Processing Layer Design
The issues and considerations for designing an application's processing or 
business logic layer, such as whether to perform this logic in the Web or EJB 
tier, are the same whether or not you use a Web service.
We do not address these business logic design issues here, since much of this
discussion has already been covered in the book 
Designing Enterprise Applica 
tions with the J2EE
 Platform, Second Edition
, and you can refer to that book for
general guidelines and recommendations. You should also refer to the BluePrints
Web site at 
 for recommendations on designing
an application's business processing logic. 
In addition to these general guidelines, there are some specific issues to keep
in mind when designing the processing layer of a Web service.
Keep the processing layer independent of the interaction layer
. By keeping 
the layers independent and loosely coupled, the processing layer remains ge 
neric and can support different types of clients, such as Web service clients, 
classic Web clients, and so forth. To achieve loose coupling between the lay 
ers, consider using delegate classes that encapsulate the access to business 
Bind XML documents to Java objects in the interaction layer
. There are 
times when your Web service expects to receive from a client an XML docu 
ment containing a complete request, but the service's business logic has no 
need to operate on the document. On these occasions, it is recommended that 
the interaction layer bind the XML document contents to Java objects before 
passing the request to the processing layer. Since the processing logic does not 
have to perform the XML to Java conversion, a single processing layer can 
support XML documents that rely on different schemas. This also makes it 
easy to support multiple versions of an XML schema.
Keep in mind that your processing logic can operate on the contents of an
XML document received from a client. Refer to  Handling XML Documents in a
Web Service  on page 105, which highlights issues to consider when you pass
XML documents to your business processing logic.
Depending on your application scenario, your processing layer may be
required to work with other Web service peers to complete a client's request. If so,
your processing layer effectively becomes a client of another Web service. Refer
to Chapter 5 for guidelines on Web service clients. In other circumstances, your

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