Software Design 2e - DIM

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Proxy221The GoF classify this pattern as Object Structural, whereas under the scheme used inBuschmann et al. it is classified as Access Control. The core set of headings fromSection 10.2.2 can then be used to describe this as follows.NameAlso known asProblemSolutionExampleApplicabilityStructureProxySurrogateProxy addresses the problem where direct provision of access toan actual object will represent a significant overhead in some form(loading time, space, communication, etc.). As such, the use ofproxy may be particularly relevant when developing distributedsystems.This is to provide a representative of the object, and to let the userobject communicate with the proxy rather than the real thing. Theproxy provides the same interface as the actual object and ensurescorrect access to this object, while possibly performing additionaltasks such as enforcing access protection rules. Where necessary, itmay organize the creation and deletion of instances of the ‘real’object.The GoF offer the example of a complex image in a word processingdocument. Rather than incurring the overhead of loading the(potentially) large and complex drawing object to produce animage on the screen, it may be possible to employ a much simplerproxy object that ensures that key properties (such as position andboundaries) are correctly represented in the screen image. So thisexample of a proxy may just display an outline box to representthe image so that the user may adjust text flow, positioning, etc.However, if further information is needed, or if it is necessary tomanipulate the image itself, these can be invoked in the original viathe proxy. Buschmann et al. offers a rather different example whichis based upon optimizing repeated accesses to remote databases toobtain images which may be large and complex. Here, the proxyplays a role which is closer to the concept of a cache and, like acache, it is clearly most effective in situations where the predominantaccess forms are for reading rather than for updating.Proxy is concerned with access to relatively complex objects whichmay be in a different space, such as a remote database (remoteproxy), involve significant overheads in object creation, as with theexample of the graphical image (virtual proxy), or where differentaccess rights may be involved (protection proxy).Figure 10.3 illustrates the idea of proxy using a class diagram.Normally, the client requests will be serviced by the proxy, whichrelays these to the original where necessary. Figure 10.4 demonstratesthis role in terms of a sequence diagram that describes onepossible way of working.The design pattern
222<strong>Design</strong> patternsFigure 10.3The proxy design pattern (class diagram).Figure 10.4The proxy design pattern (sequence diagram).ImplementationKnown UsesRelated PatternsConsequencesSince this aspect is not directly relevant to the theme of this book,we omit such a detailed level of discussion.Both the GoF and Buschmann et al. provide examples here, withthe latter being the more extensive and varied and, in particular,relating more strongly to web-based forms of implementationwhere this pattern is quite widely employed.The Adapter (GoF(139)) and Decorator (GoF(175)) patterns areboth concerned with interfacing issues.There is some discussion of these in both reference texts. Perhapsthe most obvious is the correspondence to the use of a cache in asituation where an object is extensively updated. In this context,
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Software Design
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Software DesignDavid Budgen
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viContentsChapter 4 Design Qualitie
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viiiContentsChapter 16 Designing wi
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xPreface to the second editiondescr
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xiiPreface to the First EditionWhy
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xivPreface to the first editionto t
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xviPublisher’s AcknowledgementsWe
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chapter 13The Nature of the Design
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Design is just as important with so
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Initialobservations7More systematic
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development. However, it should als
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From this relatively simple case st
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nnWhere possible, the panels used i
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It might well be argued that while
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system being developed. Clearly, in
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case study, the outline plan on squ
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nnnWicked problems do not have an e
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on to Pittsburgh. Airline B is the
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262.1 What is software?The software
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28nChangeability. Software suffers
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30The software design processFigure
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32The software design processthat a
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38The software design processproces
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40The software design processform p
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44The software design process2.3 De
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463.1 A context for designDesign in
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48Design in the software developmen
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54A personal itchDesign in the soft
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60nthe associated role of requireme
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chapter 463Design Qualities4.1 The
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is virtually impossible to modify i
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measurable characteristic that can
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69Assessing design qualityFigure 4.
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nnnnreliabilityefficiencymaintainab
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Table 4.1The cognitive dimensions73
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system. The results of this study u
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nnnmodules are easy to replace;each
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Table 4.3The principal forms of coh
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Identifying attributes that are the
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Our study of the nature of the desi
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nTechnical design reviews can provi
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chapter 589Describing a Design Solu
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91Figure 5.1Example of a ‘physica
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term is used to describe ‘stakeho
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95Design viewpoints for softwareFig
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nnof mark-up languages such as HTML
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nnntextdiagramsmathematical express
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happens with pioneering ideas, it h
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I have termed ‘derived viewpoints
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1066.1 The need to share knowledgeT
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108Transferring design knowledge6.1
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110then briefly consider how the co
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112Transferring design knowledgethe
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114Transferring design knowledge6.2
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116mainTransferring design knowledg
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118Transferring design knowledgennn
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120Transferring design knowledgeOne
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1226.5 A unified interpretation?Tra
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124Transferring design knowledgeA s
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chapter 7127Some Design Representat
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Table 7.1The selection of black box
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131Black box notationsFigure 7.1A f
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133Black box notationsFigure 7.3mac
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135Black box notationsFigure 7.4A
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nattributes are classes of values t
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The ERD viewpointERDs are purely an
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7.2.4 The StatechartLike the STD th
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145Black box notationsFigure 7.13St
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7.2.5 The Jackson Structure Diagram
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149Black box notationsFigure 7.17 A
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153Figure 7.21The UML class notatio
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155Use case (title indicates the ac
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157cardrejectedprocesscardcard acce
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159White box notationsFigure 7.27A
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only the presence of the latter is
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In its most simple form, such a com
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NarrativeUser starts web browserUse
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If anything, pseudocode is used rat
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Many notations can be conveniently
Page 188 and 189: 171Developing a diagramFigure 7.38E
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Page 231 and 232: 21410.1 Design by template and desi
Page 233 and 234: 216Design patternspatterns for obje
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Page 256: Further reading239Wirth N. (1971).
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Page 267 and 268: 250Incremental designFunctionalityT
Page 269 and 270: 252Incremental designBusinessstudyF
Page 271 and 272: 254Incremental designalthough it ma
Page 273 and 274: 256ExercisesIncremental design12.1
Page 275 and 276: 25813.1 Origins, development and ph
Page 277 and 278: 260Structured systems analysis and
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272Structured systems analysis and
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288ExercisesStructured systems anal
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29014.1 Some background to JSPJacks
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292Jackson Structured Programming (
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chapter 15315Jackson System Develop
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317The JSD modelFigure 15.1Top-leve
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319Figure 15.3Generic form of ESD.t
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321Figure 15.6Process 1.The SSD for
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323The JSD processFigure 15.9 The J
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Entity analysis (the entity-action
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327The JSD processFigure 15.13The J
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329The JSD processFigure 15.14 The
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331The JSD processFigure 15.15 The
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333The JSD processFigure 15.16The c
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Information function step335From a
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The situation is analogous to that
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Exercises33915.1 Construct an ESD t
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34216.1 The ‘object concept’Des
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344Designing with objectsThe evolut
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346Designing with objectsFigure 16.
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348Designing with objectsFor the ob
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352 Executable units of code(subpro
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354A summary of the core concepts o
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356Designing with objectsnnnnnnnorg
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358Designing with objectsA rather l
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360Designing with objects16.2.1 Sur
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364closely at the representation pa
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366Designing with objectsAn empiric
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368Designing with objectsof a frame
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370Designing with objects16.3.2 Des
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372Designing with objectsDepartment
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376Designing with objects1. The obj
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378Designing with objectsdescribe a
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380Designing with objectsnnapproach
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382Designing with objectssequence d
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384Designing with objectsnnhow is t
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386step, it is those analysis-relat
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388Designing with objectsStep 7: De
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390Designing with objectsbehaviour,
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392indicate the degree of effort. A
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394Designing with objectsThe UP wor
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398Designing with objectsSimilarly,
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400Designing with objectswww.omg.or
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40217.1 The component conceptCompon
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404Component-based designmore than
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406Component-based designFigure 17.
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408Component-based designto address
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410Component-based designnto decomp
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412Component-based design4. Archite
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41417.3 Designing componentsCompone
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416Component-based designSourceInde
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418Component-based designwww.sei.ed
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42018.1 The case for rigourA formal
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422A formal approach to designmight
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424A formal approach to designFigur
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426A formal approach to design18.2.
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428 Table 18.1 Set operations used
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430A formal approach to designClear
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432InitResSystA formal approach to
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438A formal approach to designThis
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44018.2 Write simple Z specificatio
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44219.1 What is software now?Whithe
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444Whither software design?Figure 1
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446Whither software design?Figure 1
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448Whither software design?Another
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451BibliographyAbbott R.J. (1983).
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Buxton J. and McDermid J. (1992). H
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Guindon R. (1990). Designing the de
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Perry D.E. and Wolf A.L. (1992). Fo
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Ward P.T. and Mellor S.J. (1985). S
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462IndexCcall and return (architect
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464Indexgraphical user interface (G
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466Indexprogram design 290program i
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468WYIndexwalkthrough 81waterfall m
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Software Design 2e - DIM

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