A sequence diagram shows the interactions between objects in a system. It displays objects arranged along a timeline, with messages passed between objects over time. Key elements include object lifelines that run vertically, messages shown as horizontal arrows, and activation boxes that indicate when an object is processing a message. Sequence diagrams are useful for illustrating the flow of control in response to events generated by external actors on the system.

1. System Sequence Diagrams
By Ashwini G. Munagekar
9922946280

2. Sequence Diagram
• Vertical line is called an object’s lifeline
– Represents an object’s life during interaction
• Object deletion denoted by X, ending a lifeline
– Horizontal arrow is a message between two objects
• Order of messages sequences top to bottom
• Messages labeled with message name
– Optionally arguments and control information
• Control information may express conditions:
– such as [hasStock], or iteration
• Returns (dashed lines) are optional
– Use them to add clarity
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3. System Sequence Diagram (SSD)
For a use case scenario, an SSD shows:
• The System (as a black box)
• The external actors that interact with System
• The System events that the actors generate
• SSD shows operations of the System in response
to events, in temporal order
• Develop SSDs for the main success scenario of a
selected use case, then frequent and salient
alternative scenarios
:System
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Syntax of Sequence Diagram
4

5. 5
Sequence Diagram - Objects
• A life line illustrates what is happening to an
object in a chronological fashion.
:Name
Life line
Activation
Object
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6. 6
Sequence Diagram – Time & Messages
• Messages are used to illustrate communication between
different active objects of a sequence diagram.
:Name1 :Name2
Message Two
Actor
Message One
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7. SSD for Process Sale scenario
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8. From Use Case to
Sequence System Diagram
How to construct an SSD from a use case:
1. Draw System as black box on right side
2. For each actor that directly operates on
the System, draw a stick figure and a lifeline.
3. For each System events that each actor
generates in use case, draw a message.
4. Optionally, include use case text to left of diagram.
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9. Room Management System
How UML is used – software specification
• Showing which activities the software supports
– Sometimes also useful to note those it doesn’t support
Person
check in
check out
Receptionist
pay bill
use-case diagram with system boundary
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10. Scenario with sequence diagram
• Interactions between actors:This is a form of instance diagram: it just shows one
example of what could occur
• Used to work out use-case implementation at high level
– No internal details of system
Person arrives at desk,
gives name.
R gets map of available
Rooms, chooses one.
R enters Person’s details
on Room Management
system against chosen
room.
R gives key to Person.
jo : Person chris: Receptionist rms: Room Manager System
arrive
get room map
choose room
person details
get details
key
confirm room assignment
time
Sequence diagram
use-case check in
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11. 11
Sequence diag. from use case
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12. Example: use cases to SSD
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A Sequence Diagram
member:
LibraryMember
book:Book
:Book
Copy
borrow(book)
ok = mayBorrow()
[ok] borrow(member)
setTaken(member)
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A Sequence Diagram
member:
LibraryMember
book:Book
:Book
Copy
borrow(book)
ok = mayBorrow()
[ok] borrow(member)
setTaken(member)
X-Axis (objects)
Y-Axis
(time)
Object
Life Line
message
Activation box
condition
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Object
• Object naming:
– syntax: [instanceName][:className]
– Name classes consistently with your class
diagram (same classes).
– Include instance names when objects are
referred to in messages or when several
objects of the same type exist in the diagram.
• The Life-Line represents the object’s life
during the interaction
myBirthdy
:Date
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Representing objects
• squares with object type, optionally preceded
by object name and colon
– write object's name if it clarifies the diagram
– object's "life line" represented by dashed vert. line
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Messages
• An interaction between two objects is
performed as a message sent from one
object to another (simple operation call,
Signaling, RPC)
• If object obj1 sends a message to another
object obj2 some link must exist between
those two objects (dependency, same
objects)
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Messages (Cont.)
• A message is represented by an arrow between the
life lines of two objects.
– Self calls are also allowed
– The time required by the receiver object to process the
message is denoted by an activation-box.
• A message is labeled at minimum with the message
name.
– Arguments and control information (conditions, iteration)
may be included.
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Return Values
• Optionally indicated using a dashed arrow
with a label indicating the return value.
– Don’t model a return value when it is obvious
what is being returned, e.g. getTotal()
– Model a return value only when you need to refer
to it elsewhere, e.g. as a parameter passed in
another message.
– Prefer modeling return values as part of a method
invocation, e.g. ok = isValid()
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20. 20
• message (method call) indicated by horizontal arrow
to other object
– write message name and arguments above arrow
– dashed arrow back indicates return
– different arrowheads for normal / concurrent
(asynchronous) methods
Messages between objects
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• Sequence diagrams have two features:
– Object lifeline
• Vertical dashed line that represents the existence of an object over
a period of time
– Focus of control
• A tall, thin rectangle that shows the period of time during which an
object is performing an action, either directly or through a
subordinate procedure
Sequence Diagrams
21

22. 22
Lifetime of objects
• creation: arrow with
'new' written above it
– notice that an object
created after the start of
the scenario appears
lower than the others
• deletion: an X at bottom
of object's lifeline
– Java doesn't explicitly
delete objects; they fall
out of scope and are
garbage-collected
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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
finish
– nest to indicate recursion
Activation
Nesting
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24. 24
Types of Messages
• Synchronous (flow interrupt until the message has
completed.
• Asynchronous (don’t wait for response)
• Flat – no distinction between sysn/async
• Return – control flow has returned to the caller.
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Synchronous Messages
• Nested flow of control, typically implemented
as an operation call.
– The routine that handles the message is
completed before the caller resumes execution.
:A :B
doYouUnderstand()
Caller
Blocked
return
(optional)
yes
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Object Creation
• An object may create another object via a
<<create>> message.
:A :B
<<create>>
Constructor
:A
<<create>>
:B
Preferred
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Object Destruction
• An object may destroy another object via a
<<destroy>> message.
– An object may destroy itself.
– Avoid modeling object destruction unless memory
management is critical.
:A :B
<<destroy>>
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Control information
• Condition
– syntax: ‘*‘ expression ’+’ message-label
– The message is sent only if the condition is true
– example:
• Iteration
– syntax: * * ‘*‘ expression ‘+’ + message-label
– The message is sent many times to possibly
multiple receiver objects.
[ok] borrow(member)
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Control Information (Cont.)
• Iteration examples:
:Driver
*[until full] insert()
:Bus
The syntax of
expressions is
not a standard
:CompoundShape :Shape
*draw()
draw()
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Control Information (Cont.)
• The control mechanisms of sequence
diagrams suffice only for modeling simple
alternatives.
– Consider drawing several diagrams for modeling
complex scenarios.
– Don’t use sequence diagrams for detailed
modeling of algorithms (this is better done using
activity diagrams, pseudo-code or state-charts).
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31. Identifying the right Actor
• In the process Sale example, does the customer
interact directly with the POS system?
• Who does?
• Cashier interacts with the system directly
• Cashier is the generator of the system events
• Why is this an important observation?
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32. Naming System events & operations
• System events and associated system operations
should be expressed at the level of intent
• Rather than physical input medium or UI widget
• Start operation names with verb (from use case)
• Which is better, scanBarCode or enterItem?
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33. By Ashwini s. Deshpander -
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Sequence Diagram
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34. Sequence Diagram
Time
Increasing
-->
All lines should be horizontal to indicate instantaneous actions. Additionally if
ActivityA happens before ActivityB, ActivityA must be above activity A
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35. By Ashwini s. Deshpander -
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Components
Objects: aStudent is a specific instance of the
Student class
Specific
Instance of an
Object
Generic (unnamed)
objects
Generic (unnamed)
objects of class type Seminar
and Course
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Components
lifeline
execution
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Components
Return value
Method call
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Components
c : Client
: Transaction
o : ODBCProxy
create()
setActions(a, b, c)
setValues(a, 3, 4)
setValues(b, c, 7)
(committed)
destroy()
Synchronous message
Asynchronous message
create()
destroy()
Return message
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39. 39
Understanding Sequence Diagrams (SSDs)
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Sequence Diagram: Example
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print(doc,client)
Example 2
Client
:PrintServer :Queue
:Printer
Proxy
enqueue(job)
status
Printing A
Document
job=dequeue()
[job]print(job.doc)
[job] done(status)
Repeated
forever with 1
min
interludes
Active
object
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42. 42
Example sequence diagram
sd Example
loop
StoreFront Cart Inventory
AddItem
ReserveItem
PlaceItemInOrder
Checkout
ProcessOrder
ConfirmOrder
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43. Sequence Diagram: Example
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44. Sequence Diagram: Example
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45. Sequence Diagram: Example
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46. 46
Sequence diagrams – an example
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47. 47
Sequence diagrams –
same example, more details
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48. 48
Sequence diagrams –
an example with replicated messages
– An iteration over objects is indicated by an asterisk preceding
the message name
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49. 49
Sequence diagrams –
an example with object deletion
– If an object’s life ends, this is shown with an X at the end of the
lifeline
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50. 50
Detailed Sequence Diagram
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51. 51
Potential Objects for Cancel an Order
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52. Sequence Diagram / Communication
Diagram
52
an Order an Order Line aProduct aCustomer
calculatePrice
getQuantity
getProduct
aProduct
getPricingDetails
calculateBasePrice
calculateDiscounts
getDiscountInfo
found
message
return
self-call
activation
lifeline
message
participant
an Order aCustomer
an Order Line aProduct
1: calculatePrice
1.5.1: getDiscountInfo
1.3: getPricingDetails
1.4: calculateBasePrice()
1.5: calculateDiscounts()
1.1: getQuantity()
1.2: getProduct()
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53. 53
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54. System Sequence Diagram
for Deposit Money Use Case
Customer
Money Detection
Hardware
CRT Display
Vending Machine
Controller
Deposit Money
[Idle State]
Display Welcome
Message
Display Amount
Send Amount
Prompt for Selection
Start Timer
[If Timer > 30 seconds and Invalid or No Selection]
Return Money
Money
Dispenser
Stop
Timer
[Idle State]
Display Welcome
Message
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