Fault-tolerant valid XML
ABSTRACT
XML is inherently fragile. Future-proofing complex mission critical app's represents an IT dichotomy. 3-schema architecture offers a solution.
The 'three-schema' model formalised by the International Standard's Organisation (ISO) and the American National Standards Institute in the late 1980s was originally drafted to provide a very powerful way of insulating application developers from unknown influences. The principle was based on capturing the known, and providing extensible interface layers for the unknown. The architecture, which formed the basis for 3-tier (and later n-tier) application architectures, separated applications into external, conceptual, and internal schemas. The external schema is the logical user's view of data; the conceptual schema is the business view of the data; the internal schema is the storage of the data.
The most notable benefits of 3-schema architecture when applied rigorously to application development (with the help of compliant toolsets) are the rapid application development times, and the significantly reduced maintenance costs. In short, deployed application environments can be insulated from change, or 'future-proofed'.
The parallels with XML application development are striking. XML-based applications are complicated to develop and configure, and they are fragile. Small changes to the requirements for validating objects can break complete environments. This paper applies the rules and principles of 3-schema architecture to complex XML application development, and turns up some surprising results.