07-Uml Sequence Diagrams

8/3/2019 07-Uml Sequence Diagrams

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TCSS 360, Spring 2005Lecture Notes

UML Sequence Diagrams

Relevant Reading:UML Distilled, Third Edition , Chapter 4

M. Fowler


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UML sequence diagrams

sequence diagram : an "interaction diagram" that models asingle scenario executing in the systemperhaps 2nd most used UML diagram (behind class diagram)

A sequence diagram is an interaction diagram that emphasizesthe time ordering of messages. It shows a set of objects andthe messages sent and received by those objects.

Graphically, a sequence diagram is a table that shows objects

arranged along the X axis and messages, ordered in increasingtime, along the Y axis.

relation of UML diagrams to other exercises:CRC cards -> class diagramuse cases -> sequence diagrams


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Key parts of a sequence diag.

participant : an object or entity that acts in the sequencediagramsequence diagram starts with an unattached "foundmessage" arrow

message : communication between participant objects

the axes in a sequence diagram:

horizontal: which object/participant is actingvertical: time (down -> forward in time)


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Sequence Diagram

An object in a sequence diagram is rendered

as a box with a dashed line descending from it.

The line is called the object lifeline , and itrepresents the existence of an object over a

period of time.

an Order Line


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Sequence diag. from use case


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o a ora on agramSequence Diagram

Both a collaboration diagram and a sequence diagram derivefrom the same information in the UMLs metamodel, so you cantake a diagram in one form and convert it into the other. Theyare semantically equivalent.


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Representing objects

squares with object type, optionally precededby object name and colonwrite object's name if it clarifies the diagramobject's "life line" represented by dashed vert. line


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message (method call) indicated by horizontal arrowto other objectwrite message name and arguments above arrow

dashed arrow back indicates returndifferent arrowheads for normal / concurrent (asynchronous)

methods

Messages between objects


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Indicating method calls

activation : thick box over object's life line;drawn when object's method is on the stack

either that object is running its code, or it is on the

stack waiting for another object's method to finishnest to indicate recursionActivation

Nesting


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linking sequence diagrams

if one sequence diagram is too large or refers toanother diagram, indicate it with either:an unfinished arrow and commenta "ref" frame that names the other diagram

when would this occur in our system?

Verify customer credit

refCustomer Info

Approved?


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Example sequence diagram

sd Example

loop

StoreFront Cart Inventory

AddItemReserveItem

PlaceItemInOrder

Checkout

ProcessOrderConfirmOrder


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(De)centralized system control

What can you say about the control flow of each of the following systems?centralized?distributed?


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Flawed sequence diagram 1

What's wrong with this sequence diagram? (Look atthe UML syntax and the viability of the scenario.)


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What's wrong with this sequence diagram?


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What's wrong with this sequence diagram?

:Computer :PrintServer :Printer :Queue

print(file)[if printer free] print(file)

[else] enqueue(file)


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Why not just code it?

Sequence diagrams can be somewhat close tothe code level. So why not just code up thatalgorithm rather than drawing it as a sequence

diagram?a good sequence diagram is still a bit above the level of thereal code (not EVERY line of code is drawn on diagram)sequence diagrams are language-agnostic (can be

implemented in many different languagesnon-coders can do sequence diagramseasier to do sequence diagrams as a teamcan see many objects/classes at a time on same page(visual bandwidth)

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07-Uml Sequence Diagrams