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Introduzione ai casi d’uso Adriano Comai 1999
Pag. 1www.analisi-disegno.com
Use Cases: an Introduction
Adriano Comai 1999
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 2www.analisi-disegno.com
Goals of this Introduction
• To provide basic information about use cases
• To provide suggestions about their use in application development
• These points are treated in more depth, with exercises, in the course “Requirements and Use Cases Definition”:
http://www.analisi-disegno.com/a_comai/corsi/sk_req.htm
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 3www.analisi-disegno.com
Use Cases
• Proposed by Ivar Jacobson (book published in 1992)
• New terminology, but a long practised technique (study of operational scenarios of system usage)
• Use cases are the “ways” in which a system can be used (the functions which the system provides to its users)
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 4www.analisi-disegno.com
Use Case : Point of View
• It starts usually with a request from an actor to the system
• It ends with the production of all the answers to the request
• It defines the interactions (between system and actors) related to the function
A use case describes a function from the point of view of its users:
The point of view to take into account must be the actor’s, not the one of the system
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 5www.analisi-disegno.com
Use Cases vs (Internal) Functions
Use Cases
• call someone
• receive a call
• send a message
• memorize a number
• ….
Point of view: USER
Internal Functions
• transmit / receive
• energy (battery)
• user I/O (display, keys, ...)
• phone-book mgmt.
• …..
Point of view: DESIGNER
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 6www.analisi-disegno.com
Use Cases vs (Internal) Functions
Internal functions• specialized front-ends
• common front-end
• pre-application controls
• contract management
• system monitoring
Use Cases
• customer:– orders (payment, buy
stocks, etc.)
– inquiries
– contract
• administrator:– verify anomalies
customer banking systems
administrator
electronicbanking
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 7www.analisi-disegno.com
Use cases as Interaction
• Use cases can be described as an interaction scenario (a dialogue) between the users and the system:– customer asks for a list of products
– system shows available products
– customer chooses products she wants
– system shows total cost of selected products
– customer confirms order
– system communicates acceptance of order
• Attention must be given to the interaction, not to internal system activities
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 8www.analisi-disegno.com
Use Cases Identification
1 Identify system users (“actors”)
2 (For each actor) discover in which ways the actor will use the system, starting from the goals the actor has to achieve
3 (For each use case) clarify how the the activity starts, the answers the actors expect from the system, the sequence of the interaction
Use cases help to discover requirements
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 9www.analisi-disegno.com
Requirements and Use Cases
(functional) requirement : a function, or a characteristic of a function, requested by the customer or by some other stakeholders of the system
use case: a way the system can be used by a user (actor)
• Each use case can satisfy many functional requirements
• A functional requirement can be related to many use cases
• Each use case can have many non-functional requirements associated to it
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 10www.analisi-disegno.com
Prototype and Use Cases
• For each use case there is an interaction between actors and system, realized through interfaces
• It is a good idea to prototype the interaction (especially between system and human beings) during use case definition :
– use case and prototype are complementary ways to depict the interaction
– UI prototype helps to clarify use case sequence
– use case description helps to discover needed links between interfaces
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 11www.analisi-disegno.com
Use Case (Jacobson’s Definitions)
• (in a business system): “A sequence of transactions in a system whose task is to yield a result of measurable value to an individual actor of the business system”
• (in an information system): “A behaviourally related sequence of transactions performed by an actor in a dialogue with the system to provide some measurable value to the actor”(Jacobson 1995)
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 12www.analisi-disegno.com
Use Cases and Transactions
Each use case can be implemented by a sequence of transactions in the system, in order to provide the aswers needed by the actor
customer open account
Transactions:* verify existence of customer in DB* insert new customer* scan signature of customer* insert new account
Introduzione ai casi d’uso Adriano Comai 1999
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Actors
• Entities interacting with the system, through messages
• Actors can be:
– human beings
– organizations
– other systems (both hardware and software)
• Each actor is a class, and may have many instances– Ex. Actor “customer” is a class; every individual customer is a
member of the class
Introduzione ai casi d’uso Adriano Comai 1999
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“Business Actors” vs. “Information System Actors”
A model independent from any organizational and technological solution
Order Clerk places an order
The model of a specific organizational and technological solution
Customer places an order
Introduzione ai casi d’uso Adriano Comai 1999
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“Business Actors” vs. “Information System Actors”
• Independent from specific organizational and technological solutions (“Business Model”)
• Dependent on a specific organizational and technological solution (“Information System Model”)
Actors can be defined following two points of view, both important and legitimate, corresponding to two different levels of abstraction :
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 16www.analisi-disegno.com
Business / IS Model
From the point of view of the customer, the order clerk is a part of the system (as a mediator, an interface)
customer
places an order
administration
order clerk
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 17www.analisi-disegno.com
Mediation Forms
• The interface between actor and system changes• The logic core does not change
customer
request of statement of account
phone
clerk
internet
ATM
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 18www.analisi-disegno.com
Mediation Forms
• “human” interface:• pros: flexibility, able to adapt to the specific actor• cons: cost, absence of uniformity
• “automated” interface:• pros: cost, uniformity• cons: not able to understand requests that are
either not-predefined, or specified in an unexpected way
Introduzione ai casi d’uso Adriano Comai 1999
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Use Cases and Scenarios
• Base Scenario : (usually) implies success, and a linear development of the use case
• Alternate Scenarios : can imply success or failure, with various complications
• we do not need (it would be very expensive) a detailed analysis of every possible use case scenario (combination of variances)
• but we must discover every variance that can bring to the failure of the use case, or that needs a specific treatment
Introduzione ai casi d’uso Adriano Comai 1999
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example: open account
1 Customer goes to the bank to open an account2 Clerk welcomes customer and gives explanations3 If customer accepts rates she gives her personal info4 Clerk verifies if customer is already known to the bank5 Clerk opens a new account
6 Clerk gives the customer an account number
Variances:3 (a) If customer does not accept, use case ends3 (b) If the account is opened by many people, it is necessary to give
personal info of every holder
4 (a) If customer (one of the set of customers) is not known, clerk registers her,asks for the signature, scans the signature
Introduzione ai casi d’uso Adriano Comai 1999
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example: open account - more detail
(5) Clerk opens a new account
1 Clerk starts transaction “open account”
2 System asks customers’ codes
3 Clerk inputs codes
4 System shows corresponding personal info, and asks for rates
5 Clerk inputs rates, and confirms
6 System prints contract, with new account number
Variances:
3 (a) if system does not know a customer, or displays unexpected data, clerk can correct or stop the transaction
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 22www.analisi-disegno.com
Use Case Levels
Use cases are a specific way to represent functions, and they can describe objects at different levels:
• system
• subsystem
• component (or class)
Whatever the level, use cases define a behaviour of the object they describe, without revealing its internal structure
Introduzione ai casi d’uso Adriano Comai 1999
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Use Cases at the “Business Event” Level
• Update Web catalog
• Customer Info Request
• Customer Order
• Payment
• Assistance Request
It is possible to identify and describe use cases at the level of events triggered by actors (each use case implies every needed answer to the event):
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 24www.analisi-disegno.com
Use Cases at the “Transaction” Level
• Create Order
• Read Order
• Update Order
• Delete Order
It is possible to identify and describe use cases at the level of each transaction, down to the point of atomic operations (CRUD) on the classes of the system:
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 25www.analisi-disegno.com
From which Level to Start?
For a medium-sized system we can have:
• about a dozen use cases at the “business event” level– complete functions from the point of view of actors
– meaningful delivery (and system test) units
• more than one hundred use cases at the “transaction” level– internal system functions
– can be too fragmented to be used as a basis of understanding between designers and with system customers
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 26www.analisi-disegno.com
Use Case Role
Requirements Use case:Customer Order
Seller
Customer
Ordine
DataArrivoNumeroPrezzo
verifica( )evadi( )
Cliente
nomeindirizzo
StabilisciCredito( )10..*
Analysis & Design Models
Test cases
Delivery Units
Introduzione ai casi d’uso Adriano Comai 1999
Pag. 27www.analisi-disegno.com
Are Use Cases OO?
• Were “Invented” in an Object Oriented context
• Describe system functions from the point of view of the external actors (like OO messages)
• Do not reveale the internal structure of the system
• Are the best starting place for OO design
…but…
• Can be used in a non-OO development process
• OO theory is not needed to understand and to use them