Compass provides the ability to map XML structure to the underlying Search Engine through simple XML mapping files, we call this technology XSEM (XML to Search Engine Mapping). XSEM provides a rich syntax for describing XML mappings using Xpath expressions. The XSEM files are used by Compass to extract the required xml elements from the xml structure at run-time and inserting the required meta-data into the Search Engine index.

At the core of XSEM supports is XmlObject abstraction on top of the actual XML library implementation. The XmlObject represents an XML element (document, node, attribute, ...) which is usually the result of an Xpath expression. It allows to get the name and value of the given element, and execute Xpath expressions against it (for more information please see the XmlObject javadoc).

Here is an example of how XmlObject is used with Compass:

CompassSession session = compass.openSession();
// ...
XmlObject xmlObject = // create the actual XmlObject implementation (we will see how soon)
session.save("alias", xmlObject);

An extension to the XmlObject interface is the AliasedXmlObject interface. It represents an xml object that is also associated with an alias. This means that saving the object does not require to explicitly specify the alias that it will be saved under.

CompassSession session = compass.openSession();
// ...
AliasedXmlObject xmlObject = // create the actual XmlObject implementation (we will see how soon)
session.save(xmlObject);

Compass comes with support for dom4j and JSE 5 xml libraries, here is an example of how to use dom4j API in order to create a dom4j xml object:

CompassSession session = compass.openSession();
// ...
SAXReader saxReader = new SAXReader();
Document doc = saxReader.read(new StringReader(xml));
AliasedXmlObject xmlObject = new Dom4jAliasedXmlObject(alias, doc.getRootElement());
session.save(xmlObject);

And here is a simple example of how to use JSE 5:

CompassSession session = compass.openSession();
// ...
Document doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(new InputSource(new StringReader(xml)));
AliasedXmlObject xmlObject = NodeAliasedXmlObject(alias, doc);
session.save(xmlObject);

Up until now, Compass has no knowledge of how to parse and create an actual XmlObject implementation, or how to convert an XmlObject into its xml representation. This is perfectly fine, but it also means that systems will not be able to work with XmlObject for read/search operations. Again, this is perfectly ok for some application, since they can always work with the underlying Resource representation, but some applications would still like to store the actual xml content in the search engine, and work with the XmlObject for read/search operations.

Compass XSEM support allows to define the xml-content mapping (defined below), which will cause Compass to store the xml representation in the search engine as well. It will also mean that for read/search operations, the application will be able to get an XmlObject back (for example, using CompassSession#get operation).

In order to support this, Compass must be configured with how to parse the xml content into an XmlObject, and how to convert an XmlObject into an xml string. Compass comes with built in converters that do exactly that:

Most of the time, better performance can be achieved by pooling XmlContentConverters implementations. Compass handling of XmlContentConverter allows for three different instantiation models: prototype, pool, and singleton. prototype will create a new XmlContentConverter each time, a singleton will use a shared XmlContentConverter for all operations, and pooled will pool XmlContentConverter instances. The default is prototype.

Here is an example of a Compass schema based configuration that registers a global Xml Content converter:

<compass-core-config ...
   <compass name="default">

       <connection>
           <file path="target/test-index" />
       </connection>

       <converters>
           <converter name="xmlContentMapping"
              type="org.compass.core.converter.mapping.xsem.XmlContentMappingConverter">
              <setting name="xmlContentConverter.type"
                       value="[fully qualified class name of XmlContentConverter]" />
              <setting name="xmlContentConverter.wrapper" value="prototype" />
           </converter>
       </converters>

   </compass>
</compass-core-config>

And here is an example of a DTD (settings) based configuration file:

<!DOCTYPE compass-core-configuration PUBLIC ...
<compass-core-configuration>
   <compass>
      <setting name="compass.converter.xmlContentMapping.type">
         org.compass.core.converter.mapping.xsem.XmlContentMappingConverter
      </setting>
      <setting name="compass.converter.xmlContentMapping.xmlContentConverter.type">
         [fully qualified class name of XmlContentConverter]
      </setting>
      <setting name="compass.converter.xmlContentMapping.xmlContentConverter.wrapper">
         prototype
      </setting>
   </compass>
</<compass-core-configuration>

And last, here is how it can be configured it programmatically:

settings.setGroupSettings(CompassEnvironment.Converter.PREFIX,
   CompassEnvironment.Converter.DefaultTypeNames.Mapping.XML_CONTENT_MAPPING,
   new String[]{CompassEnvironment.Converter.TYPE, CompassEnvironment.Converter.XmlContent.TYPE},
   new String[]{XmlContentMappingConverter.class.getName(), XPP3ReaderXmlContentConverter.class.getName()});

Note, that specific converters can be associated with a specific xml-object mapping, in order to do it, simply register the converter under a different name (compass.converter.xmlContentMapping is the default name that Compass will use when nothing is configured), and use that name in the converter attribute of the xml-content mapping.

If Compass is configured with an Xml Content converter, it now knows how to parse an xml content into an XmlObject. This allows us to simplify more the creation of XmlObjects from a raw xml data. Compass comes with a wrapper XmlObject implementation, which handles raw xml data (non parsed one). Here is how it can be used:

Reader xmlData = // construct an xml reader over raw xml content
AliasedXmlObject xmlObject = RawAliasedXmlObject(alias, xmlData);
session.save(xmlObject);

Here, Compass will identify that it is a RawAliasedXmlObject, and will used the registered converter (or the one configured against the xml-content mapping for the given alias) to convert it to the appropriate XmlObject implementation. Note, that when performing any read/search operation, the actual XmlObject that will be returned is the onc the the registered converter creates, and not the raw xml object.

XML/Search Engine mappings are defined in an XML document, and maps XML data structures. The mappings are xml centric, meaning that mappings are constructed around XML data structures themselves and not internal Resources. If we take the following as a sample XML data structure:

<xml-fragment>
     <data>
         <id value="1"/>
         <data1 value="data11attr">data11</data1>
         <data1 value="data12attr">data12</data1>
     </data>
     <data>
         <id value="2"/>
         <data1 value="data21attr">data21</data1>
         <data1 value="data22attr">data22</data1>
     </data>
</xml-fragment>

We can map it using the following XSEM definition file:

<?xml version="1.0"?>
<!DOCTYPE compass-core-mapping PUBLIC
      "-//Compass/Compass Core Mapping DTD 2.0//EN"
      "http://www.compass-project.org/dtd/compass-core-mapping-2.0.dtd">

<compass-core-mapping>

  <xml-object alias="data1" xpath="/xml-fragment/data[1]">
      <xml-id name="id" xpath="id/@value" />
      <xml-property xpath="data1/@value" />
      <xml-property name="eleText" xpath="data1" />
  </xml-object>

  <xml-object alias="data2" xpath="/xml-fragment/data">
      <xml-id name="id" xpath="id/@value" />
      <xml-property xpath="data1/@value" />
      <xml-property name="eleText" xpath="data1" />
  </xml-object>

  <xml-object alias="data3" xpath="/xml-fragment/data">
      <xml-id name="id" xpath="id/@value" />
      <xml-property xpath="data1/@value" />
      <xml-property name="eleText" xpath="data1" />
      <xml-content name="content" />
  </xml-object>

</compass-core-mapping>

The mapping definition here shows three different mappings (that will work with the sample xml). The different mappings are registered under different aliases, where the alias acts as the connection between the actual XML saved and the mappings definition.

An xml-object mapping can have an associated xpath expression with it, which will narrow down the actual xml elements that will represent the top level xml object which will be mapped to the search engine. A nice benefit here, is that the xpath can return multiple xml objects, which in turn will result in multiple Resources saved to the search engine.

Each xml object mapping must have at least one xml-id mapping definition associated with it. It is used in order to update/delete existing xml objects.

In the mapping definition associated with data3 alias, the xml-content mapping is used, which stores the actual xml content in the search engine as well. This will allow to unmarshall the xml back into an XmlObject representation. For the first two mappings (data1 and data2), search/read operations will only be able to work on the Resource level.

<xml-object
       alias="aliasName"
       sub-index="sub index name"
       xpath="optional xpath expression"
       analyzer="name of the analyzer"
 />
     all?,
     sub-index-hash?,
     xml-id*,
     (xml-analyzer?),
     (xml-boost?),
     (xml-property)*,
     (xml-content?)
 

<xml-id
      name="the name of the xml id"
      xpath="xpath expression"
      value-converter="value converter lookup name"
      converter="converter lookup name"
/>

<xml-property
      xpath="xpath expression"
      name="optionally the name of the xml property"
      store="yes|no|compress"
      index="tokenized|un_tokenized|no"
      boost="boost value for the property"
      analyzer="name of the analyzer"
      reverse="no|reader|string"
      override="true|false"
      exclude-from-all="no|yes|no_analyzed"
      value-converter="value converter lookup name"
      converter="converter lookup name"
/>

<xml-analyzer
         name="property name"
         xpath="xpath expression"
         null-analyzer="analyzer name if value is null"
         converter="converter lookup name"
   >
   </xml-analyzer>

<xml-analyzer
     name="property name"
     xpath="xpath expression"
     default="the boost default value when no property value is present"
     converter="converter lookup name"
>
</xml-analyzer>

<xml-content
       name="property name"
       store="yes|compress"
       converter="converter lookup name"
 >
 </xml-content>