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UML Sequence Diagrams
2 December 2010
7M822
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UML Diagrams
• Structure diagram
–
–
–
–
–
Class diagram
Object diagram
Component diagram
Deployment diagram
Package diagram
• Behaviour diagram
– Use Case diagram
– Activity diagram
– Interaction diagram
• Sequence diagram
• Communication diagram
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In a nutshell: what are interactions?
• A collection of communications between instances,
including all ways to affect instances, like operation
invocation, as well as creation and destruction of
instances
• The communications are partially ordered (in time)
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Interaction Diagram Guide
• Show interactions between instances in the model
– graph of instances (possibly including links) and stimuli
– existing instances
– creation and deletion of instances
• Kinds
– Sequence diagram (temporal focus)
– Communication diagram (structural focus)

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Interaction Diagrams
Sequence Diagram
x
y
Communication Diagram
z
1.1: a
1.2: c
x
a
y
b
1.1.1: b
c
z
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Different Kinds of Arrows
Procedure call or other kind of nested
flow of control (synchronous message)
Return
Asynchronous message
Asynchronous message in UML 1.x
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Lifeline & Messages
• The vertical line is called the object’s lifeline. The lifeline represents
the object’s life during the interaction.
• Each message is represented by an arrow between the lifelines of two
objects.
• The order in which these messages occur is shown top to bottom.
• Each message is labeled at minimum with the message name; also
arguments and some control information can be included.
• A self-back is a message that an object sends to itself, by sending the
message arrow back to the same lifeline.
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Sequence Diagram / Communication Diagram
an Order
an Order Line
aProduct
aCustomer
calculatePrice
getQuantity
participant
found
message
1: calculatePrice
lifeline
getProduct
1.5.1: getDiscountInfo
1.4: calculateBasePrice()
1.5: calculateDiscounts()
aProduct
getPricingDetails
an Order
aCustomer
activation
return
1.1: getQuantity()
1.2: getProduct()
calculateBasePrice
1.3: getPricingDetails
message
self-call
an Order Line
calculateDiscounts
aProduct
getDiscountInfo
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Sequence Diagram vs. Communication Diagram
• Sequence diagrams are used when you want to look at
the behavior of several objects within a single use case.
Sequence diagrams are good at showing collaborations
among the objects.
• Communication diagrams emphasize the data links
between the various participants in the interaction.
With communication diagrams we can show how the
participants are linked together.
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Elements Sequence Diagrams
Participant
: Object
Participant
/ Object
Lifeline
Message
Return
Activation
Message
Lifeline
Self back
Return
{}
Constraint
Comment
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Next Concepts Sequence Diagrams 1of2
:Computer
:PrinterServer
:Printer
:Queue
control guard
Print
Print(file)
[printer available] Print(file)
[printer working] Save(file)
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Next Concepts Sequence Diagrams 2of2
:Computer
constraint
:PrinterServer
:Printer
Print
Print(file)
{
Print(file)
a
b-a < 5 sec
b
b'-b < 1 sec
b'
}
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: Order Entry
Window
: Order
: Order Line
: Stock Item
SEQUENCE
DIAGRAM
: Reorder
Item
: Delivery
Item
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: Transaction
: Transaction
Coordinator
: first
Transaction
Checker
Concurrent
Processes
and
Activations
: second
Transaction
Checker
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: Transaction
: Transaction
Coordinator
: first
Transaction
Checker
: second
Transaction
Checker
SEQUENCE
DIAGRAM:
Check Failure
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Example: A Booking System
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Use Case Description: Change Flt Itinerary
• Actors: traveler, client account db, airline
reservation system
• Preconditions: Traveler has logged in
• Basic course:
– Traveler selects ‘change flight itinerary’ option
– System retrieves traveler’s account and flight itinerary from client account
database
– System asks traveler to select itinerary segment she wants to change;
traveler selects itinerary segment.
– System asks traveler for new departure and destination information;
traveler provides information.
– If flights are available then …
– …
– System displays transaction summary.
• Alternative course:
– If no flights are available then…
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Sequence Diagram: Change Flight Itinerary
Traveler
: Booking System
Client Account DBMS
Airline Reservation System
change flight itinerary
get customer account
get itinerary
present itinerary
select segment
present detailed info
update information
available flight
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Collaboration Diagram: Change Flt Itinerary
1: change flight itinerary
5: select segment
7: update information
2: get customer account
3: get itinerary
: Booking System
4: present itinerary
Traveler 6: present detailed info
Client Account DBMS
8: available flight
Airline Reservation System
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