• Purpose:
  • Represent dynamic interactions between objects/classes
• Focus:
  • The message (i.e., method call) sequence

• Purpose:
  • Represent objects, roles, or classes over time and consist of a rectangle (representing the object/class) and a dashed line (representing time)
• Visualization:
  • 
• Notes:
  • The class of the object can be omitted
  • The name/identifier can be omitted (in which case the instance is said to be anonymous/unnamed)
  • Instances should be underlined and roles/classes should not be (but this can be difficult to achieve with some tools)

• Purpose:
  • The sender waits for the receiver to process the message
• Visualization:
  • 
• Notes:
  • The return need not be explicit
  • The activation/focus of control need not be included
  • The sequence number need not be included
  • Sequence numbers need not be hierarchical

• Purpose:
  • The sender continues executing immediately after the message is sent
• Visualization:
  • 
• Notes:
  • Asynchronous messages do not have returns (in the normal sense)

• Defined:
  • A significant amount of time relative to the process of interest
• Visualization:
  • 
• Notes:
  • Delays should only be modeled when it is important to do so
  • Always check carefully for potential synchronization problems! (For example, what problems might arise in this example?)

• Creation:
  • Use a Create Message
• Destruction:
  • Use a Destroy Message
• Example:
  • 

• An object can send itself a message
  • 
• Messages can be recursive
  • 

• Found Messages have a sender that is not modeled
  • 
• Lost Messages have a receiver that is not modeled
  • 

• Purpose:
  • Used to group sets of messages together
• Some Types:
  • alt (Alternatives like: if-then, switch)
  • opt (Optionals like: if)
  • loop
  • par (Parallel Execution)
  • assert (Assertions)
• Example:
  • 

• Purpose:
  • Represent dynamic interactions between objects/classes
• Notes:
  • Are functionally equivalent to sequence diagrams
  • Are better at handling large numbers of objects because the vertical dimension is not used to represent time
  • Require sequence numbers to represent time

An Example (Centralized Control)