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A Rationale-based Model for Architecture Design Reasoning

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6.7. AREL usability<br />

AE by ≪AEsupersede≫ links. As mentioned in the last section, all ≪ARtrace≫ links in<br />

a superseded AE are made non-current.<br />

Similarly, ≪ARsupersede≫ link connects current AR to superseded AR. Non-current<br />

or superseded ARs retain historical links (i.e. ARtrace.currFlag=FALSE) to AEs so that<br />

past ≪ARtrace≫ relationships are not lost. The replacement or the current AR would<br />

maintain the current ≪ARtrace≫ links (i.e. ARtrace.currFlag=TRUE) to all AEs.<br />

The introduction of eAREL in the architecture model creates complexity in the model<br />

because multiple versions of AR and AE and their relationships are kept. However, it<br />

is possible to manage this complexity through proper tool implementation by selectively<br />

showing or hiding the in<strong>for</strong>mation as required. When architects need to trace architecture<br />

evolution, the hidden relationships can be exposed through specific tool functions.<br />

6.6.5 AREL well-<strong>for</strong>medness in UML<br />

Given that AREL relies on UML extension, of which there is no support <strong>for</strong> checking<br />

model well-<strong>for</strong>medness, we require some methods to check the integrity of AREL so that<br />

any mistakes can be detected. In particular, the following well-<strong>for</strong>medness rules of AREL<br />

must be validated:<br />

• <strong>Architecture</strong> decision - each architecture rationale AR must have an input AE and<br />

an outcome AE. A decision node AR can neither be a root node nor a leaf node in<br />

the AREL model.<br />

• <strong>Architecture</strong> element - an architecture element AE cannot be linked to another<br />

architecture element AE without an architecture rationale AR.<br />

• Acyclic graph - the graph must be acyclic.<br />

The AREL Tool carries out consistency checking to ensure that the AREL model is<br />

well-<strong>for</strong>med. This is described in details in Chapter 11.<br />

6.7 AREL usability<br />

As discussed in Chapter 3, there are challenges in creating a design rationale method that<br />

can easily be used by practitioners. The existing design rationale methods that we have<br />

reviewed do not have all the required features (see Table 3.1). We intend to overcome these<br />

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