Chapter 4 XML Processing

145

Design time

(1) Design Inter 

nal XML Schema 

Internal XML

Schema

External XML

Schema

(2) Design Style 

Sheets

Data Model 

Classes

Runtime

Transform 

Style Sheet

 XML 

Document 

Data Model 

Adaptation Layer

Objects

Figure 4.5

Meet in the Middle Approach for Mapping Schemas to Data Model 

Classes

The second approach (Figure 4.5), which introduces a transformation or adap 

tation layer between the application's data model and logic and the external sche 

mas, is particularly suitable for applications that may have to support different

external schemas. Having a transformation layer leaves room for supporting addi 

tional schemas and is a natural way to account for the evolution of a particular

schema. The challenge is to devise an internal schema that is sufficiently generic

and that not only shields the application from external changes (in number and

revision) but also allows the application to fully operate and interoperate. Typi 

cally, such an internal schema either maps to a minimal operational subset or

common denominator of the external schemas, or it maps to a generic, universal