Transition Links

Transition links represent an activity diagram's control flow. They connect nodes in which activities occur, such as actions, decisions, subdiagrams, and iterators, and connect these nodes to start and completion symbols. Transition links can contain business object probes that monitor business object instance values.

Note:
In activity diagrams, transition links do not represent data flow. Data passes from node to node when one node sets a variable and another accesses the variable. The activity diagram's data-flow mechanism is comparable to that of a class or program, in which code sets a variable that other code uses. This model differs from the one used by event-passing modeling tools, which show data moving along links.

Process Designer Express provides both orthogonal and free-form transition links. Use the orthogonal links whenever possible. Use the free-form links when you cannot get the desired shape from the orthogonal links. Right-click a link to see a context menu that shows the orthogonality of the link. Use this context menu to toggle a link between orthogonal and free form.

How many links can coexist?

Table 23 displays the number of incoming and outgoing links that different types of nodes can have.

Table 23. Permitted incoming and outgoing links by node type
Node type Incoming links Outgoing links
Action Unlimited One
Decision One Seven
Subdiagram or iterator Unlimited One

Creating a transition link

To create a transition link, the two symbols that you want to connect must be available on the workspace.

To add a transition link to an activity diagram:

  1. In the Symbols toolbar, click the Transition Link button.
  2. In the workspace, click the bottom edge of the symbol where you want the transition link to start.
  3. Click the top edge of the symbol where you want the transition link to end.

Process Designer Express lets you place valid connections between symbols. It does not permit you to link two symbols for which a link is invalid. Rather than displaying an error message, Process Designer Express does not permit the invalid transition link to be made.

Process Designer Express indicates whether an attempt to place a transition link on a symbol is valid. When you position the mouse pointer (with the link) on the edge of a symbol, the mouse pointer changes to a plus sign within a circle if the connection is valid. You can then click and place the connection on the symbol. If the connection is not valid, the mouse pointer does not change to a plus sign and you cannot place the connection.

Cancelling a link

Abort or cancel a transition link by pressing the Escape (ESC) key. Each press of the ESC key undoes the last segment of the connection line. Using the ESC key is the only way to cancel a connection attempt for which there is no valid symbol in the activity diagram.

For example, suppose you have placed two symbols, a start symbol and an end symbol, in a diagram. You then select a transition link and click the start symbol. The transition-link line segment appears, connected to the start symbol. However, there are no valid symbols or ports to which you can connect the transition-link line. At this point, pressing the ESC key is the only way to cancel the connection activity and continue the editing session.

Defining transition link properties

After the transition link appears in the activity diagram, you can define its properties in the Link Properties dialog (see Figure 29).

Figure 29. Link Transition Properties dialog box


The Link Properties dialog identifies the transition link with a name at the top of the dialog in the following format:

LinkTransition_UID
 

where UID is the unique identifier for the link.

The properties of a transition link include its label, definition, and whether it contains a business object probe.

To define transition link properties:

  1. Display the Link Properties dialog.

    You can display this dialog in any of the following ways:

    The Link Properties dialog displays. Figure 29 shows a sample Link Properties dialog.

  2. Optionally, specify the label and description for this transition link.

    For more information on link labels, see "Labeling a link".

  3. Click Apply to save the link properties.

Labeling a link

Link labels can be instrumental in ensuring the readability of the activity diagram. Try to capture the decision logic in the same way that you would when labeling a decision node in a flow chart. Logically named link labels explain the scenario flow. For example:

To label a transition link, enter the text of the link label in the Label box of the Link Properties dialog. The label appears in the activity diagram exactly as it does in the text box. Use carriage returns in the text box to break a label into multiple lines so that it does not overlap other links.

Using business object probes

A business object probe monitors business object instance values during runtime. The probe is placed on a transition link during the creation of an activity diagram, and is activated or deactivated during runtime through System Manager's Collaboration Properties dialog box.

Note:
Business object probes cannot be used on the incoming transition link for a decision node or on a service call link.

By default, a business object probe appears as a red square on the transition link in an activity diagram. In Figure 30, the Default branch link contains a business object probe.

Figure 30. Transition link with business object probe


You can use a business object probe to monitor any business object specified in the Ports and Triggering Events tab of the Template Definitions window. For each business object, you can choose the specific attributes you want to monitor. All of the instance values for these attributes are presented in a report provided by System Monitor.

Perform the following tasks to add a business object probe:

  1. Ensure that Process Designer Express is open and that the activity diagram is displayed.
  2. Right-click the transition link to which you want to add the business object probe.
  3. From the context menu, click Properties. The Link Transition Properties dialog box opens.
  4. Click the Enable Business Object Probe checkbox.
  5. Click the plus sign (+) next to the business object you want to monitor to display that object's list of attributes.
  6. Select the specific attributes you want to monitor.
  7. Click Apply to save your changes.
  8. During runtime, use System Manager to enable or disable the business object probe as needed.

Modifying a transition link

You can disconnect and reconnect transition links as well as modify the appearance of the transition-line segments.

Note:
Modifying line segments does not apply to orthogonal links.

To ensure that transition-line segments have only right angles, hold down the Shift key while creating a new line segment.

Copyright IBM Corp. 2003, 2004