UML : MODELS, VIEWS, AND DIAGRAMS

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Roderick Copeland
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1 UML : MODELS, VIEWS, AND DIAGRAMS Purpse and Target Grup f a Mdel In real life we ften bserve that the results f cumbersme, tedius, and expensive mdeling simply disappear in a stack f paper n smene's desk. We might ask why this is s. Tw factrs greatly influence the result f mdeling: fr whm d we create the mdel and fr what purpse is it suppsed t be used. If we dn't discuss and define these aspects sufficiently, we run the risk f creating mdels that dn't cntain what is imprtant t the user. In ther wrds, if details are nt emphasized and mitted apprpriately, the mdel is rendered wrthless. T define the purpse and target grup the fllwing questins shuld be answered: Hw much business expertise can we expect? Can we assume basic knwledge f the subject, r d we have t explain the fundamentals f the mdel's events and prcesses? What amunt f detail des the target grup need? What level f cmplexity des the mdel permit? If prcesses and systems are subject t cnstant changes, a highly detailed mdel might be unrealistic. This is because, mst f the time, it is nt pssible t maintain thse mdels in a satisfactry manner. A less detailed mdel requires less effrt t develp and update, but it is als less precise. Hw much time des the target grup have t read and interpret the mdel? Prevent yur mdel frm disappearing in a stack f paper n smene's desk by chsing the apprpriate level f detail and cmplexity; therwise, nbdy might have enugh time t read it. What language can be used in the mdel? Des the target grup understand technical business terms? D they understand IT terminlgy? Let's clarify with an easy example: If a bttle filled with water is labeled 'water', virtually anyne wh can read will understand the bttle's cntent. Hwever, if the bttle is labeled 'H 2O' even thugh this is crrect we

2 reach a much smaller grup f peple, fr example, the wrkers f a chemistry lab. Yet, the additinal benefit is that it shws the cmpsitin f the cntent: hydrgen and xygen. In either case, yu will have t decide what 'label' is mst apprpriate fr yur target grup. What level f abstractin shuld yu chse? The less abstract a mdel, the mre cmprehensible, and clear it is fr the user. This is because a less abstract mdel is clser t the user's actual use and language. On the ther hand, mdels with a high level f abstractin are mre reusable and they are mre easily cnverted int IT systems. We can als prve mre accurately that they are crrect. IT specialists prbably manage highly abstract mdels best. Users, n the ther hand, might pull their hair ut if asked t deal with a mdel like that. Practical Tips Cmprmises have t be made between the level f abstractin, clarity, and the amunt f detail used fr a mdel. It is pssible t develp several mdel cmpnents, differing in degree f frmality and detail, in rder t satisfy different target grups. In this way cmmunicatin between mdel builders, custmers, users, and develpers can be facilitated much mre easily. It is imprtant nt t 'verd' it, but t adjust the mdel t its target grups and their uses. Analysis r design patterns are example mdels that describe cmmn design and mdeling methds. Prcess f Analysis Figure 2.4 shws the prcess f analysis, which cnsists f btaining, representing, and verifying facts:

3 Figure 2.4 Prcess f Analysis This is the jb f the analyst. The prcess f analysis prduces a specificatin that cmes frm the mdel and ther representatins. The analyst wrks with knwledge carriers, such as custmers, users, and dmain experts: Facts are btained by cllabratin between analysts and dmain experts in which knwledge carriers cntribute dmain knwledge and analysts cntribute methdlgical knwledge. Facts are represented in diagrams and dcuments, which are usually prepared by the analyst. Facts are verified nly by knwledge carriers, since they alne can decide if the presented facts are crrect. Verificatin is abslutely essential. Withut it we might have pretty diagrams, but the prbability is high that the facts represented are faulty. In simple terms: develpment f a mdel withut verificatin is abslutely wrthless! Practical Tips It is impssible t develp and verify a usable mdel withut mastering the technical fundatins f a tpic. Where d we find these knwledge carriers wh knw smething abut the systems that we want t mdel? We have had gd experiences with the fllwing grups f peple: Peple wh are invlved in perfrming, perating, and cntrlling business prcesses Users f similar r related IT systems Custmers, wh are ften critical and creative knwledge carriers

4 Business Partners Dmain Experts Management External Observers Several helpful techniques have prven t be useful fr the analysis and understanding f business prcesses: Observing emplyees at wrk Participating in the investigated business prcesses Taking the rle f an utsider (e.g. f a custmer) Carrying ut surveys Cnducting interviews Brainstrming with everyne invlved Discussing with dmain experts Reviewing existing frms, dcumentatin, specificatins, handbks, and wrk tls Describing the rganizatinal structure and wrkflw management (rganizatin charts, etc.) Diagrams as Views Each particular UML diagram crrespnds t ne view f a mdel f a system. Depending n the type f diagram used, different aspects are either emphasized r mitted. All the different views cmbined result in a gd mdel f a system. Mst f the UML diagrams are graphs (as shwn in Figure 2.5), implying that they cnsist f elements that are cnnected thrugh lines:

Figure 2.5 Diagram as graphs T read diagrams, yu have t knw what types f elements and lines are allwed and what they mean. We'll explain this fr the diagrams we use in the fllwing chapters.

5 Figure 2.5 Diagram as graphs T read diagrams, yu have t knw what types f elements and lines are allwed and what they mean. We'll explain this fr the diagrams we use in the fllwing chapters. Even cmputer-aided sftware engineering (CASE) tls treat UML diagrams as views. They use adatabase in which the infrmatin abut the mdel is stred. Each diagram shws as a view a part f that infrmatin. In this way, the CASE tl helps t preserve the cnsistency f each view. If, fr example, the name f a class is changed in a class diagram, the statechart diagram f that class is autmatically updated: Figure 2.6 CASE tl as database The mdel database is what fundamentally differentiates a CASE tl frm a graphical prgram (Figure 2.6). Any UML diagram can be generated easily with paper and pencil r a graphical prgram. In this case, hwever, the varius diagrams are nthing mre than drawings. Only the use f a CASEtl with a database, accrding t UML specificatins, permits cnsistent cllectin, management, and mdificatin f mdel infrmatin.

6 UML prvides its wn database mdel: the UML meta-mdel, a cmpnent f the UML specificatins. All elements fund in UML diagrams, as well as the descriptins f these elements, are cntained in the UML meta-mdel. It states, fr example, that a class can have attributes and methds. This "data mdel" f UML as a language, is the fundatin f the mdel databases f all UML CASE tls. Unfrtunately, many CASE tls are hungry fr resurces, expensive, prly develped, cumbersme, and require extensive training. Despite this, except fr very small prjects, their use is wrthwhile.

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