A Rationale-based Model for Architecture Design Reasoning

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8.2. The architecture rationalisation method indicates no risk and one indicate total risk. Outcome Certainty Risk (OCR) measures the risk or the uncertainty level of the architectural design meeting the desired outcome, represented by the Architecture Benefits Index (ABI). Basically it indicates the level of risk of not meeting the desired outcome by the architecture design at a decision point. In other words, it indicates if motivational reasons are satisfied by the architecture decision. Implementation Certainty Risk (ICR) measures the risk or the uncertainty that there are no unexpected issues in the implementation of the architectural design. In other words, ICR represents the architect’s assessment of the uncertainty that issues may occur during the design, development or implementation phases, thus affecting the Architecture Cost Index (ACI). For instances, does the team have the knowledge to carry out the implementation? Is the technology stable enough to support this implementation? Using example 1 as an illustration, the architect feels that the OCR for MySQL is higher than Oracle, hence MySQL is given the OCR risk 0.4 and Oracle is given the OCR risk 0.2. This assessment basically is an estimation that the architect feels that there is a higher chance that there be some issues with MySQL. The ICR risk of MySQL is 0.3 and Oracle is 0.2. This indicates that when it comes to implementation, Oracle has less risk than MySQL because developers are more familiar with Oracle and there are more documentation provided by Oracle and various sources including Oracle local technical support to guide the developers. Evaluation of risks is an important and useful method for architecture modelling because it provides a means to discover and identify uncertainties. We further argue that it should be done as an ongoing and incremental activity during the architecture construction process. Some research work suggests that software architecture risk is evaluated when a model is relatively complete [50], we suggest that risk analysis should be carried out during the entire decision making process. Designers can investigate the decision issue in more details if they feel that there are major uncertainties in the decision. Risk awareness helps designers to be mindful of assumptions and unknowns that might affect the design. Architecture cost-benefit ratio Although the costs, benefits and risks give us some measurements to understand the different perspectives of a design alternative, it is difficult to make comparisons between alternative design. As such, we require a utility function to allow us to compute an index to compare alternative designs. The cost-benefit ratio in ARM provides a value to each design alternative to help decision making. The calculation takes into account the risks that are involved in each design option. 141
8.2. The architecture rationalisation method Quantitative rationalization using these metrics may be used at each decision point during architecture design. It can be supported by qualitative arguments that are captured in QLR. The Expected Benefit (EB) is the Architecture Benefit Index (ABI) discounted by the potential impact of not meeting the benefits which is represented by OCR. As such, the benefit is discounted by its risk, i.e. 1 − OCR. The expected Benefit (EB) is therefore: EB = (1 − OCR) ∗ ABI Expected Cost (EC) is the Architecture Cost Index (ACI) amplified by the risk of design implementation which is represented by ICR. As such, the cost increases according to the risk, i.e. 1 + ICR. The expected Cost (EC) is therefore: EC = (1 + ICR) ∗ ACI Quantitative rationale evaluation uses Cost-Benefit Ratio (CBR) to measure the ratio of expected benefits to costs at a particular decision point. Cost-Benefit Ratio (CBR) is expressed as follows: CBR = EB EC = (1 − OCR) ∗ ABI (1 + ICR) ∗ ACI Given the CBR is represented by benefits over cost, thus the higher the CBR ratio, the better the expected benefit ratio. Given two alternative architectural designs that deal with the same set of requirements, the design that has a higher CBR ratio is necessarily the better option because the architect expect a higher return from choosing this option. CBR is an index to indicate expected returns of each design, architects need to assign the ratings using a uniform scale to make comparisons possible between options. Table 8.2: The Expected Architecture Cost and Benefit Ratio ABI OCR EB ACI ICR EC CBR MySQL 3.5 0.4 2.1 3.5 0.3 4.55 0.46 Oracle 9 0.2 7.2 7 0.2 8.4 0.85 Using example 1, the expected benefit rating of MySQL and Oracle are 2.1 and 7.2 respectively. Similarly, the expected cost of MySQL is lower than Oracle, 4.55 and and 8.4 respectively (see table 8.2). Using the expected return formula, the CBR of MySQL and Oracle are 0.46 and 0.85 respectively. Since Oracle has a higher expected return, it can be chosen as the platform for implementing the CRM system. The quantitative approach using QNR has three merits: (a) architecture rationale is quantified at each decision point; (b) CBR takes into account risks or uncertainties in architecture modelling; (c) the soundness of each architecture decision can be assessed and measured for evaluation and verification. Using this approach, a quantitative comparison 142
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A Rationale-based Model for Archite
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maintenance. These techniques can h
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Declaration This thesis contains no
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2.2.6 DoD Architecture Framework .
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5.4.6 Risk assessment in architectu
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8.2 The architecture rationalisatio
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12.2 Contributions . . . . . . . .
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6.10 ≪AR≫ and ≪AAR≫ Stereot
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11.13A Process to Synchronise Chang
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5.13 Design Rationale Helps Evaluat
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that architecture design is highly
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1.2. Research motivations and resea
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1.2. Research motivations and resea
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1.4. Structure of the thesis the ch
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Part I Problem Analysis 10
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2.1. Industry practice of design ra
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2.2. Architecture frameworks and de
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Chapter 3 Related work in design ra
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3.2. Why do we need design rational
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3.2. Why do we need design rational
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3.3. Existing methods for capturing
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3.4. Other related studies 3.3.7 Qu
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3.4. Other related studies evolutio
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3.5. How well design rationale meth
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3.5. How well design rationale meth
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Chapter 4 Research methodology and
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4.1. Software engineering research
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4.2. The chosen research and valida
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Part II Architecture Design Rationa
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on a regular basis. Their inputs ha
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5.1. Architecture rationale in the
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5.2. Survey methodology data collec
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5.3. Survey findings experience). I
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5.3. Survey findings Table 5.1: Fre
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5.3. Survey findings Business Goals
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5.3. Survey findings 5.3.5 Document
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5.3. Survey findings Table 5.8 summ
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5.3. Survey findings We used Spearm
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5.3. Survey findings previous desig
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5.4. Discussion of findings Since t
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5.4. Discussion of findings or cons
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5.5. Limitations and use design rat
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Part III The Representation and App
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6.1. A conceptual model for design
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6.2. Architecture Rationale and Ele
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6.2. Architecture Rationale and Ele
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6.3. Architecture elements Figure 6
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11.1. Capturing architecture design
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11.1. Capturing architecture design
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11.2. Checking AREL models Figure 1
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11.3. Tracing AREL models Figure 11
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11.4. Analysing AREL with BBN • S
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11.5. Limitations Figure 11.13: A P
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Chapter 12 Conclusions 12.1 Summary
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12.2. Contributions reasoning metho
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12.2. Contributions implementations
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12.3. Future work architecture deci
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12.3. Future work to examine the co
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BIBLIOGRAPHY [10] B. W. Boehm, Soft
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BIBLIOGRAPHY [37] A. Eden and R. Ka
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BIBLIOGRAPHY [62] J. D. Herbsleb an
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BIBLIOGRAPHY [89] N. Lassing, D. Ri
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BIBLIOGRAPHY [115] D. L. Parnas,
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BIBLIOGRAPHY [142] Z. Simsek and J.
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BIBLIOGRAPHY [167] W. F. Tichy, N.
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Appendix A AREL Tool User Manual 1
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4.1 AREL Check Model When the Check
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4.2 AREL Model Tracing AREL model t
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Figure 9 - result of downward trace
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To initiate the graph’s creation,
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onto the diagram 14 Assign values t
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Useful Definitions The following ar
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) Some projects only c) Not at all
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19. I revisit architecture design d
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Complaint Procedure If you have any
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Privacy Protection Only investigato
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B. Exploring design reasoning. 1. W
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3. Why does the system use asynchro
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C. General Comments on AREL Modelli
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IEEE Computer Society Press. A. Tan
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