A Rationale-based Model for Architecture Design Reasoning

6.2. Architecture Rationale and Elements Linkage (AREL)
∀r ∈ AR, there exists a cause e ∈ AE such that (e, r) ∈ P L and an effect e ′ ∈ AE
such that (r, e ′ ) ∈ P L;
2. no subset of links in P L form a directed cycle.
According to Definition 1, an AR is connected to a minimum of two AE nodes through
links, one from a cause AE and one to an effect AE. As such, the links to the AEs then
represent the cause and the effect of the design decision represented by AR. Clause 2 of
Definition 1 specifies that insertion of a link is not allowed if it would result in a directed
cycle of links. Essentially, this means that the links maintain the integrity of the causal
modelling whereby something cannot be the cause of itself, directly or indirectly.
Using Definition 1, a basic form of the model construct is {AE 1 , AE 2 , ...} → AR 1 →
{AE a , AE b , ...} where AE 1 , AE 2 etc. are the inputs or the causes of a decision AR 1 ,
and AE a , AE b etc. are the outcomes or the effects of the decision. Figure 6.2 shows a
UML representation of the AREL model of the relationship between a motivational input
AE, a decision AR and a resulting AE. The cardinality in the relationship shows that
the motivational AE and the resulting AE must be a non-empty set linked by the single
decision AR. The uniqueness constraint in the diagram specifies that each instance of AE
in the relationship cannot appear more than once.
Figure 6.2: A Causal Relationship between AEs and an AR
The directional links of ≪ARtrace≫ represents the causal relationships in Figure 6.2.
An AE causes AR by motivating or constraining the decision, and the AR results-in an
outcome AE by having a design rationale to justify the design. Conversely, an outcome
AE depends on AR which in turn depends on an input AE. The causal relationship is the
basis for forward tracing and the reverse is the implied dependency relationship which is
the basis for backward tracing. Together they provide a means to navigate and explain the
architecture design. This causal relationship simplifies and replaces similar relationships
such as “creates” and “achieves”’ in argumentation-based methods (see Chapter 3). This
is advantageous because the model becomes simpler and the complexity of traversal for
knowledge retrieval is reduced.
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