Behavioral Diagrams

Use Case Diagrams

The first diagram to look at is the Use Case diagram. The main ingredients for this type of diagram are use cases and actors, together they define the roles that users can play within a system. They are associated to the tasks, or use cases, they are involved in. It is often used in early stages of the design process to collect the intention requirements of a project.

If you are not well-acquainted with UML yet, remember that a use case is not just a bubble noted in the diagram. Along with this bubble, there should be a description of the use case, a typical scenario, exceptional cases, preconditions etc. These can either be expressed in external texts using regular text processing tools, requirements tools or file cards. It can be and is often refined using other diagrams like a sequence diagram or an activity diagram that explain its main scenarios. The basic description of a use case can also be inserted in the Documentation tab of the Details pane.

Extension points can be added to a use case via the extension point button in the Details pane or via the extension point rapid button.

Sequence Diagrams

A sequence diagram is an easily comprehensible visualization of single scenarios or examples of business processes with regard to their behavior in time. It focuses on when the individual objects interact with each other during execution. The diagram essentially includes a timeline that flows from the top to the bottom of the diagram and is displayed as a dotted line. The interaction between objects is described by specifying the different kinds of messages sent between them. Messages are called stimuli. They are displayed as arrows; the diverse arrowheads stand for different kinds of messages (see below).

The following diagram shows a typical example:

Objects

After creating or changing objects, they are automatically arranged in the Diagram pane. You can specify which of the objects is to have control by enabling the corresponding check box 'Focus of control' in the Properties tab. Afterwards, you'll see how the graphical representation of this object changes: it gets a thick border and its lifeline is no longer a dashed line but a solid rectangular area.

Self messages

In Poseidon, all stimulus types, except create stimuli, can be created as self stimuli. In the case of a stimulus to itself, the arrow starts and finishes on the object's lifeline. A self stimulus is created by dropping the stimulus target point on the source object.

Selecting an operation

A call stimulus is regarded as a procedure call and can be connected with any operation provided by the receiving object, depending on its type. This is achieved by connecting the stimulus with an action that will cause the class operation to be called. The following two figures show an example for selecting an operation and attaching actual arguments to the call.

After selecting the stimulus in the diagram, the Details pane shows the properties of the stimulus. It is there that you have to open the dispatched action field displayed in the Details pane, which is directly below the name field of the stimulus. This causes the Details pane to change the view to the properties of the action.

The properties of the action allow you to select an operation and edit the arguments attached to the procedure call. The set of possible operations includes all operations of the receiving object's class, as well as any operations inherited from direct and indirect superclasses or interfaces. If an operation is selected, the name of the action is updated according to the name of the operation and the given values of the arguments. An empty argument value is displayed as an 'x'. Keep in mind that you cannot edit the name field while an operation is selected.

Activations

An activation shows the period of time during which an object will perform an action, either directly or through a subordinate procedure. It is represented as a tall thin rectangle with the top aligned with its point of initiation and the bottom aligned with its point of completion.

Now, let's consider how Poseidon deals with starting and terminating activations. When an object receives a stimulus, an activation is created that starts at the tip of the incoming arrow. When an object sends a stimulus, an existing activation is terminated at the tail of the outgoing arrow. There are two exceptions: First, an outgoing send stimulus does not terminate an existing activation, because it represents an asynchronous message. Second, if an object has explicitly set the focus of control, its activation will continue during the whole lifetime.

Collaboration Diagrams

Collaboration diagrams are another means for representing the interactions and relationships between objects. Unlike sequence diagrams, however, they do not focus on the timeline of interaction, but on the structural connections between collaborating objects. Of central interest are the messages and their intent, when creating a collaboration diagram. The chronological order of messages is represented by numbers preceding each message.

Statechart Diagrams

Business process models do not lend themselves to implementation in an object-oriented way. If you go the UML way, you will break down the business process and express it in terms of states for each object involved in the process. State diagrams are used to illustrate the possible states of an object and the causes and effects of those state changes within the object.

Let's take a short look at the States themselves. In the diagram toolbar you find three different symbols:

• State

  In a state diagram, each state has at least two compartments, the top one always keeping the name of the state. The name usually is an adjective describing the recent object.

  The states properties are a lot more meaningful and complex than they are in the activity diagrams. Not only does a state have ingoing and outgoing transitions, but also different actions or activities that are to be taken with it.

• Composite State

  Composite States make visual use of the second compartment that encloses refinements of the given state. Enclosed states don't have to have an initial state. Ingoing as well as outgoing transitions might be connected directly to one of them. When the corresponding object is in the composite state, it is exactly in one of the sub-states (OR relation).

  If you find yourself needing to change a simple state to a composite state, you have to delete the former and again add the new state via the toolbar.

• Concurrent State

  Concurrent States are, like the above, refinements; therefore, they are focused in the second compartment. When the corresponding object enters the concurrent state, all initial sub-states all are enabled at once (AND relation).

Editing States

Actions and activities are added from within the state's Property tab using the 'Add' button. Whenever you add an action or activity, Poseidon will automatically navigate to the new element. Upon return to the parent element, the 'Add' button will no longer be visible, rather it will be the 'Delete' button, indicating that the element has been successfully added.

To remove an existing element, use the 'Delete' button. Note that this will delete the element entirely from the model.

Some items within the Property tab will appear in blue - these are in effect hyperlinks to the element. Clicking on them will navigate to the element named.

Editing Actions and Activities

Actions and Activities are elements in themselves, just like any other part of the model. As such, editing them is done in the same way as any other element - just like an attribute is a part of a class.

For more information about the specific fields, see the Element section the Section called States in Chapter 12.

Activity Diagrams

Activity diagrams are often used to model business processes. They simply and quite plainly show how things work, and so function as a good aid to discussions of aspects of the workflow with the domain experts. These are less abstract than the often used object-oriented state diagrams.

The following example shows an activity diagram that depicts the rules and the process of paying an order. In the following example, Softsale will not accept an order if you have overdue payments open, will only allow payment by invoice if your e-mail and home address have been verified, and a few other rules. Take a closer look for yourself in order to become more familiar with the notation.