Model Driven Architecture(2)
The OMG turned its attention from Common Object Request Broker Architecture to MDA with a white paper in 2000, beginning an effort at classification and standardization and in the process producing a new lexicon, including core notions of the platform-independent model (PIM), platform-specific model (PSM) and Meta-Object Facility (MOF).
UML has a role in MDA, but models do not have to be created in UML to conform to the OMG’s MDA. Instead, both text-based and graphical languages must conform to MOF, which could be called a kind of mother language. (However, it’s hard to say what came first; MOF was abstracted from UML, and UML is one of MOF’s best examples.) MOF is general and universal enough that the disparate companies that make up the OMG do not have to agree to use the same modeling languages. They only have to agree to follow the same principles.
After requirements analysis, modeling in MDA begins with PIMs. These models are meant to capture an algorithm but to ignore the computer hardware and software that will be used to implement them. The models are intended to be pure expressions of the algorithm, with the best division between design and implementation possible.
PIMs are translated into PSMs in the next stage, “compilation,” in which the algorithms are made more ready to run. PSMs have features that correspond to the capabilities of hardware and software. In addition, PIMs may have been“marked” to facilitate this transformation. Finally, PSMs are translated into actual code, which will run on actual hardware, or on actual middleware on actual hardware.
2. Automated Code
The vision behind MDA is to automate as much of the process of code generation as possible, leaving designers to focus on the algorithm instead. Using tools and languages that will make the design as good as possible, the implementation will come later and will have as little of the pesky interference of fallible humans as possible.
If the underlying hardware or operating system is changed, the model is simply "recompiled" or "retranslated" to work on the new system. If maintenance is required, the model serves as an accurate document of how the algorithm works, and only the model needs to be maintained. The idea is that implementation and coding errors will be minimized.
Fear that this process will produce inefficient code is similar to the reaction when the compilation of third- generation languages was compared to the direct writing of assembly code, which generated a fear that in many cases proved to be unfounded.
The OMG lists several large projects on its Web site that have successfully used automated MDA to produce efficient, successful code with fewer resources than would have been used in more traditional projects. These include work done for Wisconsin's Department of Workforce Development, Postgirot Bank AB, Looking Glass Networks Inc., Credit Suisse Group and others.
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模型驅動的體系結構(2)
在2000年的一份白皮書中，OMG將其注意力從“公共對象請求代理體系結構”轉向MDA，開始了分類和標準化的研究，在這個過程中產生了新的專用詞匯，如獨立于平臺的模型（PIM）、特定平臺的模型（PSM）和元對象功能（MOF）等核心概念。
UML在MDA中起作用，但不一定非要用UML建立模型來符合OMG的MDA。相反，基于文本和圖形的語言必須符合MOF，這可以叫做一種母語言（但是，很難說哪個先有，MOF是從UML中抽象出來的，UML是MOF最好例子之一）。MOF非常通用、萬能，以至于組成OMG的不同公司不必同意使用相同的建模語言。它們只是必須同意遵循相同的原則。
在需求分析之后，用MDA建模與PIM同時開始。這些模型意味著俘獲了一個算法但忽略了實現它們的計算機硬件和軟件。模型更像算法的純表達式，是設計和可能的實現之間的最佳分割。
下一步，PIM會被翻譯成PSM，即“編譯”，在此過程中，使算法更多地做好運行的準備。PSM擁有與軟硬件的功能相對應的特性，而且，為了方便這個轉化，PIM事先已經被做了“標記”。最后，PSM翻譯被成實際代碼，它將在實際的硬件上運行或者在實際硬件的中間件上運行。
2. 自動編寫代碼
MDA背后的愿景就是盡可能多地使代碼生成的過程自動化，讓設計師集中在算法上。利用那些能使設計盡可能好的工具和語言，以后實現時盡可能少地讓易出錯的人類干預。
如果基礎的硬件或操作系統變了，模型只是簡單地重新編譯或重新轉換，就能在新系統上工作。如果需要維護，模型起到了實際文檔的作用，說明算法是如何工作的，而且只有模型需要維護。這個設想使實現和編碼錯誤減至最少。
對這個過程會產生低效代碼的擔憂，與把第三代語言的編譯與直接手工編寫匯編碼進行比較時的反應相似，業已證明，這種擔心在大多數情況下是多余的。
在其網站上，OMG列出了幾個已經成功使用自動MDA生成高效的、成功的代碼的一些大項目，它們比更傳統項目用到更少的資源，這些項目包括威斯康星州勞動部、Postgirot銀行、窺視鏡網絡公司、瑞士信用集團等。(全文結束)
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