Loose vs. Tight Coupling(2)

One key feature of SOA is the use of loosely coupled connections. Traditionally, connections between applications or between applications and services have been tightly coupled, as with CORBA (Common Object Request Broker Architecture). The difference is important.

Eric Van der Vlist, author of XML Schema: The W3C's Object-Oriented Descriptions for XML (O'Reilly, 2002), describes the differences between the two types of coupling with this analogy: In a tightly coupled system, you have direct control over the operation. For example, flipping a wall switch to turn on a light is a tightly coupled system. However, making a telephone call to order a book is a loosely coupled system. It could be tightly coupled only if you had access to the button controlling the printer that will print the book you order.

Tightly coupled systems are usually fast and safe, and the risk of transmission errors is very low. Loosely coupled systems, on the other hand, are usually more error-prone but also more flexible. The clerk you talk to on the phone may misunderstand the ISBN number of the book you want to order or make an error while entering it. But if you don't remember the ISBN number, you can still tell the clerk that you want the latest book on the World Wide Web Consortium's XML schema by a guy with a Dutch name from a publisher that puts pictures of animals on its book covers—and when you do that, you've got a good chance of being understood.

Tight coupling tends to make component maintenance and reuse much more difficult, because a change in one component automatically means changes in others. Similarly, tight coupling makes extra work when an application has to adapt to changing business requirements, because each modification to one application may force developers to make changes in other connected applications.

In general terms, a Web service is a type of SOA in which interfaces are based on standardized Internet protocols. In addition, except for binary data attachment, Web service messages must be in XML. Generally speaking, a Web service is little more than an SOA that uses Simple Object Access Protocol and the Web Services Description Language. However, an SOA doesn't require the use of Web services as we understand them, and some types of Web services can be deployed without an SOA. (The End)

參考譯文

松散耦合與緊密耦合 (2)

SOA的一項關鍵特性就是使用松散耦合連接。傳統上，應用程序與應用程序之間或者應用程序與服務之間的連接是緊耦合的，如用CORBA(公共對象請求代理體系結構)那樣。此差異是很重要的。

《 XML Schema: The W3C's Object-oriented Descriptions for XML》一書(O'Reilly公司2002年出版) 的作者 Eric Van der Vlist有下列比喻來描述這兩類耦合之間的差異:在緊耦合系統中，你直接控制操作。例如，扳動墻上的開關接通電燈就是緊耦合系統，而利用電話訂購書就是松耦合系統。只有當你訪問控制打印機(該打印機將打印你訂購的書籍)的按鈕時，才可能是緊耦合。

緊耦合系統通常比較快、也安全，傳輸誤差的風險非常低。另一方面，松散耦合系統通常比較容易出錯，但比較靈活。當你在電話上通話時對方職員可能會聽錯你要訂購書的 ISBN編號、或者在輸入時出錯。如果你不記得ISBN編號，你還可以告訴對方職員，你要一本由一名有荷蘭名字的人寫的有關3W聯盟的XML范式的新書，該書的出版社通常將動物的圖片放在書的封面上，當你這樣做時，你就得到了被理解的好機會。

緊耦合往往會使部件的維護和重復使用變得非常困難，因為一個部件中的修改就自動意味著其他部件中的修改。同理，在應用程序不得不適應變化了的業務要求時，緊耦合就要做額外的工作，因為一個應用程序中的一次修改將迫使開發人員對其他相連接的應用程序進行修改。

從一般意義上講， Web Services 就是一類 SOA ，其接口都是基于標準化了的互聯網協議。此外，除二進制數據附件， Web Services 消息必須用 XML 語言。通常講， Web Service 只是一個用了 SOAP (簡單對象訪問協議)和 Web Services 描述語言的 SOA 。但是， SOA 不要求使用我們所理解的 Web Services ，有些類型的 Web Services 不用 SOA 也能部署。