First draft documentation

== Simple concept presentation: ==
Store fragments of metadata as reference using XLink. Remote fragments are not editable in the metadata editor. The source document has to be modified.

=== Main principle ===
Metadata stored in DB
{{{
<gmd:contact>
<gmd:CI_ResponsibleParty xlink:href= »xml/codelist/CI_ReponsibleParty.xml#xpointer(//gmd:CI_ResponsibleParty[id='XXXX'])xmlns(gml=http://www.isotc211.org/2005/gmd) »/></gmd:contact>
<gmd:contact>
}}}


Metadata after XLink resolver operation:
{{{
		<gmd:CI_ResponsibleParty xlink:href= »xml/codelist/CI_ReponsibleParty.xml#xpointer(//gmd:CI_ResponsibleParty[id='XXXX'])xmlns(gml=http://www.isotc211.org/2005/gmd) »>
			<gmd:individualName gco:nilReason="missing">
				<gco:CharacterString/>
			</gmd:individualName>
			<gmd:organisationName>
                                    ...
		</gmd:CI_ResponsibleParty>
</gmd:contact>
}}}

== Configuration ==
In xml/schemas/iso19139/schema-suggestion.xml, catalogue administrator could define which element should propose a popup to select a remote resource and set the xlink for this elements.

 * To add a new xlink popup on an element add a new field <field name="nsPrefix:elementName" mode="xlink|xlinkOnly"/>
 * Different modes are available:
  * xlink: will allow classic editor AND xlink popup
  * xlinkOnly: will only allow xlink popup
 * If not set default editor is used.

For example :
{{{
	<!-- xLink : as defined in XSD, all PropertyType having an
		attribute group set to gco:ObjectReference could be linked
 		using xlink attributes. 
 		
		For each elements allows user to use default editor or 
		defined a remote resource using an xlink.		
	-->
	<field name="gmd:contact" mode="xlink"/>
	<field name="gmd:citedResponsibleParty" mode="xlink"/>
	<field name="gmd:pointOfContact" mode="xlinkOnly"/>
	<field name="gmd:distributionFormat" mode="xlink"/>
	<field name="gmd:descriptiveKeywords" mode="xlink"/>
	<field name="gmd:referenceSystemInfo" mode="xlink"/>
}}}	


Then if a field is set with mode = xlink, GUI is :
TODO
is mode = xlinkOnly, GUI is:
TODO

== Metadata storage and XLink resolution ==
Metadata view :
 * Get Metadata from DB
 * Resolve XLink
 * View via XSL

Metadata edit :
 * Get Metadata from DB
 * Resolve XLink
 * (optional) XSD validation
 * (optional) schematron validation
 * Metadocument to be processed for editing

The XLink resolver is used before index, export, view, edit operation.



== XLink resolver ==
 * search only XLinked tags
 * handle missing fragement, timeout (not implemented yet)
 * cache XLink fragment (not implemented yet)
 * support xpointer (not implemented yet) - use specific service to search and retrieve remote fragment in iso19139 format.



= Questions =
== Which elements to be reference by XLink ? ==
 * contact
 * keyword
 * CRS
 * ... ?

== Where to store remote fragments ? ==
 * XML files (in metadata format like iso or not)


== Others ==
 * Allow editing of remote fragments in GeoNetwork ?
 *