148

Designing XML Based Applications

The developer may have to use the application's data model to create a set of

matching schemas, which would then be exposed as part of the application's inter 

face. This third approach is often used when existing or legacy applications need

to expose an XML based interface to facilitate a loosely coupled integration in a

broader enterprise system. This technique is also known as legacy adapters or

wrappers. In these cases, the application's implementation determines the inter 

face and the schemas to expose. In addition, this approach can be combined with

the meet in the middle approach to provide a proper adaptation layer, which in

turn makes available a more interoperable interface that is, an interface that is not

so tightly bound to the legacy application. See Chapter 6 for more information on

application integration.

4.3.4.2

Flexible Mapping

In complement to these approaches, it is possible to map complete documents or

map just portions of documents. Rather than a centralized design for mapping from

an external schema to a well defined internal schema, developers can use a decen 

tralized design where components map specific portions of a document to an ade 

quate per component internal representation. Different components may require

different representations of the XML documents they consume or produce. Such a

decentralized design allows for flexible mapping where: 

  A component may not need to know the complete XML document. A compo 

nent may be coded against just a fragment of the overall document schema.

  The document itself may be the persistent core representation of the data mod 

el. Each component maps only portions of the document to transient represen 

tation in order to apply their respective logic and then modifies the document 

accordingly.

  Even if the processing model is globally document centric (see  Choosing Pro 

cessing Models  on page 151), each component can if adequate locally im 

plement a more object centric processing model by mapping portions of the 

document to domain specific objects.

  Each component can handle the document using the most effective or suitable 

XML processing technique. (See  Choosing an XML Processing Program 

ming Model  on page 164.)