The Dynamic Model

In the functional specification scenarios are simple scripts that describe typical "conversations" between actors and the system.

Scenarios as Scripts

These scripts are easy for all stakeholders to understand, but aren't precise enough to specify how the use cases should be implemented.

The dynamic model uses sequence and/or activity diagrams to specify use case scenarios.

Scenario as sequence diagram

Sequence diagrams show the interactions between collaborating objects. These objects can be actors and use cases.

Begin by reviewing the Sequence Diagrams lecture.

Here's the main scenario for transfer funds:

Here's the insufficient funds scenario:

In theory we could combine these two scenarios into a single diagram using an alternate fragment:

Keep in mind that all stakeholders should be able to understand these scenarios, so multiple simple sequence diagrams is better than a single complicated one.

Scenarios as a statechart diagram

The ultimate elaboration of a use case is done using a statechart diagram.

First review the Statechart Diagram lecture.

In a use case elaboration states are usually wait states: the system is waiting for a user input. Transitions are triggered by the arrival of the input. Here's a typical transition in a use case elaboration:

Upon entering the "wait for input 1" state input 1 is requested. When it arrives the statechart machine transitions to the "wait for input 2" state is input 1 is valid. For variety, the diagram shows the request for input 2 as an effect labeling the transition rather than an entry action.

Here's a statechart diagram for the transfer funds use case: