You should already have a short, step-by-step description of the use-case flow of events. This is created in the Activity: Find Actors and Use Cases. Use this step-by-step description as a starting point, and gradually make it more detailed.
Decide on the following points before describing the use cases so that you are consistent across use cases:
"The actor chooses one of the following, one or more times:
a) . . .
b) . . .
c) . . ."
Concentrate on describing what is done in the use case, not how specific problems internal to the system should be solved. When working with object models, you may have to complement the description with details about how things work, so do not make the description overly detailed at this point. Describe only what you believe will be stable later on.
If a use case's flow of events has become too encompassing or complex, or if it appears to have parts that are independent of one another, split it into two or more use cases.
When you write the descriptive text, refer to the glossary. As fresh terms evolve from new concepts, include them in the glossary. Do not change the definition of a term without informing the appropriate project members.
The Content of a Flow of Events Description
A flow of events description explores:
"The use case can start when the function 'Administer Order' is activated by a user."
"To create a new order, the user activates the function 'New' and then specifies the following mandatory data concerning the order: name, network elements (at least one), and type of measurement function. Optional data can also be specified concerning the order: a comment (a small textual description). The user then activates the function 'Ok,' and a new order is created in the system."
Note: You must be explicit regarding the data exchanged between the actors and the use case; otherwise, the customer and the users will probably not understand the use-case description.
"The user activates the function 'Modify' to modify an existing order, and specifies an order number (small integer). The system then initializes an order form with the name of the order, its network elements, and its type of measurement function. This data is retrieved from a secondary storage device."
"The use case ends when the function 'Exit' is activated by the Orderer."
You should also describe odd or exceptional flows of events. An exceptional flow is a subflow of the use case that does not adhere to the use case's normal or basic behavior. This flow may nevertheless be necessary in any complete description of the use case's behavior. A typical example of an exceptional flow is the one given in the first example. If the use case receives some unexpected data (that the actor is not the one expected in that particular context) it terminates. Having the wrong actor and terminating prematurely are not in the typical flow of events.
Other "do's and don'ts" to consider when you describe a use case include:
A use case's flow of events can be divided into several subflows. When the use case is activated the subflows can combine in various ways if the following holds true:
Part of the description of the use case Withdraw Money in an automated teller machine system could be "The amount of money the client wants to withdraw from the account is compared to the balance of the account. If the amount of money exceeds the balance, the client is informed and the use case terminates. Otherwise, the money is withdrawn from the account."
You must describe all these optional or alternative flows. It is recommended that you describe each subflow in a separate supplement to the Flow of Events section, and should be mandatory for the following cases:
If a subflow involves only a minor part of the complete flow of events, it is better to describe it in the body of the text.
"This use case is activated when the function 'administer order' is called for by either of the actors Orderer or Performance Manager Administrator. If the signal does not come from one of these actors, the use case will terminate the operation and display an appropriate message to the user. However, if the actor is recognized, the use case proceeds by....."
You can illustrate the structure of the flow of events with an activity diagram.
For more information, see Guidelines: Use Case, structure of the flow of events.
Create use-case diagrams showing the use case and its relationships to actors and other use cases. A diagram of this type functions as a local diagram of the use case, and should be related to it. Note that this kind of local use-case diagram is typically of little value, unless the use case has use-case relationships that need to be explained, or if there is an unusual complexity among the actors involved.
For more information, see Guidelines: Use-Case Diagram.
Any requirements that can be related to the use case, but that are not taken into consideration in the Flow of Events of the use case, should be described in the Special Requirements of the use case. Such requirements are likely to be nonfunctional.
For more information, see Guidelines: Use Case, special requirements.
Develop a communication protocol if the actor is another system or external hardware. The description of the use case should state if some existing protocol (maybe even a standardized one) is to be used. If the protocol is new, you must fully describe it during object-model development.
A precondition on a use case explains the state the system must be in order for the use case to be possible to start.
In order for an ATM system to be able to dispense cash, the following preconditions must be satisfied:
These would all be valid preconditions for the use case Dispense Cash.
Take care to describe the system state; avoid describing the detail of other incidental activities that may have taken place prior to this use case.
Preconditions are not used to create a sequence of use cases. There will never be a case where you have to first perform one use case, then another, in order to have a meaningful flow of events. If you feel there is a need to do this, it is likely that you have decomposed the use-case model too much. Correct this problem by combining the sequentially dependent use cases into a single use case. If this makes the resulting use case too complex, consider techniques for structuring use cases, as presented in Structure the Flow of Events of the Use Case above, or in the Activity: Structure the Use-Case Model.
For more information, see Guidelines: Use Case, Preconditions and Postconditions.
A postcondition on a use case lists possible states the system can be in at the end of the use case. The system must be in one of those states at the end of the execution of the use case. It is also used to state actions that the system performs at the end of the use case, regardless of what occurred in the use case.
If the ATM always displays the ‘Welcome’ message at the end of a use case, this could be documented in the postcondition of the use case.
Similarly, if the ATM always closes the customer’s transaction at the end of a use case like Withdraw Cash, regardless of the course of events taken, that fact should be recorded as a postcondition for the use case.
Postconditions are used to reduce the complexity and improve the readability of the flow-of-events of the use case.
Under no circumstances should postconditions be used to create a sequence of use cases. There should never be a case where you have to first perform one use case, then another, in order to have a meaningful flow of events. If you feel a need to do this, the sequentially dependent use cases should be combined into a single use case. If this makes the combined use case too complex, consider techniques for structuring use cases, as presented in Structure the Flow of Events of the Use Case above, or in the Activity: Structure the Use-Case Model.
For more information, see Guidelines: Use Case, Preconditions and Postconditions.
If the use case is to be extended by another use case, you need to describe what the extension points are (see Guidelines: Use Case, extension points).
Review and discuss the use case with the stakeholders, so that they have a clear understanding of the use case and agree on its description.
The use-case description is complete only when it describes everything the use case performs, implements, or otherwise allows from beginning to end. Before you finish, check that the use case exhibits the properties that characterize it as a "good" use case. See checkpoints for use cases and use-case reports in Activity: Review Requirements.