UML Sequence Diagrams

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UML Sequence Diagrams

• MIE456 - Information Systems Infrastructure II
• Vinod Muthusamy
• November 19, 2004

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UML diagrams

• Static models
• Class diagram
• Describe types of objects and their relationships
• Static model
• But software is about behaviour which is dynamic
• Dynamic models
• Show interactions between components
• Dont show software structure or dependencies as
  clearly

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Dynamic models

• 3 types of dynamic models
• State diagram
• Collaboration diagram (type of interaction
  diagram)
• Sequence diagram (type of interaction diagram)
• State diagram
• Describe how state changes in response to events
• State is the properties of an object
• Interaction diagrams
• Show flow of messages between objects
• Useful for describing procedural flow, finding
  race conditions
• Collaboration and sequence diagrams show same
  information
• Collaboration diagrams emphasize roles and their
  interactions
• Sequence diagrams emphasize time (useful for
  real-time applications)

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Running example

• Cellular phone
• Will show
• Static UML model
• Class diagram
• Dynamic UML models
• Collaboration diagram
• Sequence diagram

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Class diagram

• Dialer gather digits, emit tones
• Cellular Radio communicate with cellular network
• Button, Speaker, Microphone, Display hardware

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The dynamic interactions

• Dynamic models are based on a use case
• Use case Make Phone Call
• User presses the digit buttons to enter the phone
  number.
• For each digit, the display is updated to add the
  digit to the phone number.
• For each digit, the dialer generates the
  corresponding tone and emits it from the speaker.
• User presses Send
• The in use indicator is illuminated on the
  display
• The cellular radio establishes a connection to
  the network.
• The accumulated digits are sent to the network.
• The connection is made to the called party.

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Collaboration diagram

• Objects with names
• Note 2 Button objects
• Arrows are messages
• Sequence numbers tell order
• Links must correspond to class diagram

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Sequence diagram
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Sequence diagram syntax

• Vertical axis time (increases down)
• Horizontal axis components (no order)
• Objects or classes
• Rectangles across the top
• Class name MyClass
• Object name MyObjectMyClass
• Anonymous object MyClass
• Lifeline of an object
• Dashed vertical line from object
• The time an objects exists

• Messages
• Arrows between objects
• Messages ordered vertically (no need for sequence
  numbers)
• Procedure call, RMI, JDBC,
• Activations
• Hollow vertical rectangles
• Show duration of execution
• Directly or by subordinate
• Shows call stack
• Return message
• Implicit at end of activation
• Explicit with a dashed arrow
• Iterations
• Rectangle to enclose messages in iteration
• Looping condition at bottom

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Object lifetimes and asynchronous messages

• Object creation
• Message arrow pointing to object rectangle
• Object destruction
• A large X
• Can be in response to a message
• Object can destroy itself at the end of an
  activation
• Asynchronous messages
• Half arrow
• No return value expected
• Activation of caller may end before return
  message
• Activation of callee continues
• End of callee activation does not imply return
  message
• Nicely shows concurrent interactions
• Hard to show in Collaboration diagram

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Race conditions
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Race conditions (2)

• E.g. an object receives two messages
• Order of arrival changes behaviour
• Only one order leads to correct behaviour
• CellularRadio expects Answer() not Connect(pno)
• Two states when Send can be pressed
• To make outgoing call (after dialling digits)
• To answer incoming call
• State diagram can be useful here
• To help realize there is a race condition
• To specify what should happen
• Angled arrows
• Show message delay
• Can show race conditions clearer

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Timing constraints

• Useful in real-time applications
• May be useful to specify race condition behaviour
• Two ways to specify
• Note message labels

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Linking sequence diagrams

• Use dangling arrows
• Notes clarify other end of dangling arrow
• Try to group common sub-sequences together

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More syntax

• Actor
• Conditional
• Specify gt 1 case
• Branch messages
• Label arrows
• Note does not imply concurrent processing
• Concurrent lifelines
• For conditionals
• For concurrency
• Recursion

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Some notes for Milestone 2

• Use proper syntax for object/class names, message
  arrows, etc.
• Show full lifelines (including possible creation
  and destruction) and activations
• Show all message names, and parameters
• Show explicit return
• Try to keep message flow left to right (not
  always possible in complex applications)
• Add notes for clarity if necessary
• Learn to use an appropriate tool to draw the
  diagrams

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Conclusions

• Sequence diagrams show interactions of a use case
• A visual coding technique
• Make time explicit
• Very useful for real-time applications
• Often iterate between static and dynamic models
  during design phase
• E.g. sequence diagrams can help identify new
  responsibilities for a class or need for a new
  class (changes class diagrams)

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References

• Figures are from
• Object Mentor UML articles
• UML Class Diagrams by Robert C. Martin
• UML Collaboration Diagrams by Robert C. Martin
• UML Sequence Diagrams by Robert C. Martin
• http//www.objectmentor.com/resources/listArticles
  ?keytopictopicUML
• OMG Unified Modeling Language, v1.5 Specification
• http//www.omg.org/technology/documents/formal/uml
  .htm