XML Reference Guide

Mar 14, 2003

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In the world of XML validation, there are lots of choices. You can use the original method, Document Type Definitions, which are defined with XML itself, but there are lots of limitations there. You can use XML Schemas, but many developers complain that they're too complicated. You can use RELAX NG, which appears to be gathering steam lately. Or you can use Schematron.

Schematron is an interesting way of validating XML files because it is, quite frankly, not a schema at all. Where the previously mentioned validation methods define a structure for the document and attempt to validate the doucment itself against it, Schematron does no such thing. Instead, Schematron is almost more of a reporting tool, defining situations (based on XPath expressions) in which the tool is to provide either positive or negative feedback regarding the structure of the document.

For example, consider our typical validation target document:

<?xml version="1.0"?>
<order orderid="THX1138" customerNumber="3263827">
    <lineitem itemid="C33">
       <item>3/4" Hex Bolt</item>
       <quantity>36</quantity>
       <unitprice currency="dollars">.35</unitprice>
    </lineitem>
    <lineitem itemid="M48">
       <item>Condenser</item>
       <quantity>1</quantity>
       <unitprice currency="dollars">2200</unitprice>
    </lineitem>
    <delivery>Overnight</delivery>
</order>

We can say several things about this document:

The order must have an orderid and a customerNumber attribute.
An order must have at least one lineitem.
An order must have a delivery method.
A lineitem must have an itemid attribute.
A lineitem must have an item, quantity and unitprice child.
A unitprice must have a currency attribute.

We can also add ideas that have more to do with the business rules involved than the actual data, such as:

If the order contains more than one lineitem, provide free shipping.

Schematron enables us to create a document that provides information against which the document should be checked. It's based on three basic ideas: patterns, assertions, and reports.

A pattern is used to find the specific information we're checking against. An assertion is much like a validation test. If it fails, the checker outputs a message. A report is the opposite; if its test passes, the checker outputs a message. Let's see how this might work in an actual document. Consider this simple Schematron document:

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.ascc.net/xml/schematron">
    <title>Schematron order validation</title>
    <ns prefix="orders" uri="http://www.example.com/orderSystem"/>
    <pattern name="Order Structure" id="Order">
        <rule context="/order">
            <assert test="@orderid">An order must have an 
                  associated orderid.</assert>
            <assert test="@customerNumber">An order must be 
                  associated with a customer.</assert>
            <assert test="delivery">An order must have an 
                  associated delivery method.</assert>
            <report test="lineitem[2]">Customer should receive 
                  free shipping.</report>
        </rule>
    </pattern>
</schema>

Here we've created a single pattern with a single rule intended to check the order element. The rule sets the context, just as an XSLT template's match attribute sets the context in an XSLT style sheet. We've added two assertions and a report, so if we were to remove the customerNumber attribute and validate the file, we'd receive the following message from a tool such as Topologi's XML validator:

Order Structure

An order must be associated with a customer. 

/order[1]
<order orderid="THX1138">...</> 

Customer should receive free shipping. 

/order[1]
<order orderid="THX1138">...</>

In this case the test for the first assertion was true, so the message is not shown. Because we removed the customerNumber, however, the second test comes up false, the equivalent of an error. As a result, the tool outputs the associated message.

For a report, the situation is just the opposite. There are at least two lineitems, so the report test comes up true, and the tool outputs the message.

We can also create multiple rules for a single pattern:

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.ascc.net/xml/schematron">
    <title>Schematron order validation</title>
    <ns prefix="orders" uri="http://www.example.com/orderSystem"/>
    <pattern name="Order Structure" id="Order">
        <rule context="/order">
            <assert test="@orderid">An order must have an 
                            associated orderid.</assert>
            <assert test="@customerNumber">An order must be 
                             associated with a customer.</assert>
            <report test="lineitem[2]">Customer should receive 
                             free shipping.</report>
        </rule>
        <rule context="//lineitem">
             <assert test="@itemid">A line item must have an 
                             itemid.</assert>
             <assert test="item">A line item must have an item 
                             name.</assert>
             <assert test="quantity">A line item must have an 
                             associated quantity.</assert>
             <assert test="unitprice">A line item must have a unit 
                             price.</assert>
             <assert test="unitprice/@currency">A unit price must 
                             have an associated currency.</assert>
        </rule>
    </pattern>
</schema>

Or multiple patterns:

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.ascc.net/xml/schematron">
    <title>Schematron order validation</title>
    <ns prefix="orders" uri="http://www.example.com/orderSystem"/>
    <pattern name="Order Structure" id="Order">
        <rule context="/order">
            <assert test="@orderid">An order must have an 
                             associated orderid.</assert>
            <assert test="@customerNumber">An order must be 
                             associated with a customer.</assert>
            <report test="lineitem[2]">Customer should receive 
                             free shipping.</report>
        </rule>
    </pattern>
    <pattern name="Line Item Structure" id="Item">
        <rule context="//lineitem">
            <assert test="@itemid">A line item must have an 
                             itemid.</assert>
            <assert test="item">A line item must have an item 
                             name.</assert>
            <assert test="quantity">A line item must have an 
                             associated quantity.</assert>
            <assert test="unitprice">A line item must have a 
                             unit price.</assert>
            <assert test="unitprice/@currency">A unit price must 
                             have an associated currency.</assert>
        </rule>
    </pattern>
</schema>

Schematron also enables you to embed dynamic information within the output. For example, if we were to check for a non-existant attribute, we could provide information not only on where the problem exists within the structure, but within the document:

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.ascc.net/xml/schematron">
    <title>Schematron order validation</title>
    <ns prefix="orders" uri="http://www.example.com/orderSystem"/>
    <pattern name="Order Structure" id="Order">
        <rule context="/order">
            <assert test="@orderid">An order must have an 
                             associated orderid.</assert>
            <assert test="@customerNumber">An order must be 
                             associated with a customer.</assert>
            <report test="lineitem[2]">Customer should receive 
                             free shipping.</report>
        </rule>
    </pattern>
    <pattern name="Line Item Structure" id="Item">
        <rule context="//lineitem">
            <assert test="@itemid">A line item must have an 
                             itemid.</assert>
            <assert test="item">A line item must have an item 
                             name.</assert>
            <assert test="quantity">A line item must have an 
                             associated quantity.</assert>
            <assert test="unitprice">A line item must have a unit 
                             price.</assert>
            <assert test="unitprice/@currency">A unit price must 
                             have an associated currency.</assert>
            <assert test="@backordered">The <name /> shown as 
                             "<value-of select="item" /> 
                             (<value-of select="@itemid" />) " 
                             needs a backordered status.</assert>
        </rule>
    </pattern>
</schema>

Validating against this document provides the following results:

Order Structure

Customer should receive free shipping. 

/order[1]
<order orderid="THX1138" customerNumber="3263827">...</> 

Line Item Structure

The lineitem shown as " 3/4" Hex Bolt ( C33 ) " needs a backordered status. 
/order[1]/lineitem[1]
<lineitem itemid="C33">...</> 

The lineitem shown as " Condenser ( M48 ) " needs a backordered status. 
/order[1]/lineitem[2]
<lineitem itemid="M48">...</>

Schematron also enables you to create different phases, as well as nested reports, and enables you to embed Schematron information within other validation languages such as RELAX NG.

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