pix – Chap 2, Use Cases, Fig 2.12)
– Extending (blunt side) is a special use case that extends the original use case (sharp)
• “Extending use case” would not exist without the use case it is extending (the extended use case);
• Used for special cases; exceptional behaviors. Inserted behaviors
• Arrow points toward the base use case that is being extended…
– Include stereotype allows use case designers to avoid duplicating steps across multiple use cases (a reuse strategy).
• Arrow extends away from the owning parent use case.
• e.g. Authenticate User
– Note: controversial;
– Many Use Case authors do not like these and include the steps within the use case itself or use some other kind of subflow (later).
19
extending Use Case (inserted behaviors) (extended) Use Case
Included Use Case
(Particularly good for reuse!) Use Case
(Primary use case)
(Primary use case)
Exception and Alternative Paths
Last look:
Exceptions are errors that occur.
– Interactions thus contain steps to be executed.
Alternatives may / may not be close to basic course of events – just not the most likely course of events.
– No errors- but some authors treat these to document error conditions.
– Alternatives may be just as important as the basic course…Often the case.
– Some authors treat alternatives as MUCH less likely to occur. I don’t.
Alternate Paths: very important to capture them (with full interactions in most cases, too).
Exception paths should be kept separate from alternative paths.
All have a home with their use-case.
21
The Use Case Specification
Use Case Number: UC-03
Use Case Name: Edit Member Profile
Actor (s): Buyer, Seller
Maturity: Focused
Summary: This use case is started by a Buyer or a Seller. It provides the capability for one of these actors to edit their member profile.
Basic Course of Events: Actor Action
1. This use case is started when a Buyer or Seller elects to edit their member profile.
Perform S1-Login (subflow – later) 3. The Actor updates their member profile.
6. The Actor confirms that the information is correct.
A1. {Profile Change}
8. This use case concludes when the Actor receives visual confirmation of the update.
System Response
2. The System displays the
Actor’s member profile and prompts the Actor to update it.
4. The System validates the information entered by the Actor.
E1. {Validate Information}
5. The System prompts the Actor for confirmation.
7. The System updates the
Actor’s member profile to the member repository and informs the Actor that the information was updated successfully.
23
Alternate Paths: A1 Change Member Profile
At {Profile Change} the Member indicates that he/she entered incorrect information.
The System immediately returns to the step 2.
Exception Paths: E1 Handle Invalid Information
At {Validate Information} if any fields are entered incorrectly.
the System indicates the fields that were entered incorrectly and prompts the Buyer or Seller to make the necessary corrections. If errors, the flow of events is resumed at Basic Flow Step 2.
Extension Points: {Change Profile }, {Validate Information}
Triggers: The Buyer or Seller would like to edit their member profile.
Assumptions: The Buyer or Seller is aware of the steps required to edit their member profile.
Preconditions: The System is functioning properly.
Actor already has a Profile stored in the Profile Database???
Post Conditions: The member profile was successfully updated to the member repository. Actor sent email to confirm changes?
Reference - Business
Rules: See Business Rules section: 2.3.1 and 2.3.5.
Reference - Risks: See Risks List sections: 2.1 and 2.4.
Author (s): TeamA
Date: 10-31-2011
Continuing….
The Use Case Specification
A page or two of text; corresponds to a rounded oval in the Use Case diagram.
Definitely need some kind of a standard template.
Most organizations using Use Cases develop their own template format. Consistency is critical.
In all, the Use Case Specification should contain most of the following attributes:
– Name, iteration name/number; author, description, basic course of events;
alternative paths, exception paths, triggers, assumptions; preconditions;
post-conditions; references to related business rules; risks lists, and more.
All seem important.
25
The Use Case Specification -2
We have also indicated the need for an Index of Use Cases. Designate each use case as UC1, …, UCn.
Have already discussed the UC name: verb -> object
Iteration = the ‘maturity’ of the use case – and hence the maturity of our understanding of the functional
requirements.
(façade, filled, focused … many name variations…)
Descriptions –sentences describing the interaction.
The Use Case Specification -3
Basic Course of Events – (NOT in Façade)
– actor always takes first step (trigger) – always have a ‘happy path’
• Simple path; Most likely path)
– Can include a picture, if helpful. (screen shot, etc.) hyperlinks, etc.
Alternative Paths
– Other paths (may be less common)
– Each alternative and exception (below) should indicate WHERE in the basic course of events the alternative / exception emanates from. Starting point.
– Techniques:
cite the step number;
labels in the basic course of events (preferred). Discuss.
27
The Use Case Specification -4
Exception Paths
– Like alternatives but typically shown for errors.
Extension Points
– The <<extends>> relationship exists between use cases when one use case provides an optional sequence of events that is included in the other use case.
(see pg. 42, Use Case text)
– “Extension point” in the flow of events shows step name (in braces) or step number from which the extending use case extends. (more later)
– Note: the extending use case points to the extended use case.
• Again, this implies ‘inserted’ behaviors.
– Extending use case has ‘no life’ without the extended use case
The Use Case Specification -5
Triggers - entry points for a use case
Assumptions –
– document things assumed to be true (but might not be true) (Critical section.)
Preconditions –
– Things that must be in place before an interaction may occur. (part of contract between use case and outside world)
Post Condition –
– part of contract between use case and outside world; specify outcomes; independent of alternative path. e.g. transaction successfully posted.
Related Business Rules
– - Reference to business rules Related Risks addressed by this Use Case.
– Include a reference to the Risk List document
Author and Date
– at bottom; original date for each version: façade, filled, focused etc.
29
Paths and Scenarios
Use Cases – abstractions
– Really need detailed interactions – scenarios.
– Scenarios can provide details of the interactions!
Scenarios are instantiations of Use Cases:
• Use Cases with data
Book provides three definitions for scenarios:
– 1: merely an alternate path;
– 2: an instantiation of the use case with a specific path plus relevant data;
– 3: same as a use case (synonyms)
Paths and Scenarios -2
Definition #2 – the “Instance,” where the scenario is a
realization of a use case (one possible path) with included data.
Scenarios can be used
– during requirements capture for feedback to users and analysts that the use cases accurately reflect user needs, and
– in testing to test whether the system reflects the requirements.
Scenarios, then, are instances, of use cases (complete with production data) that effectively test one path through a use case.
31
Use Case Packages
A package is a general grouping of related model elements such as classes, use cases, or Actors.
Very desirable to group into packages primarily
because these packages can be used to form logical boundaries for dividing work among sub-teams.
A good rule of thumb – each package may correspond to a chapter or at least a major section in the user
manual.
– A division of functionality, responsibilities, etc……
Package Diagrams may contain use cases that are
related….These go to functionality (architecture later)
More – very important tidbits here.
The relationship between requirements and use cases is subject of much discussion.
– A use case describes a unit of behavior
– A requirement describes a law that governs behavior
– A use case can capture/satisfy/describe one or more functional requirements
– A functional requirement may be satisfied by one or more use cases…
Note that a system will have other requirements besides ‘functional,’
such as performance, maintainability, scalability, reliability… that don’t map easily to use cases….
– These tend to ‘thread themselves through Use Cases.’
– Non-functional requirements
– Some will readily be captured via the software architecture (later in course).
At end of use case modeling, you are ready to embark upon the
creation of an
Analysis Model
(or preliminary design) … (some authors call it a robust analysis…) Probably entering a second iteration within Elaboration Phase.
– This will involve very careful and highly iterative analysis of use cases and all their scenarios.
– This can only take place after the full Use Case Model is ‘completed.’
33
Read
– Use Cases for Inquiry-only systems,
– Use Cases for Requests For Proposals, etc.
DO READ THESE.
Remaining Slides are Rosenberg’s Views:
Good to see complementary treatment.
Read remaining slides on your own.
========= End Lecture ===========
35
Summary Points on Use Cases - from
Use Case Driven Modeling by Rosenberg
Essence of use case model is to capture user
requirements of a new system or based on an existing system.
We need to Detail all scenarios users will be performing
Use Case Model is at conceptual center of the process.
– Developed in cooperation with domain model – Precedes any Analysis modeling
Use Cases – sequence of actions that an actor, often a
person (role), but perhaps some other kind of external
entity, such as another system, performs within a system
to achieve a particular goal.
Summary Points on Use Cases - from
Use Case Driven Modeling by Rosenberg
A complete and unambiguous use case describes one aspect of usage of the system without presuming any specific design or implementation.
Result of UC Modeling: all required system functionality is described in the use cases!! (Recall, however, that these evolve…)
Note: that the Façade Use Cases constitute an overview or umbrella look at the functionality; identification (without
elaboration) of basic use cases – no basic course of events and many attributes not included.
37
More on Prototyping the Interface
These are the ‘More on’ slides:
Rapid prototyping can be used to demonstrate ‘proof of concept.’
Can write a rough Users Manual from the prototype and use case descriptions.
– (as if the prototype were a fully functioning system)
Prototyping normally provides the Basic Courses of
events, because rapid prototyping normally (may often) ignores many alternatives and exceptions…
My own feeling is that the prototype, however, MUST include alternative courses of events in order to fully capture the user expectation (and hence, functionality).
Prototyping Provides Basic Flow and More
39
Prototype can contain simple windows containing radio buttons or generic icons – only to show functionality..
Should have a window navigation diagram within the prototype.
Whether you build a prototype, draw mockups, or use some other technique to address the visual aspects of your system, it is very useful to link your screen designs to your use cases.
– (Can do this manually or via a visual modeling tool.)
Prototyping Provides Basic Flow and More
Prototyping Provides Basic Flow and More
Use Case text should be relatively free of presentation details.
– Field names often match up directly with attribute names in domain model (classes; entities).
– Use terms from domain!
– Many nouns / actors are classes in domain model – Interactions will be ‘control’ later…
Some controversy:
Important to do prototyping, mockups, etc. before you start writing use cases or
in conjunction with your use cases…Otherwise, you may spend lots of extra time trying to pin down your user expectations….
41
‘ Basic course of action’ is the main ‘start to finish path’
the user will follow.
– Alternate course of actions represent other used paths
Goal is NOT to build an elegant use case –
Goal is to ACCOUNT for everything user might do!
The more well-defined the system behavior is
(modeled from the interactions in use case and other design drawings later), the easier it will be to build the system.
Developing Use Cases is sometimes referred to as a
‘behavioral approach’ to capturing requirements.
Actually, this is correct.
More on Basic Course of Events…
42
Factoring out Commonality in Usage
Generalization - …
Includes – involves making one use case take full use of another use case. (more later) See page 42 for illustration)
– Goes to ‘reuse’ = a prime objective of use case modeling.
– Example I like: login or authentication use case may be
‘included.’ (factored out – not decomposition)
Association is defined as a stereotype. Put
<<includes>> on arrow.
Extends – allows a modeler to insert one use case into another use case thus ‘extending’ the second use case. (use <<extends>> on arrow)
– Can show ‘optional system behavior’ or behavior that is executed only under certain circumstances. (The ‘extends’) – Defined as a stereotype Put <<extends>> on association
arrow.
– <<extends>> points to use case that is extended (from
43
More on Extends and Includes
We normally do not generalize use cases, because generalization is most useful for building elaborate structures dominated by abstract use cases….(But some people do…)
Don’t spend lots of time worrying to use includes, extends …..
Don’t spend lots of time with long, involved use case templates.
(I like the <<extend>> and <<include>> associations
to assist in factoring functionality associated with Use
Cases…)
Common Mistakes
10. Write functional requirements instead of usage scenario text
9. Describe attributes and methods in too much detail rather than usage
8. Write the use cases too tersely (won’t do much good!)
7. Divorce yourself completely from the user interface
– Big mistake!!!
6. Avoid explicit names for your boundary objects. (more later)
5. Write using a perspective other than the user’s, and doing so in passive voice.
4. Describe only user interactions; ignore system responses.
3. Omit text and references for alternative courses of action
2. Focus on something other than what’s ‘inside’ a use case, such as how you get there (precondition) or what happens afterward (post-condition).
1. Spend a month deciding whether to use <<include>> or
<<extend>> associations.
