Schematron
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In Markup Languages, Schematron is a rule-based validation language for making assertions about the presence or absence of patterns in XML trees. It is a simple and powerful structural schema language expressed in XML using a small number of elements and XPath. The Schematron schema XML is then processed into normal XSLT code for deployment anywhere that XSLT can be used.
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[edit] Advantages
Because of its rule-based nature, Schematron's specificity is very strong. For example, it can require that the content of an element be controlled by one of its siblings. Or it can request or require that the root element, regardless of what element that is, must have specific attributes. Schematron can even specify required relationships between multiple XML files.
On top of these specific rules are validation error messages expressed in natural language instead of cryptic error codes. This can make error diagnosis by end-users much easier than some other schema languages.
[edit] Uses
Schematron's elegant design of a expressing constraints through a simple, XPath-based language but deployed as XSLT code, make it practical for applications such as the following:
- Adjunct to Structural Validation
- by testing for co-occurrence constraints, non-regular constraints, and inter-document constraints, Schematron can extend the validations able to be expressed in languages such as DTDs, RELAX NG or XML Schema.
- Lightweight Business Rules Engine
- Schematron is not a comprehensive, Rete rules engine, but it can be used to express rules about complex structures with an XML document.
- XML Editor Syntax Highlighting Rules
- XML Editors use Schematron rules to conditionally highlight XML files for errors.
[edit] History
Schematron was invented by Rick Jelliffe. He described Schematron as "a feather duster to reach the parts other schema languages cannot reach".
[edit] Schematron as an ISO Standard
- Schematron has been standardized to become part of
- ISO/IEC 19757 - Document Schema Definition Languages (DSDL) - Part 3: Rule-based validation - Schematron.
[edit] Namespace Standard
Any files that use ISO/IEC FDIS 19757-3 should use the following namespace:
http://purl.oclc.org/dsdl/schematron
[edit] Sample Rule
Schematron rules are very simple to create using a standard XML editor or XForms application. The following is a sample rule:
<schema xmlns="http://purl.oclc.org/dsdl/schematron"> <pattern name="Contract date must be in the past"> <rule context="Contract"> <assert test="ContractDate < current-date()">ContractDate must be in the past.</assert> </rule> </pattern> </schema>
This rule checks to make sure that the ContractDate XML element has a date that is before the current date. If this rule fails the validation will fail and an error message which is the body of the assert element will be returned to the user.
[edit] Implementation
Schematron source files are usually transformed into XSLT files (using XSLT) and placed in an XML Pipeline. This allows workflow process designers to build and maintain rules using standard XML manipulation tools.
For example an Apache Ant task can be used to convert Schematron rules into XSLT files.
[edit] See also
- XML Schema Language Comparison - Comparison to other XML Schema languages.
- Service Modeling Language - Service Modeling Language uses Schematron.
