156
Designing XML Based Applications
uments. (See  Receiving Requests  on page 89 and  Formulating Responses  
on page 98.) Moreover, when the processing layer of a Web service imple 
ments an object centric processing model, the interaction layer may be re 
quired to map XML documents to or from domain specific objects before 
delegating to the processing layer by using one of the three approaches for 
mapping XML schemas to the application data model presented in  Mapping 
Schemas to the Application Data Model  on page 143.
  In the processing layer of a Web service when implementing a document 
centric processing model. (See  Handling XML Documents in a Web Service  
on page 105.) In such a case, the processing layer may use techniques such as 
flexible mapping or XML componentization. See  Flexible Mapping  on 
page 148 and  XML Componentization  on page 149.
With the object centric processing model when XML document content is
mapped to domain specific objects the application applies its business logic on
the domain specific objects rather than the documents. In this case, only the inter 
action logic may handle documents. However, in the document centric model, the
application business logic itself may directly have to handle the documents. In
other words, some aspects of the business model may be expressed in terms of the
documents to be handled.
There are drawbacks to expressing the business model in terms of the docu 
ments to be handled. Doing so may clutter the business logic with document pro 
cessing related logic, which should be hidden from application developers who
are more focused on the implementation of the business logic. It also introduces
strong dependencies between the document's schemas and the business logic, and
this may cause maintainability problems particularly when handling additional
schemas or supporting new versions of an original schema (even though those are
only internal schemas to which documents originally conforming to external
schemas have been converted). Additionally, since there are a variety of APIs that
support various XML processing models, such a design may lock the developer
into one particular XML processing API. It may make it difficult, and ineffective
from a performance perspective, to integrate components that use disparate pro 
cessing models or APIs.
The same concerns about maintainability in the face of evolution and the
variety of XML processing models or APIs apply to some extent for the logic of
the Web service interaction layer, which may be in charge of validating exchanged






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