A Rationale-based Model for Architecture Design Reasoning

6.4. Architecture rationale
• Implementation Certainty Risk (ICR) - measures the risk of not achieving the implementation
with a probability between 0 and 1.
• Cost Benefit Ratio (CBR) - measures the ratio between expected benefits to the
expected costs of the design decision
These estimates are used to assess the options at each decision point. They are the
quantifiable justifications for selecting a design option. We have provided a brief introduction
to QNR here. A detailed descriptions of their formulations and applications are
described in Section 8.2.2.
6.4.3 Alternative architecture rationale
In terms of the argumentation-based design rationale approach, an alternative design is
referred to as an option or a position. The association to support or refute an option
is modelled explicitly. This leads to complicated decision models. The AREL model
simplifies this by encapsulating the options and their arguments within the decision. In
other words, zero or more alternative design options (i.e. AAR) are contained within an
AR. AAR itself may contain entities such as arguments, design objects and behavioural
diagrams (see Figure 6.5). This information allows architects to understand and verify the
discarded design options, their relative merits and demerits, which have been considered
in the decision process.
Both AR and AAR are implemented by the package entity in UML. An AR is a
container of AAR. This multi-level structure hides the complexity of a decision and
makes it easy for searching and retrieval. The multi-layered information is exposed only
when they are required.
6.4.4 Avoiding cyclic decisions in AREL models
During the design process, architects may inadvertently cause a reasoning cycle to form as
design decisions are incrementally made. For instance, this could happen when different
teams of architects are designing simultaneously. Figures 6.6(a) and 6.6(b) show two cyclic
examples which violate the AREL definition (Section 6.2). In Figure 6.6(a), we can see
that if AR3 was to be created and added to the AREL model, with AE3 as an input
and AE1 as an outcome, this would create a directed cycle in the graph. The dotted
arrows denote the illegitimate ≪ARtrace≫ that would result in a cyclic graph, i.e. from
AE3 through to AR3, AE1, AR1, AE2, AR2, and back to AE3. Similarly, Figure 6.6(b)
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