A Rationale-based Model for Architecture Design Reasoning

10.3. Reasoning about change impact with AREL
• Step 3 – evidence is inserted in C4 2 7 (i.e. its volatility is set to 100%) to reflect
that the design object has to change. This is because when the asynchronous messaging
design changes, its error detection mechanism would have to change as well.
The MCP process will handle error processing immediately. In this way, the error
detection in C4 2 7 must change. 5 The change in C4 2 7 has an impact on C6 0 0
further down the network in that alarms would no longer be needed to be set for
each payment message. The volatility of C6 0 0 thus rises from 51.2% to 77%.
• Step 4 – evidence is inserted in C6 0 0 to indicate changes in the alarm setting
processing. Since there are no other architecture design that depends on it, there is
no further change impact analysis that is required. As a result of this change, the
posterior probability of the invalidity of AR15 has increased from 80% to 98.7%.
Figure 10.9 shows the posterior probabilities of the full network after all predictive and
diagnostic reasoning have been performed, i.e. combining all the evidence for nodes C4 2 9,
C4 2 4, C4 2 7 and C6 0 0. Note that the posterior probabilities would be the same
regardless of the order in which the evidence was added, or if evidence for all five nodes
were added together. The change in the probabilities over the sequence of reasoning steps
is clearly crucial to supporting the human reasoning by the architect who is performing
the what-if analysis.
Figure 10.9: A BBN Model of Combined Reasoning
5 C4 2 8 error recovery and all its associated architecture decisions and design are part of the original
architecture design. A change in C4 2 9 will affect this part of the system. The change impacts are not
shown here to simplify the illustration.
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