sequence diagram ppt

1. Sequence DiagramsSequence Diagrams
Preeti MishraPreeti Mishra
Course InstructorCourse Instructor

2. Also Known as:
Interaction Diagrams
Event Trace Diagram
Scenarios
Message Diagram
Sequence Diagrams

3. Scenarios
• A scenario is a sequence of events that occurs
during one particular execution of a system.
• A scenario can include all events in the system or
can only include these events that are generated
by a certain object in that system.
• A scenario can be a historical record of executing
or simulating the execution of a system or an
object.

4. Interaction Diagrams
• UML Specifies a number of
interaction diagrams to model
dynamic aspects of the system
• Dynamic aspects of the system
– Messages moving among objects/
classes
– Flow of control among objects
– Sequences of events

5. Dynamic Diagram
Types
• Interaction Diagrams - Set of objects or roles and
the messages that can be passed among them.
– Sequence Diagrams - emphasize time ordering
– Communication Diagrams - emphasize structural
ordering
• State Diagrams
– State machine consisting of states, transitions,
events and activities of an object
• Activity/Swimlane Diagrams
– Emphasize and show flow of control among objects

6. Sequence Diagrams
• Describe the flow of messages,
events, actions between objects
• Show concurrent processes and
activations
• Show time sequences that are not easily
depicted in other diagrams
• Typically used during analysis and design to
document and understand the logical flow of
your system
Emphasis on time ordering!Emphasis on time ordering!

7. Sequence Diagrams
• Each event transmits information from one object
to another.
• Example:
– dial tone begins event transmits a signal from
the phone line to the caller.
• In Event Trace:
– Identify sender and receiver objects of each
event.
• Event Trace Diagram (ETD) shows the sequence of
events and the object exchanging these events.
• ETD shows object as a vertical line and each event
as a horizontal line from the sender to the receiver.

8. Event Trace Diagram for a
Phone Call
Caller Phone line Callee
caller lifts receiver
dial tone begins
dials (5)
dial tone ends
dials (5)
dials (5)
dials (1)
dials (2)
dials (3)
dials (4)
ringing tone
tone stop
phone rings
answers phone
ringing stops
phone connectedphone connected
callee hangs up
connection brokenconnection broken

9. Sequence diagrams
• 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

11. Definition:
A sequence shows a series of messages
exchanged by A selected set of objects in
temporally limited situation, With emphasis on
the chronological course of events.
Objects are shown by vertical lifelines. This
highlights The chronological sequence of the
message.
Time runs from top to bottom.
Sequence Diagrams
Definition

12. Sequence Diagram
Components
Two major components
– Active objects
– Communications between these active objects
• Messages sent between the active objects

13. Sequence Diagram -
Objects
• A life line illustrates what is happening to an object in a
chronological fashion.
:Name
Life line
Activation
Object

14. 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

15. Active Objects
Actor
– A person or system that derives benefit
from and is external to the system
– Participates in a sequence by sending
and/or receiving messages

16. Components
lifeline
execution

17. Components
Return value
Method call

18. Sequence Diagram
Active objects
– Any objects that play a role in the system
– Participate by sending and/or receiving
messages
– Placed across the top of the diagram
– Can be:
• An actor (from the use case diagram)
• Object/class (from the class diagram) within the
system

19. Communications
between Active Objects
Messages
– Used to illustrate communication between different
active objects of a sequence diagram
– Used when an object needs
• to activate a process of a different object
• to give information to another object
A message is represented by an arrow between the life
lines of two objects.
– Self calls are allowed
A message is labeled at minimum with the message name.
– Arguments and control information (conditions, iteration) may be
included.

20. 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)

21. 21
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.

22. Other Elements
of Sequence Diagram
 Lifeline
 Focus of control (activation box or
execution occurrence)
 Control information
 Condition, repetition

23. Sequence Diagram
 Lifeline
 Denotes the life of actors/objects over
time during a sequence
 Represented by a vertical line below each
actor and object (normally dashed line)
 For temporary object
 place X at the end of the lifeline at the
point where the object is destroyed

24. Sequence Diagram
 Focus of control (activation box)
 Means the object is active and using
resources during that time period
 Denotes when an object is sending or
receiving messages
 Represented by a thin, long rectangular
box overlaid onto a lifeline

25. Control Information
 Condition
 syntax: ‘[‘ expression ’]’ message-label
 The message is sent only if the
condition is true
[ok] borrow(member)

26. 1) Set the context.
a) Select a use case.
b) Decide the initiating actor.
Steps for Building a Sequence Diagram

27. Steps for Building
a Sequence Diagram
1. Set the context
2. Identify which objects and actors will participate
3. Set the lifeline for each object/actor
4. Lay out the messages from the top to the bottom
of the diagram based on the order in which they
are sent
5. Add the focus of control for each object’s or
actor’s lifeline
6. Validate the sequence diagram

28. Iterate
Iterate the Model
• Keep on doing this until
you, your customer,
and your engineers are
happy with the model

29. 2) Identify the objects that may participate in the
implementation of this use case by completing
the supplied message table.
a) List candidate objects.
1) Use case controller class
2) Domain classes
3) Database table classes
4) Display screens or reports
Steps for Building a Sequence Diagram

30. Steps for Building a Sequence Diagram
2) Identify the objects (cont.)
b) List candidate messages. (in message analysis
table)
1) Examine each step in the normal scenario
of the use case description to determine
the messages needed to implement that
step.
2) For each step:
1) Identify step number.
2) Determine messages needed to complete this
step.
3) For each message, decide which class holds
the data for this action or performs this action
3) Make sure that the messages within the table

31. Steps for Building a Sequence Diagram
2) Identify the objects (cont.)
c) Begin sequence diagram construction.
1) Draw and label each of the identified actors and
objects across the top of the sequence diagram.
2) The typical order from left to right across the top is the
actor, primary display screen class, primary use case
controller class, domain classes (in order of access),
and other display screen classes (in order of access)
3) Set the lifeline for each object/actor

32. 4) Lay out the messages from the top to the
bottom of the diagram based on the order
in which they are sent.
a) Working in sequential order of the message
table, make a message arrow with the message
name pointing to the owner class.
b) Decide which object or actor initiates the
message and complete the arrow to its lifeline.
c) Add needed return messages.
d) Add needed parameters and control
information.
Steps for Building a Sequence Diagram

33. 5) Add the focus of control (activation box) for
each object’s or actor’s lifeline.
6) Validate the sequence diagram.
Steps for Building a Sequence Diagram

34. Summary
• Sequence diagrams model object interactions with an
emphasis on time ordering
• Method call lines
– Must be horizontal!
– Vertical height matters!
“Lower equals Later”
– Label the lines
• Lifeline – dotted vertical line
• Execution bar – bar around lifeline when code is running
• Arrows
– Synchronous call (you’re waiting for a return value) – triangle
arrow-head
– Asynchronous call (not waiting for a return) – open arrow-head
– Return call – dashed line