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Re: Separate data from rules ... is the XML Schema 1.1 <assert

• From: Philip Fearon <pgfearo@googlemail.com>
• To: "xml-dev@lists.xml.org" <xml-dev@lists.xml.org>
• Date: Mon, 15 Jun 2009 22:52:37 +0100

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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• References:
  • Separate data from rules ... is the XML Schema 1.1 <assert> elementa step backwards?
    • From: "Costello, Roger L." <costello@mitre.org>