EXPRESS & STEP in Practice — EXPRESS Language Foundation

ISO 10303-28 defines an XML representation for EXPRESS data, providing an alternative to the Part 21 clear-text format for applications that work with XML-based toolchains. Part 28 maps EXPRESS schemas to XML Schemas (XSD) and EXPRESS instance data to XML documents that conform to those schemas.

This module covers the concepts, structure, and configuration options of the STEP Part 28 format.

Why XML?

While Part 21 is the traditional STEP exchange format, XML offers several advantages in modern computing environments:

Web compatibility — XML is natively supported by web browsers, web services, and most enterprise integration platforms
Schema validation — XML Schema (XSD) provides built-in validation of document structure
Tool ecosystem — a vast ecosystem of XML parsers, transformers (XSLT), and query tools (XPath, XQuery) is available
Human readability — like Part 21, XML documents are text-based and can be inspected directly

Part 28 defines a systematic mapping from EXPRESS constructs to XML Schema constructs, ensuring that every valid EXPRESS data set can be represented as a valid XML document.

Mapping Concepts

A Part 28 configuration defines how an EXPRESS schema maps to an XML Schema:

Entities → XML elements
Attributes → XML attributes or nested elements
Data types → XML Schema types
Relationships → XML references or nested elements

The result is an XML Schema definition and corresponding XML document instances.

XML Schema Example

A typical Part 28 XML Schema maps an EXPRESS schema to a target namespace:

<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema targetNamespace="http://www.example.org/ifcXML/IFC2X2_FINAL"
  xmlns:ifc="http://www.example.org/ifcXML/IFC2X2_FINAL"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  xmlns:ex="urn:iso10303-28:ex"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  elementFormDefault="qualified" attributeFormDefault="unqualified">
  <xs:import namespace="urn:iso10303-28:ex" schemaLocation="ex.xsd" />
  <xs:element name="uos" type="ifc:uos" substitutionGroup="ex:uos" />
  <xs:complexType name="uos">
    <xs:complexContent>
      <xs:restriction base="ex:uos">
        <xs:choice minOccurs="0" maxOccurs="unbounded">
          <xs:element ref="ifc:Ifc2DCompositeCurve" />
          <xs:element ref="ifc:IfcActionRequest" />
          ...
        </xs:choice>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>
</xs:schema>

The ex namespace (urn:iso10303-28:ex) contains common definitions shared across all Part 28 documents.

XML Document Structure

A Part 28 XML document is governed by:

Configuration — defines how the EXPRESS schema maps to XML
governing_schema — the EXPRESS schema being represented
Schema population — the actual data instances
governed_sections — sections of the document controlled by specific schemas

The root element contains UOS (Units Of Structure) instances — the top-level container for all data.

Configuration Options

Part 28 provides flexible configuration options that control how EXPRESS data is serialized to XML:

<configuration id="default">
  <option
    naming-convention="preserve-case"
    exp-type="root"
    exp-attribute="double-tag"
    tagless="true"
    flatten="false"
    sparse="false">
  </option>
  <entity select="product">
    <attribute select="LongName" name="FullName"></attribute>
    <inverse select="IsDecomposedBy"></inverse>
  </entity>
</configuration>

Key configuration options:

naming-convention — controls case mapping: preserve-case, initial-upper, or camel-case
exp-type — how EXPRESS types are represented: root, value, or unspecified
exp-attribute — how attributes are encoded: double-tag, attribute-tag, entity-tag, or attribute-content
tagless — omit entity wrapper tags when possible
flatten — flatten nested structures
sparse — omit attributes with unset values

Per-entity overrides allow fine-tuning the mapping for specific entities, such as renaming attributes or excluding inverse attributes.

Reading and Writing Part 28 Documents

The typical workflow for working with Part 28 involves:

Define a configuration — specify how the EXPRESS schema maps to XML, or use a default configuration
Export to XML — serialize a data population as an XML document conforming to the generated XML Schema
Import from XML — parse an XML document and create EXPRESS instances in a data management system

Both full and partial models can be exported and imported. Partial exports are useful for exchanging subsets of a large model, such as all instances matching a particular query.

Advantages and Limitations

Advantages

Standards-based — fully compliant with ISO 10303-28
XML toolchain compatibility — can be processed with standard XML tools (XSLT, DOM, SAX, etc.)
Web-friendly — suitable for HTTP-based data exchange and REST APIs
Schema validation — XML Schema validation catches structural errors before data processing begins

Limitations

Verbosity — XML is typically more verbose than Part 21 for the same data, resulting in larger file sizes
Performance — parsing large XML documents can be slower than parsing Part 21 files
Configuration complexity — the many configuration options can be daunting; understanding which combination suits a particular use case requires experience

Choosing Between Part 21 and Part 28

The choice depends on the use case:

Part 21 is preferred for traditional STEP data exchange between CAD/CAM/CAE systems, where the format has been established for decades and tool support is mature.
Part 28 is preferred when integrating with XML-based systems, web services, or enterprise service buses, or when the data needs to be processed by standard XML tools.

Many organizations use both formats — Part 21 for system-to-system data exchange within the engineering domain, and Part 28 for publishing data to enterprise systems or web-based applications.

STEP Part 28 XML Format