Re: [xml-dev] Separate data from rules ... is the XML Schema 1.1 <assert

[Philip Fearon <pgfearo@googlemail.com>]

Roger,

In this example, the 'classification' attribute applied to an element
is describing the content of that element rather than being data
itself.

The 'classification' attribute value is therefore metadata, and thus a
prime candidate for use in a business rule.

XML Schema 1.1 <assert> has to be a step in the right direction.

Phil Fearon
http://www.sketchpath.com

On Mon, Jun 15, 2009 at 10:10 PM, Costello, Roger L.<costello@mitre.org> wrote:
>
> Hi Folks,
>
> In the book Business Rules Applied, Barbara von Halle presents the Business Rules Methodology, which fundamentally is about separating rules from data: [1]
>
>    A business rules methodology specifically
>    removes rule considerations from the data
>    and process tracks.
>
>    The data and process tracks are reduced in
>    content because they are devoid of rule
>    details.
>
>    You deliver a data structure with minimal
>    rules in it.
>
>
> WHAT IS A BUSINESS RULE?
>
> Consider this XML document:
>
>    <?xml version="1.0"?>
>    <Document classification="secret">
>        <Para classification="unclassified">
>          One if by land, two if by sea;
>        </Para>
>        <Para classification="confidential">
>          And I on the opposite shore will be,
>          Ready to ride and spread the alarm
>        </Para>
>        <Para classification="unclassified">
>          Ready to ride and spread the alarm
>          Through every Middlesex, village and farm,
>        </Para>
>        <Para classification="secret">
>          For the country folk to be up and to arm.
>        </Para>
>   </Document>
>
> Here's an example of a business rule:
>
>    Security Classification Rule: No paragraph
>    may have a classification higher than the
>    overall document's classification.
>
>
> THE XML SCHEMA 1.1 <assert> ELEMENT
>
> XML Schema 1.1 introduces a new element: the <assert> element. It may be used to express business rules.
>
> The Security Classification Rule may be expressed like this:
>
>    <element name="Document">
>        <complexType>
>            <sequence>
>                <element name="Para" type="ParaType" maxOccurs="unbounded">
>                </element>
>            </xs:sequence>
>            <attribute name="classification" type="classificationLevels"
>                       use="required"/>
>            <assert test="if (@classification eq 'top-secret') then
>                             not(Para/@classification != 'top-secret') else
>                          if (@classification eq 'secret') then
>                             not(Para/@classification = 'top-secret') else
>                          if (@classification eq 'confidential') then
>                             not(Para/@classification = 'top-secret') and
>                             not(Para/@classification = 'secret') else
>                          if (@classification eq 'unclassified') then
>                             not(Para/@classification = 'top-secret') and
>                             not(Para/@classification = 'secret') and
>                             not(Para/@classification = 'confidential') else
>                          true()" />
>        </complexType>
>    </element>
>
> Notice the use of the <assert> element to express the security classification business rule.
>
> This example illustrates the co-mingling of data and rules. According to the Business Rules Methodology this is not advisable (it hinders the ability of a business to make changes).
>
> Up till this moment in XML's history the separation of data and rules have been enforced by the technologies:
>
>   - Data structure and vocabulary was
>     expressed and managed using XML Schema
>     or RELAX NG, and
>
>   - Rules were expressed and managed using
>     Schematron.
>
> The <assert> element changes things. Rather than technology enforcing the separation of data and rules, it is incumbent on developer discipline to enforce the separation.
>
> Is the XML Schema 1.1 <assert> element a step backwards?
>
>
> /Roger
>
>
> [1] Business Rules Applied by Barbara von Halle, p. 66-67. For more information on the Business Rules Methodology see:
>
>  - Business Rules and Information Systems by Tony Morgan
>  - Principles of the Business Rules Approach by Ronald G. Ross
>  - Wikipedia: http://en.wikipedia.org/wiki/Business_rules_approach
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