Reuse using Abstract Patterns

Roger Costello

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Motivation for Abstract Patterns

• Consider the pattern in Organization-XYZ-Security-Classification-Policy.sch
• The pattern is "hardcoded" to validate XML instance documents that have a root element whose name is "Document," each child element has the name "Para," and each element has an attribute with the name "classification," e.g. Organization-XYZ.xml
• Other XML instance documents may have the same pattern - a security label on the root element and a security label on each child element - but use a different name for the root element, a different name for the child elements, and a different name for the attribute, e.g. Organization-ABC.xml

Motivation for Abstract Patterns

• Schematron abstract patterns enable us to express patterns in a way that is independent of specific tag and attribute names.

How to Create an Abstract Pattern

• On a pattern element add an attribute, abstract="true"
• On each rule use a "parameter reference" for the value of the context attribute. (A parameter reference looks like a variable)
• On each assertion use a parameter reference for the value of the test attribute.

Example Abstract Pattern

1. <sch:pattern abstract="true" id="Abstract-Security-Classification-Policy">
2. <sch:rule context="$child-element-labeled-top-secret">
3. <sch:assert test="$require-top-secret-root-element" />
4. If there is a <sch:name /> labeled "top-secret"
5. then the document's label must be top-secret.
6. </sch:assert>
7. </sch:rule>
8. ...
9. </sch:pattern>

• View the entire abstract pattern: Abstract-Security-Classification-Policy-Pattern

How to use an Abstract Pattern

• On a pattern element add an "is-a" attribute. Its value is the id of a abstract pattern.
• For each parameter reference in the abstract pattern, assign it a value using a param element.

Example of using an Abstract Pattern

1. <sch:pattern is-a="Abstract-Security-Classification-Policy" id="Organization-XYZ-Security-Classification-Policy">
2. <sch:param name="child-element-labeled-top-secret"
3. value="Para[@classification='top-secret']" />
4. ...
5. </sch:pattern>

• View the entire pattern: Organization-XYZ-Security-Classification-Policy-Pattern

Information Patterns!

• Science is all about identifying patterns, and representing the patterns abstractly.
• Abstract representations allow us to contemplate the properties of a whole class of things, rather than treating each thing on a case-by-case basis.
• For example, long ago people noticed that then an object is lifted and then released, it falls down to earth. It doesn't matter whether the object is a rock, or a feather, or anything else. All objects exhibit the same pattern of behavior.

Information Patterns!

• The abstract patterns in Schematron challenge us to become information scientists: identify patterns in information, create an abstract representation of the pattern, and then for each instance relate it to the abstract represenation.

Put Abstract Pattern in a Separate File

• An abstract pattern has greatest value when it is used by multiple schemas.
• So let's move it into a separate file, where it can be included by other schemas.
• Here's the abstract pattern in its own file: Abstract-Security-Classification-Policy-Pattern
• Here's Organization-XYZ using the abstract pattern: Organization-XYZ.sch
• Here's Organization-ABC also using the abstract pattern: Organization-ABC.sch