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System Modeling

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System modeling

Ohjelmistokehityksessä mallilla tarkoitetaan systeemin abstrahointia, eli yleiskäsitteen muodostamista yksinkertaistettuna kuvana. Yksinkertaistaminen on kuitenkin lavea käsite. Malli voi olla formaali kuvaus systeemin toiminnasta, rakenteesta ja käyttäytymisestä tai määrittelemättömään merkistöön perustuva yksinkertaistettu kuvaus.

Mallien tarve on selkeä. Malli on työkalu, jolla lopputulosta voidaan arvioida ennen valmistumista ja jolla valmistamista voidaan ohjata. Arvioimiseen liittyy olennaisesti seikka, joka on vahvasti ollut esillä kurssin jokaisessa tähänastisessa aihealueessa. Käytännön ohjelmistotuotantoon liittyy erottamatta loppukäyttäjä, asiakas. Systeemin varhaisen tason mallintaminen on välttämätön edellytys suunnittelutyössä asiakkaan kanssa tai asiakasta varten.

Yksinkertaistaminen jättää pois yksityiskohtia. Kuvaus luodaan tällöin valitun näkökulman mukaan. Näkökulmia voidaan karkeasti rajata funktionaalisiin, organisaatio-, informaatio- ja infrastruktuurinäkökulmiin. Asiakasta ajatellen tämä tarkoittaa informaation tuottamista kuulijaa ajatellen yhteistyön parantamiseksi. Ohjelmointia ajatellen näkökulman valinta riippuu kehitysprosessista. Projektinhallinta, kehitys ja dokumentointi voi perustua vahvasti tai löyhästi mallin päälle. Liian laaja mallintaminen ei ole mielekästä ellei arkkitehtuuri sitä vaadi.

Sommervillen Software Engineering –kirjan System modeling –kappaleen jäljiltä mallintamisesta jäi hieman hajanainen kuva. Käytännön ohjelmistokehitys on henkilökohtaisesti melkoisen vieras asia. Kirjan pohjalta maallikolle muodostuu käsitys loputtomista työtavoista, dokumentointityyleistä ja prosesseista. Kirja ja luennot painottavat kuitenkin käsitystä vaihtoehtoisista tavoista ja ohjelmistokehityksen olemuksesta käsityötaitona eikä eksaktina tieteenä. Tässäkin yhteydessä, täytyy siis mallintamisesta saatavan hyödyn ylittää siihen käytetyt resurssit. Mallien hyödyn realisoituminen malliperustaisena ohjelmistokehityksenä onkin omasta mielestäni eräitä kurssin mielenkiintoisimpia aihealueita.

Jos malleista saadaan tuotettua lopullista ohjelmakoodia, automaattisesti tai vähemmän automaattisesti, mallinnus muuttuu houkuttelevammaksi kaikille osapuolille. Työskentely voidaan aloittaa perinteistä ohjelmointia korkeammalla abstraktiotasolla tuottaen heti osittain valmiita esimerkkejä. Ulkopuolisten tahojen kanssa kommunikointi helpottuu luonnosten avulla, joista saadaan lisäkäsittelyllä tuotettua valmista ohjelmakoodia. Eri alojen asiantuntijat pääsemät myös syvemmin mukaan kehitystyöhön kehittämällä ylemmän tason malleja ja liiketoimintaratkaisuja. Käytettävyys- ja ulkoasuosaajat eivät joudu piinaamaan ohjelmoijia vaatimuksillaan ja luonnoksillaan, vaan luonnoksista saadaan generoitua lopullista tuotetta.

Ohjelmoinnin historiassa näkyy kehityksenä abstraktiotason nouseminen. Kolmannen sukupolven ohjelmointikielissä ohjelmoijalta piilotetaan konepellin alla tapahtuvia toimintoja, jolloin keskittymisen voi suunnata olioiden määrittelyyn muistipaikkojen määrittelyn sijasta. Abstraktio tasoa nostamalla on onnistuttu tuottamaan yhä monimutkaisempia ohjelmistoja. Myös ohjelmakoodin uudelleenkäytettävyys on noussut ja helpottunut muun muassa kattavien komponenttikirjastojen takia. Abstraktiotason noustessa koodin kirjoitus muuttuu yhä triviaalimmaksi osaksi kompleksisessa järjestelmässä. Mallipohjainen ohjelmistokehitys on looginen vaihe ohjelmointikielien kehityksessä ja abstraktiotason nostamisessa.

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