Precise Use Cases - Part 2

David Gelperin, LiveSpecs Software

5. A sample PUC

The following example is derived from a specification appearing in [22]. Additional examples can be found in [10].

Case Name: Get [new] Seat on Reserved Flight

Risk Factors: Frequency of occurrence: 0 to 2 times per reservation

Impact of failure: likely case - low, open seating is a workaround

worst case - medium, open seating in a large plane with many expensive seats is likely to anger important passengers

Case Conditions:

Invariants: None

Preconditions:For reservation system, status is active

For passenger, system access status is signed on

Interactions

Basic course:

Passenger	Web-based Airline Reservation System
1. requests seat assignment	2. requests a reservation locator
3. provides a (corrected) reservation locator alternative	4. searches for reservation
Until (reservation located or all reservation locator strategies tried), repeat 3 to 4
	5. offers seating alternatives, unless a. reservation not located or b. seat previously assigned or c. no seats are available or d. no seats are assignable
6. selects a seating alternative	7. assigns selected seat unless a. no seating alternative selected
	8. If (For reservation, seat previously assigned) Post-conditions -- For flight, previously assigned seat is available Endif
	9. confirms assignment SUCCESS EXIT

Basic course Conditions

Invariants:

For reservation system, status is active

For passenger, system access status is signed on

For passenger & flight, a reservation exists and can be located

Pre-conditions:

For flight, some seats are assignable

Passenger wants to get or change seat assignment

Post-conditions:

For flight, seating alternative was selected

For reservation, selected seat is assigned

Alternative Courses:

Exception Handlers (EH):

EH 1 - 5a (reservation not located)

EH1 Invariants:

For reservation system, status is active

For passenger, system access status is signed on

For passenger, reservation not located

 

Passenger	Web-based Airline Reservation System
	1. Offers help making reservation FAILURE EXIT

EH 2 - 5b (seat previously assigned)

EH2 Invariants:

For reservation system, status is active

For passenger, system access status is signed on

For passenger & flight, a reservation exists and can be located

For reservation, seat previously assigned

Passenger	Web-based Airline Reservation System
	1. offers to change seat assignment
2. wants a. to change seat assignment b. no change	3. If (Passenger wants to change seat assignment), CONTINUE Else, provides help and SUCCESS EXIT

EH3 - 5c or 5d (no seats are available or assignable)

EH3 Invariants:

For reservation system, status is active

For passenger, system access status is signed on

For passenger & flight, a reservation exists and can be located

For flight, no seats are assignable

Passenger	Web-based Airline Reservation System
	1. If (check in), places passenger on standby Post-cond -- For passenger, name on standby list Else, advises when assignment will be possible Endif FAILURE EXIT

EH4 - 7a (no seating alternative selected)

EH4 Invariants:

For reservation system, status is active

For passenger, system access status is signed on

For passenger & flight, a reservation exists and can be located

For flight, some seats are assignable

For reservation, no seating alternative selected

Passenger	Web-based Airline Reservation System
	1. If (For reservation, seat previously assigned) SUCCESS EXIT Endif
	2. If (day of flight), advises to get assignment at check in Else, advises to try later Endif FAILURE EXIT

Figure 1. Example of the reserved seat functionality in a web-based airline reservation system

6. Comments on the example

The use of explicit exception conditions assures that each exception handler has a visible point of origin. A point of origin is marked by an individual exception condition preceded by the keyword unless. For example in step 7 of the basic course, some alternative must be selected before the system can assign a seat. An exception handler must deal with the situation when no seat is selected. A point of origin may be in any course of action. In the example above, steps 5 and 7 in the basic course contain points of origin, while the exception handlers do not.

Precise use cases encourage the explicit identification of valid input alternatives. When there are only a few simple alternatives, they may be provided directly within the step as they are in the second step of exception handler 2. Otherwise, the alternatives should be specified in an associated glossary of input alternatives. The alternatives referenced in step 3 of the basic course would be found in such a glossary.

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