Validating Co-Constraints

Roger Costello

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Example: Checking Classifications

• Consider this XML instance document:

1. <?xml version="1.0"?>
2. <Document classification="secret">
3. <Para classification="unclassified">
4. One if by land, two if by sea;
5. </Para>
6. </Document>

• Security Classification Policy: For the instance document to be valid no <Para> element may have a classification value higher than the <Document>'s classification value.

Classification Values

• Here are the possible classification values, from highest to lowest:
  1. top-secret
  2. secret
  3. confidential
  4. unclassified

Rule #1

• Here is a Schematron rule which implements a portion of the above Security Classification Policy:

1. <sch:rule context="Para[@classification='top-secret']">
2. <sch:assert test="/Document/@classification='top-secret'">
3. If there is a Para labeled "top-secret" then the Document
4. must be labeled top-secret
5. </sch:assert>
6. </sch:rule>

• Read as: Within the context of a <Para> element whose classification value is 'top-secret', I assert that the classification value of the <Document> element must be 'top-secret'.

Rule #2

• Here is a Schematron rule which implements a second portion of the above Security Classification Policy:

1. <sch:rule context="Para[@classification='secret']">
2. <sch:assert test="(/Document/@classification='top-secret') or
3. (/Document/@classification='secret')">
4. If there is a Para labeled "secret" then the Document
5. must be labeled either secret or top-secret
6. </sch:assert>
7. </sch:rule>

• Read as: Within the context of a <Para> element whose classification value is 'secret', I assert that the classification value of the <Document>element must be either 'secret' or 'top-secret'.

Rule #3

• Here is a Schematron rule which implements a third portion of the above Security Classification Policy:

1. <sch:rule context="Para[@classification='confidential']">
2. <sch:assert test="(/Document/@classification='top-secret') or
3. (/Document/@classification='secret') or
4. (/Document/@classification='confidential')">
5. If there is a Para labeled "confidential" then the Document
6. must be labeled either confidential, secret or top-secret
7. </sch:assert>
8. </sch:rule>

• Read as: Within the context of a <Para> element whose classification value is 'confidential', I assert that the classification value of the <Document> element must be either 'top-secret', 'secret' or 'confidential'.

Pattern

• The three rules collectively implement the Security Classification Policy:

1. <sch:pattern name="Security Classification Policy">
2. <sch:p>A Para's classification value cannot be more sensitive
3. than the Document's classification value.</sch:p>
4. <sch:rule context="Para[@classification='top-secret']">...</sch:rule>
5. <sch:rule context="Para[@classification='secret']">...</sch:rule>
6. <sch:rule context="Para[@classification='confidential']">...</sch:rule>
7. </sch:pattern>

• Note that a pattern can be annotated using the <sch:p> element.
• Note that a pattern element wraps the rule elements, and the pattern element is given a name.

Schematron Schema

• Here is the Schematron Schema that implements the Security Classification Policy:

1. <?xml version="1.0"?>
2. <sch:schema xmlns:sch="http://www.ascc.net/xml/schematron">
3. <sch:pattern name="Security Classification Policy">
4. <sch:p>A Para's classification value cannot be more sensitive
5. than the Document's classification value.</sch:p>
6. <sch:rule context="Para[@classification='top-secret']">...</sch:rule>
7. <sch:rule context="Para[@classification='secret']">...</sch:rule>
8. <sch:rule context="Para[@classification='confidential']">...</sch:rule>
9. </sch:pattern>
10. </sch:schema>

• View check-classifications.sch (example01 folder).

Validation

• Let's validate this XML instance document against the Schematron schema:

1. <?xml version="1.0"?>
2. <Document classification="secret">
3. <Para classification="unclassified">...</Para>
4. <Para classification="confidential">...</Para>
5. <Para classification="unclassified">...</Para>
6. <Para classification="secret">...</Para>
7. </sch:schema>

• View valid-document.xml (example01 folder).

Use Oxygen XML

• Let's use Oxygen XML as the validator.
• Add the Oxygen processing instruction to the instance document:

1. <?xml version="1.0"?>
2. <?oxygen SCHSchema="check-classifications.sch" type="xml"?>
3. <Document classification="secret">
4. <Para classification="unclassified">...</Para>
5. <Para classification="confidential">...</Para>
6. <Para classification="unclassified">...</Para>
7. <Para classification="secret">...</Para>
8. </Document>

Use Oxygen XML (cont.)

• Start Oxygen XML.
• Drag and drop the XML instance document into Oxygen XML.
• Validate by clicking the red checkmark on the toolbar.

Use Oxygen XML (cont.)

Clicking on the red checkmark in the toolbar results in Oxygen XML validating the instance document against the Schematron schema.

Example Invalid Instance

• This instance document does not conform to the Classification Security Policy: invalid-document.xml (example01 folder).
• Validate it using Oxygen XML and see the errors it generates.

Oxygen PI Not Necessary

• It is not necessary to add the Oxygen Processing Instruction (PI) to instance documents
• "Note that we provide support for external association (based on filename, root element namespace or name), in Options -- Preferences -- Editor -- Default Schema Association and also support for validation against an external schema, see the "External Validation" action from the tool bar or from the menu Document -- Validate -- External Validation." (George Bina)

Use Topologi

• Let's use Topologi as the validator.
• There's no need to add a processing instruction to the instance document.
• On the left side of the tool, browse to the example01 folder, likewise for the right side of the tool.
• On the left side of the tool click valid-document.xml; on the right side of the tool click check-classifications.sch
• Click on the Run button. View the results.
• On the left side of the tool click invalid-document.xml; click on the Run button. View the results.

Use Topologi (cont.)

Selecting the instance document on the left, the Schematron document on the right, and then clicking the Run button results in Topologi validating the instance document against the Schematron schema.

Lab 1

• The check-classifications schema we have examined assumed that all classifications are expressed as lower-case.
• We want the schema to perform case-insensitive validation. That is, it doesn't matter whether the XML instance document uses, for example, secret, Secret, or SECRET.
• I modified two of the three rules in check-classifications.sch (lab01 folder) to perform a case-insensitive validation.
• Revise the third rule so that it also performs case-insensitive validation.

Lab 1

• Validate valid-document.xml (lab01 folder) against your Schematron schema.
• Validate invalid-document.xml (lab01 folder) against your Schematron schema.
• Use both Oxygen XML and Topologi to do the validation.
• Remember to add the Oxygen processing instruction to each XML instance document.