Tutorial

This tutorial demonstrates how to configure two complex feature types using the app-schema plugin and data from two property files.

GeoSciML

This example uses Geoscience Markup Language (GeoSciML) 2.0, a GML 3.1 application schema:

“GeoSciML is an application schema that specifies a set of feature-types and supporting structures for information used in the solid-earth geosciences.”

The tutorial defines two feature types:

  1. gsml:GeologicUnit, which describes “a body of material in the Earth”.

  2. gsml:MappedFeature, which describes the representation on a map of a feature, in this case gsml:GeologicUnit.

Because a single gsml:GeologicUnit can be observed at several distinct locations on the Earth’s surface, it can have a multivalued gsml:occurrence property, each being a gsml:MappedFeature.

Installation

  • Install GeoServer as usual.

  • Install the app-schema plugin geoserver-*-app-schema-plugin.zip:

    • Place the jar files in WEB-INF/lib.

    • The tutorial folder contains the GeoServer configuraration (data directory) used for this tutorial.

      • Either replace your existing data directory with the tutorial data directory,

      • Or edit WEB-INF/web.xml to set GEOSERVER_DATA_DIR to point to the tutorial data directory. (Be sure to uncomment the section that sets GEOSERVER_DATA_DIR.)

  • Perform any configuration required by your servlet container, and then start the servlet. For example, if you are using Tomcat, configure a new context in server.xml and then restart Tomcat.

  • The first time GeoServer starts with the tutorial configuration, it will download all the schema (XSD) files it needs and store them in the app-schema-cache folder in the data directory. You must be connected to the internet for this to work.

datastore.xml

Each data store configuration file datastore.xml specifies the location of a mapping file and triggers its loading as an app-schema data source. This file should not be confused with the source data store, which is specified inside the mapping file.

For gsml_GeologicUnit the file is workspaces/gsml/gsml_GeologicUnit/datastore.xml:

<dataStore>
    <id>gsml_GeologicUnit_datastore</id>
    <name>gsml_GeologicUnit</name>
    <enabled>true</enabled>
    <workspace>
        <id>gsml_workspace</id>
    </workspace>
    <connectionParameters>
        <entry key="namespace">urn:cgi:xmlns:CGI:GeoSciML:2.0</entry>
        <entry key="url">file:workspaces/gsml/gsml_GeologicUnit/gsml_GeologicUnit.xml</entry>
        <entry key="dbtype">app-schema</entry>
    </connectionParameters>
</dataStore>

For gsml:MappedFeature the file is workspaces/gsml/gsml_MappedFeature/datastore.xml:

<dataStore>
    <id>gsml_MappedFeature_datastore</id>
    <name>gsml_MappedFeature</name>
    <enabled>true</enabled>
    <workspace>
        <id>gsml_workspace</id>
    </workspace>
    <connectionParameters>
        <entry key="namespace">urn:cgi:xmlns:CGI:GeoSciML:2.0</entry>
        <entry key="url">file:workspaces/gsml/gsml_MappedFeature/gsml_MappedFeature.xml</entry>
        <entry key="dbtype">app-schema</entry>
    </connectionParameters>
</dataStore>

Note

Ensure that there is no whitespace inside an entry element.

Mapping files

Configuration of app-schema feature types is performed in mapping files:

  • workspaces/gsml/gsml_GeologicUnit/gsml_GeologicUnit.xml

  • workspaces/gsml/gsml_MappedFeature/gsml_MappedFeature.xml

Namespaces

Each mapping file contains namespace prefix definitions:

<Namespace>
    <prefix>gml</prefix>
    <uri>http://www.opengis.net/gml</uri>
</Namespace>
<Namespace>
    <prefix>gsml</prefix>
    <uri>urn:cgi:xmlns:CGI:GeoSciML:2.0</uri>
</Namespace>
<Namespace>
    <prefix>xlink</prefix>
    <uri>http://www.w3.org/1999/xlink</uri>
</Namespace>

Only those namespace prefixes used in the mapping file need to be declared, so the mapping file for gsml:GeologicUnit has less.

Source data store

The data for this tutorial is contained in two property files:

  • workspaces/gsml/gsml_GeologicUnit/gsml_GeologicUnit.properties

  • workspaces/gsml/gsml_MappedFeature/gsml_MappedFeature.properties

Java Properties describes the format of property files.

For this example, each feature type uses an identical source data store configuration. This directory parameter indicates that the source data is contained in property files named by their feature type, in the same directory as the corresponding mapping file:

<sourceDataStores>
     <DataStore>
         <id>datastore</id>
         <parameters>
             <Parameter>
                 <name>directory</name>
                 <value>file:./</value>
             </Parameter>
         </parameters>
     </DataStore>
 </sourceDataStores>

See Data Stores for a description of how to use other types of data stores such as databases.

Target types

Both feature types are defined by the same XML Schema, the top-level schema for GeoSciML 2.0. This is specified in the targetTypes section. The type of the output feature is defined in targetElement in the typeMapping section below:

<targetTypes>
    <FeatureType>
        <schemaUri>http://www.geosciml.org/geosciml/2.0/xsd/geosciml.xsd</schemaUri>
    </FeatureType>
</targetTypes>

In this case the schema is published, but because the OASIS XML Catalog is used for schema resolution, a private or modified schema in the catalog can be used if desired.

Mappings

The typeMappings element begins with configuration elements. From the mapping file for gsml:GeologicUnit:

<typeMappings>
    <FeatureTypeMapping>
        <sourceDataStore>datastore</sourceDataStore>
        <sourceType>gsml_GeologicUnit</sourceType>
        <targetElement>gsml:GeologicUnit</targetElement>
  • The mapping starts with sourceDataStore, which gives the arbitrary identifier used above to name the source of the input data in the sourceDataStores section.

  • sourceType gives the name of the source simple feature type. In this case it is the simple feature type gsml_GeologicUnit, sourced from the rows of the file gsml_GeologicUnit.properties in the same directory as the mapping file.

  • When working with databases sourceType is the name of a table or view. Database identifiers must be lowercase for PostGIS or uppercase for Oracle Spatial.

  • targetElement is the name of the output complex feature type.

gml:id mapping

The first mapping sets the gml:id to be the feature id specified in the source property file:

<AttributeMapping>
    <targetAttribute>
        gsml:GeologicUnit
    </targetAttribute>
    <idExpression>
        <OCQL>ID</OCQL>
    </idExpression>
</AttributeMapping>
  • targetAttribute is the XPath to the element for which the mapping applies, in this case, the top-level feature type.

  • idExpression is a special form that can only be used to set the gml:id on a feature. Any field or CQL expression can be used, if it evaluates to an NCName.

Ordinary mapping

Most mappings consist of a target and source. Here is one from gsml:GeologicUnit:

<AttributeMapping>
    <targetAttribute>
        gml:description
        </targetAttribute>
    <sourceExpression>
        <OCQL>DESCRIPTION</OCQL>
    </sourceExpression>
</AttributeMapping>
  • In this case, the value of gml:description is just the value of the DESCRIPTION field in the property file.

  • For a database, the field name is the name of the column (the table/view is set in sourceType above). Database identifiers must be lowercase for PostGIS or uppercase for Oracle Spatial.

  • CQL expressions can be used to calculate content. Use caution because queries on CQL-calculated values prevent the construction of efficient SQL queries.

  • Source expressions can be CQL literals, which are single-quoted.

Client properties

In addition to the element content, a mapping can set one or more “client properties” (XML attributes). Here is one from gsml:MappedFeature:

<AttributeMapping>
    <targetAttribute>
        gsml:specification
    </targetAttribute>
    <ClientProperty>
        <name>xlink:href</name>
        <value>GU_URN</value>
    </ClientProperty>
</AttributeMapping>
  • This mapping leaves the content of the gsml:specification element empty but sets an xlink:href attribute to the value of the GU_URN field.

  • Multiple ClientProperty mappings can be set.

In this example from the mapping for gsml:GeologicUnit both element content and an XML attribute are provided:

<AttributeMapping>
    <targetAttribute>
        gml:name[1]
        </targetAttribute>
    <sourceExpression>
        <OCQL>NAME</OCQL>
    </sourceExpression>
    <ClientProperty>
        <name>codeSpace</name>
        <value>'urn:x-test:classifierScheme:TestAuthority:GeologicUnitName'</value>
    </ClientProperty>
</AttributeMapping>
  • The codespace XML attribute is set to a fixed value by providing a CQL literal.

  • There are multiple mappings for gml:name, and the index [1] means that this mapping targets the first.

targetAttributeNode

If the type of a property is abstract, a targetAttributeNode mapping must be used to specify a concrete type. This mapping must occur before the mapping for the content of the property.

Here is an example from the mapping file for gsml:MappedFeature:

<AttributeMapping>
    <targetAttribute>gsml:positionalAccuracy</targetAttribute>
    <targetAttributeNode>gsml:CGI_TermValuePropertyType</targetAttributeNode>
</AttributeMapping>
<AttributeMapping>
    <targetAttribute>gsml:positionalAccuracy/gsml:CGI_TermValue/gsml:value</targetAttribute>
    <sourceExpression>
        <OCQL>'urn:ogc:def:nil:OGC:missing'</OCQL>
    </sourceExpression>
    <ClientProperty>
        <name>codeSpace</name>
        <value>'urn:ietf:rfc:2141'</value>
    </ClientProperty>
</AttributeMapping>
  • gsml:positionalAccuracy is of type gsml:CGI_TermValuePropertyType, which is abstract, so must be mapped to its concrete subtype gsml:CGI_TermValuePropertyType with a targetAttributeNode mapping before its contents can be mapped.

  • This example also demonstrates that mapping can be applied to nested properties to arbitrary depth. This becomes unmanageable for deep nesting, where feature chaining is preferred.

Feature chaining

In feature chaining, one feature type is used as a property of an enclosing feature type, by value or by reference:

<AttributeMapping>
    <targetAttribute>
        gsml:occurrence
    </targetAttribute>
    <sourceExpression>
        <OCQL>URN</OCQL>
        <linkElement>gsml:MappedFeature</linkElement>
        <linkField>gml:name[2]</linkField>
    </sourceExpression>
    <isMultiple>true</isMultiple>
</AttributeMapping>
  • In this case from the mapping for gsml:GeologicUnit, we specify a mapping for its gsml:occurrence.

  • The URN field of the source gsml_GeologicUnit simple feature is use as the “foreign key”, which maps to the second gml:name in each gsml:MappedFeature.

  • Every gsml:MappedFeature with gml:name[2] equal to the URN of the gsml:GeologicUnit under construction is included as a gsml:occurrence property of the gsml:GeologicUnit (by value).

WFS response

When GeoServer is running, test app-schema WFS in a web browser. If GeoServer is listening on localhost:8080 you can query the two feature types using these links:

gsml:GeologicUnit

Feature chaining has been used to construct the multivalued property gsml:occurrence of gsml:GeologicUnit. This property is a gsml:MappedFeature. The WFS response for gsml:GeologicUnit combines the output of both feature types into a single response. The first gsml:GeologicUnit has two gsml:occurrence properties, while the second has one. The relationships between the feature instances are data driven.

Because the mapping files in the tutorial configuration do not contain attribute mappings for all mandatory properties of these feature types, the WFS response is not schema-valid against the GeoSciML 2.0 schemas. Schema-validity can be achieved by adding more attribute mappings to the mapping files.

Note

These feature types are defined in terms of GML 3.1 (the default for WFS 1.1.0); other GML versions will not work.

Warning

The web interface does not yet support app-schema store or layer administration.

Acknowledgements

gsml_GeologicUnit.properties and gsml_MappedFeature.properties are derived from data provided by the Department of Primary Industries, Victoria, Australia. For the purposes of this tutorial, this data has been modified to the extent that it has no real-world meaning.