PRJ566

PRJ566System Sequence Diagrams

System Sequence Diagrams A system sequence diagram …. Illustrates input

and output events related to the system under discussion.

Larman, APPLYING UML AND PATTERNS, p. 173

System Sequence Diagrams The use case text and its implied system events

are input to a SSD (system sequence diagram).


System Sequence Diagrams Use cases describe how external actors interact

with the software system… An actor generates system events to a system,

requesting some system operation to handle the event.

The use case text implies the event…the SSD makes it concrete and explicit.


System Sequence Diagrams A system sequence diagram is a picture that

shows, for one particular scenario of a use case, the events that external actors generate, their order and the inter-system events.

All systems are treated as a black box.


Fig. 10.2

enterItem(itemID, quantity)

:System: Cashier

endSale

makePayment(amount)

a UML loop interaction frame , with a boolean guard expression

external actor to system

Process Sale Scenario

system as black box

the name could be "NextGenPOS" but "System" keeps it simple

the ":" and underline imply an instance , and are explained in a later chapter on sequence diagram notation in the UML

a message with parameters

it is an abstraction representing the system event of entering the payment data by some mechanism

description, total

return value(s) associated with the previous message

an abstraction that ignores presentation and medium

the return line is optional if nothing is returned

total with taxes

change due , receipt

makeNewSale

[ more items ]loop

Why Draw System Sequence Diagrams? The external input events—the system events are an

important part of analyzing system behavior. System behavior is a description of what a system

does, without explaining how it does it.


System Sequence Diagrams System events should be expressed at the

abstract level of intention rather than in terms of the physical input device.


System Sequence Diagrams How to name system events and operations

describe at a high level i.e. enterItem instead of scanItem ‘enter’ captures the intent of the operations without

specifying a design choice (i.e. scanning)

Fig. 10.4

e n te r I te m ( ite m ID , q u a n t ity )

s c a n ( ite m ID , q u a n t ity )

: C a s h ie r

w o rs e n a m e

b e t te r n a m e

:S y s te m

System Sequence Diagram Shows the external actors that interact directly with

the system Shows the ‘system’ Shows the system events that the actors generate Done in a chronological sequence (from first to last

event over time) Order of events should follow the order of the

scenario

System Sequence Diagrams Validates use cases

helps identify processing that has not been covered by the process described in the use case

Helps us identify objects and classes

Browse Catalog SSD

ActorSystem

Selects the Browse Catalog option on the Online Auction Main Page.

Retrieves the list of categories and displays the Category Page with a drop down list of categories and an Exit Button.

Selects an option from the drop down list

Retrieves all the items for that category and displays an Item List Page showing a table with item names with a Display Button and an Exit Button.

Selects an item and clicks the Display Button.

Retrieves the product name, it’s picture and a description and displays the information on the Item Information Page with a Place Bid Button and an Exit Button.

Clicks the Place Bid Button. Use case ends and starts the Place Bid use case.

: Shopper : System

browseCatalog

catalogName

enterCatalogName(catalogName)

itemName

enterItem(ItemName)

itemDescription,itemPrice

How Many System Sequence Diagrams?

One for each of the HD scenarios in the use case specification


PRJ566

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