Handling Conflicts in Aspectual Requirements Compositions - Transactions on Aspect-Oriented Software Development - page 154

4.1 Illustrative Example
Let us consider the following illustrative example: to use the subway each client must
possess a card in which was credited a certain amount of money. Each card can be
bought and credited at selling machines that are available at subway stations. To start
a journey, a passenger should use his/her card on an entrance machine. This machine
validates the card and allows the owner of a valid title to initiate a trip. In a similar
way, when a passenger ends her/his journey, s/he inserts the card in an exit machine.
At this moment the price to pay is calculated and debited from card. The price will
depend on the distance travelled. If the card does not have enough credit to pay for the
trip, then it must be credited at any of the available machines in the subway. The
client can request a refund of the money contained in his/her card in a selling
machine.
The application of the AORA method to this example results in a list of concerns,
each one described using the template in Table 1, and a list of match points with the
priorities of each concern given by each stakeholder. Note that the identification of
the these concerns, their description in a template, match points and corresponding
composition rules is not the aim of this paper. Here we will simply use the results of
the application of the method to handle a well identified problem (already motivated
in sect. 1 and 2).
The list of concerns we have identified is:
- Non-functional concerns: Accuracy, Compatibility, Response Time, Multi-
Access, Security and Fault tolerance. Security has been decomposed into Integrity
and Availability (based on [11]). These sub-elements can be referenced by using the
“.” notation, like Security.Integrity or Security.Availability. For simplicity we will
use Integrity and Availability instead of Security.Integrity and Security.Availability.
- Functional concerns. Enter Subway, Validate Card, Buy Card, Exit Subway,
Refund Card and Credit Card.
The list of match points derived from the concerns' templates is: Enter Subway,
Exit Subway, Buy Card, Refund Card and Credit Card.
To illustrate the use of the AHP process, we will focus on a single match point, the
Enter Subway match point. Table 5 and 6 present the Enter Subway and Response
Time descriptions, which concentrates the information regarding the concern to which
the match point under study was defined.
Based on existing work, for example [11] and [23] as well as our knowledge of the
system, the positive and negative contributions between concerns is identified. For the
Enter Subway match point, Fig. 3 illustrates the resulting schema of concern
contributions. For example, multi-access contributes negatively to response time (the
higher number of passengers the system needs to handle, the faster it needs to be),
while response time contributes positively to availability (the faster the system is, the
longer it is free to handle more passengers). Note that the contributions specified here
are symmetric, but this is not always the case. For example, usability contributes
negatively to testability, but testability contributes positively to usability (please see
[23] for more information).