Beyond schema

[Philippe.Peeters@smals-mvm.be]

Hi there,

I am quite new to XML developpement (and to this list) but I have a
question concerning the validation of XML messages (or data structure)
beyond what is allowed by a XML schema.

The problem is that we need to receive informations (forms) from various
partners. The basic format will be XML and a DTD is already defined. We
plan to move to xml schema in the near future.

When we receive the data (the xml message) we need to check the structure
(is the message valid according to the schema) but we also need to
implement  particular business rules. For exemple, a field A in the message
is required if, and only if, the value of some other field B in the message
is defined in a extrnal list of values. The DTD (and schema) can only
prescribe that the field A is optional since we cannot know in advance what
will be the content of the other fields (B in this example).
Is there some rules, standard architectures or good practices to use ? It
seems to me that there is no XML solutions to this and that we will have to
code some Java class to do the job. Another problem is that the business
rules may change quite frequently (every quarter).

Any advices ?