The best way to understand a interaction overview diagram notation is to think of it as an activity diagram, except instead of an action, a complete interaction is described by using its own diagram, as shown in Figure 10-1.

Any number of participants may be involved in the interactions that occur within the overview. To see which participants are involved across the entire overview, a lifelines subtitle is added to the diagram title, as shown in Figure 10-2.

Figure 10-1. Individual interactions are placed on an interaction overview diagram as though they were actions on an activity diagram

Figure 10-2. The lifelines subtitle shows the combined list of participants involved in the interactions within the overview

Similar to an activity diagram, the interaction overview begins with an initial node and ends with a final node. Control flows between these two nodes and passes through each of the interactions in between. However, you are limited not to just a simple sequential flow between the interactions.

Just as control flow on an activity diagram can be subjected to decisions, parallel actions, and even loops, so can the control flow on an interaction diagram, as shown in Figure 10-3.

That’s all the new notation for interaction diagrams; now it’s time to look at a practical example. To set the stage, we are going to develop an interaction overview diagram from scratch for the Create a New Regular Blog use case reusing parts from the interaction diagrams created in the previous chapters.

The big difference between the example in this chapter and the modeling in previous chapters is that with an interaction overview, we can pick and choose from the different interaction diagram types. By using an interaction overview approach, each part of the interaction is modeled using the techniques that are most effective for that part.

Pulling Together the Interactions

First, we need to decide how the interaction overview will be broken down into the most effective diagrams for each of the individual interactions, as shown in Figure 10-4.

When modeling the Select Blog Account Type, Create Regular Blog Account, and Tidy Up Author Details interactions, message order is more important than any other factor. The relevant steps can be reused from the sequence diagrams modeled in Chapter 7, as shown in Figure 10-5.

For variety’s sake, the Enter Author Details will be displayed as a communication diagram, as shown in Figure 10-6.

You could decide to represent the Enter Author Details interaction as a communication diagram simply because it is easier to understand, but there is so much similarity between sequence and communication diagrams that mixing the two on one interaction overview is not often seen; modelers tend to prefer one type of diagram or the other.

The Check Author Details interaction must enforce the timing constraint that its messages will all be executed within five seconds (see Chapter 9). The focus is on timing for this part of the overview and thanks to the fact that an interaction overview can contain any of the different types of interaction diagram, a timing diagram can be employed for the Check Author Details interaction, as shown in Figure 10-7.

Figure 10-3. Starting with the initial node, the control flow executes Interaction1, followed by Interactions 2 and 3 in parallel; Interaction4 will execute only if the condition is assessed as being true; otherwise, Interaction5 is executed 10 times in a loop before the control flow merges and the final node is reached

Figure 10-4. All three types of interaction diagram are used in this overview—sd indicates a sequence diagram, cd is for a communication diagram, and, not surprisingly, td stands for a timing diagram

Tip

An interaction overview diagram can offer an ideal place to use the alternative timing diagram notation, as shown in Figure 10-7. Since interaction overviews can get pretty big, the alternative notation often works best since it makes good use of the space available.

Figure 10-5. Some interactions to are best modeled using sequence diagrams to focus on message ordering

Figure 10-6. Enter Author Details in a communication diagram

Figure 10-7. Adding a timing diagram to show critical timing constraints for one interaction within the overview

Now that all the interactions have been added to the interaction overview, all of the participants involved are known so we can add their names to the diagram’s title, as shown in Figure 10-8.

Figure 10-8. Adding each of the participants involved in an interaction to the lifeline list in the interaction overview’s title bar

Interaction overview diagrams glue together combinations of sequence diagrams, communication diagrams, and timing diagrams, showing the higher-level picture. At this point, you’re done considering interaction diagrams, but you may want to back up and review any of the interaction diagram chapters if you weren’t clear when to use which diagram type. Sequence diagrams were covered in Chapter 7; communication diagrams were described in Chapter 8; timing diagrams were covered in Chapter 9.

Figure 10-9. Starting with an initial node and finishing with a final node, the flow of control is the thread that ties each of the interactions together