A Rationale-based Model for Architecture Design Reasoning

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8.2. The architecture rationalisation method since the architect has to decide on the trade-off between options. ARM provides a quantitative method to characterise the cost and the benefit of a decision. The quantitative rationale is captured in QNR. The Architecture Cost Index (ACI) is assigned an index to weight the cost of implementing the decision. ACI is an index (between 1 and 10) where 1 is the least cost and 10 is the highest cost. ACI is an index that takes into consideration a multitude of cost factors to provide a relative cost index between alternative rationales. The considerations for ACI weighing are the following: • Development costs - this cost takes into account the cost of development and training requirement. • Platform costs - this cost takes into account the cost for platform support such as hardware and software. • Maintenance costs - this cost takes into account routine operational maintenance and support, software maintenance, software modifiability and portability. • Potential costs - security, legal and other costs that may arise from the design should also be considered. The Architecture Benefits Index (ABI) is an index (between 1 and 10) where 1 is the least benefits and 10 is the highest benefits. It represents the relative benefits an architecture decision or design would deliver to satisfy the concerned requirements. If compromises need to be made between competing requirements, then the architect would make a judgement on the relative priority of requirements and the level of satisfaction the architectural design provides to meet the requirements. Similar assessments would be made for alternative designs for comparisons. Example 1: A decision needs to be made on which database system to use for a Customer Relationship Management (CRM) system with 2 million customers, 30 million transactions per annum (data will be on-line for 5 years) and 100 on-line users. There are two options being contemplated: MySQL or Oracle. Table 8.1 shows a list of factors being considered and the relative weightings that have been assigned to a database system with regards to this application. There are five factors to consider in each of the cost and benefit category. Given that each of the five factors have equal weighting, each can have a score of up to 2. The five factors together sum up to 10, which is the highest cost or benefit index. We use a five points scale (0, 0.5, 1, 1.5, 2) to weigh each factor. 0 being the lowest cost or benefit and 2 being the highest cost or 139
8.2. The architecture rationalisation method Table 8.1: A Comparison of the Architecture Costs and Benefits Factors MySQL Oracle Costs License Fees 0 2 Maintenance Fees 0.5 2 Training 1 1 Implementation 1 1 Integration 1 1 ACI 3.5 7 Benefits Performance 1 2 Flexibility 1 2 Reliability 0.5 2 Scalability 0.5 2 Security 0.5 1 ABI 3.5 9 benefit. In the above example, we see that the cost of MySQL license fee is 0 and the cost of Oracle is 2 because of the differences in licensing fee charges. When we sum up the cost factors, the ACI of MySQL and Oracle are 3.5 and 7 respectively indicating that MySQL has a lower cost than Oracle. The ABI of MySQL and Oracle are 3.5 and 9 respectively indicating that Oracle provides a higher benefits than MySQL. This rating reflects the opinion of the architect and is contextual given the background of a specific project and organisation. The actual cost and benefits information such as the annual maintenance fees are gathered, analysed and documented in qualitative rationale QLR for future references. Architecture risks Charette [19] suggested that risk is an uncertainty about the future, and since the future involves changes, the person facing the future will have to make a choice to accommodate the uncertainties. He suggested that risk analysis and management in software engineering share this same characteristic about risk. In order to assess the uncertainties, subjective probabilities are often used to estimate the risk [10]. When architects make a decision in architecture design, he/she faces a similar kind of uncertainties about the design. The explicit representation of uncertainties will provide a focal point for architects to deal with them. For instance, if an architect is uncertain that a design is implementable, then a more detailed analysis and design is required to reduce or remove that uncertainty. In ARM, we apply the risk assessment principle to measure the uncertainty of a decision or a choice for a design decision. ARM represents uncertainties with a ratio ranging between zero and one, where zero 140
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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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Page 115 and 116: 6.5. The extended AREL To prevent c
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10.5. Summary design object may cau
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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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A Rationale-based Model for Architecture Design Reasoning

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