Interaction Diagramming Notes

Interaction diagrams, a subset of behavior diagrams, emphasize the flow of control and data among the things in the system being modeled. For example, the sequence diagram shows how objects communicate with each other regarding a sequence of messages.

UML has four types of interaction diagrams:

• Sequence diagram
• Communication diagram
• Interaction overview diagram
• Timing diagram

Interaction overview diagram

Interaction Overview Diagram is one of the fourteen types of diagrams of the Unified Modeling Language (UML), which can picture a control flow with nodes that can contain interaction diagrams.

The interaction overview diagram is similar to the activity diagram, in that both visualize a sequence of activities. The difference is that, for an interaction overview, each individual activity is pictured as a frame which can contain a nested interaction diagram. This makes the interaction overview diagram useful to “deconstruct a complex scenario that would otherwise require multiple if-then-else paths to be illustrated as a single sequence diagram”.

The other notation elements for interaction overview diagrams are the same as for activity diagrams. These include initial, final, decision, merge, fork and join nodes. The two new elements in the interaction overview diagrams are the “interaction occurrences” and “interaction elements.”

Sequence diagram

A sequence diagram or system sequence diagram (SSD) shows process interactions arranged in time sequence in the field of software engineering. It depicts the processes involved and the sequence of messages exchanged between the processes needed to carry out the functionality. Sequence diagrams are typically associated with use case realizations in the 4+1 architectural view model of the system under development. Sequence diagrams are sometimes called event diagrams or event scenarios.

For a particular scenario of a use case, the diagrams show the events that external actors generate, their order, and possible inter-system events. All systems are treated as a black box; the diagram places emphasis on events that cross the system boundary from actors to systems. A system sequence diagram should be done for the main success scenario of the use case, and frequent or complex alternative scenarios.

A sequence diagram shows, as parallel vertical lines (lifelines), different processes or objects that live simultaneously, and, as horizontal arrows, the messages exchanged between them, in the order in which they occur. This allows the specification of simple runtime scenarios in a graphical manner.

A system sequence diagram should specify and show the following:

• External actors
• Messages (methods) invoked by these actors
• Return values (if any) associated with previous messages
• Indication of any loops or iteration area

Communication diagram

A communication diagram in the Unified Modeling Language (UML) 2.0, is a simplified version of the UML 1.x collaboration diagram.

A Communication diagram models the interactions between objects or parts in terms of sequenced messages. Communication diagrams represent a combination of information taken from Class, Sequence, and Use Case Diagrams describing both the static structure and dynamic behavior of a system.

However, communication diagrams use the free-form arrangement of objects and links as used in Object diagrams. In order to maintain the ordering of messages in such a free-form diagram, messages are labeled with a chronological number and placed near the link the message is sent over. Reading a communication diagram involves starting at message 1.0, and following the messages from object to object.

Communication diagrams show much of the same information as sequence diagrams, but because of how the information is presented, some of it is easier to find in one diagram than the other. Communication diagrams show which elements each one interacts with better, but sequence diagrams show the order in which the interactions take place more clearly.

Timing diagram

A timing diagram in the Unified Modeling Language 2.0 is a specific type of interaction diagram, where the focus is on timing constraints.

Timing diagrams are used to explore the behaviors of objects throughout a given period of time. A timing diagram is a special form of a sequence diagram. The differences between timing diagram and sequence diagram are the axes are reversed so that the time increases from left to right and the lifelines are shown in separate compartments arranged vertically.

There are two basic flavors of timing diagram: the concise notation, and the robust notation.