WFS and Geoserver - what exactly is FES and what is the correct syntax?

Question

If I want to list stored queries I get this error:

org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 735; The element type "null:ValueReference" must be terminated by the matching end-tag "</null:ValueReference>"

I found out, why this error occurs.

I created the Stored Query with this XML

<wfs:CreateStoredQuery service="WFS" version="2.0.0" xmlns="http://www.opengis.net/wfs/2.0" xmlns:wfs="http://www.opengis.net/wfs/2.0" xmlns:fes="http://www.opengis.org/fes/2.0" xmlns:ows="http://www.opengis.net/ows/1.1" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <wfs:StoredQueryDefinition id="test">
        <wfs:Parameter name="gemarkungsname" type="xsd:string"/>
        <wfs:QueryExpressionText returnFeatureTypes="KRE_ALKIS:sg_flurstueck_f" language="urn:ogc:def:queryLanguage:OGC-WFS::WFS_QueryExpression" isPrivate="false">
            <wfs:Query typeNames="KRE_ALKIS:sg_flurstueck_f">
                <fes:Filter>
                    <fes:PropertyIsEqualTo>


    <fes:ValueReference>gemarkungsname</fes:ValueReference>${gemarkungsname}</fes:PropertyIsEqualTo>
                    </fes:Filter>
                </wfs:Query>
            </wfs:QueryExpressionText>
        </wfs:StoredQueryDefinition>
    </wfs:CreateStoredQuery>

But on the server in the data directory was this XLS stored. Here you can see <null:Filter instead of <fes:Filter.

<?xml version="1.0" encoding="UTF-8"?><wfs:StoredQueryDescription xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:fes="http://www.opengis.net/fes/2.0" xmlns:wfs="http://www.opengis.net/wfs/2.0" xmlns:gml="http://www.opengis.net/gml/3.2" xmlns:ows="http://www.opengis.net/ows/1.1" xmlns:xlink="http://www.w3.org/1999/xlink" id="test">
    <wfs:Parameter name="gemarkungsname" type="xs:string"/>
    <wfs:QueryExpressionText isPrivate="false" language="urn:ogc:def:queryLanguage:OGC-WFS::WFS_QueryExpression" returnFeatureTypes="KRE_ALKIS:sg_flurstueck_f">
        <wfs:Query wfs:typeNames="KRE_ALKIS:sg_flurstueck_f">
            <null:Filter xmlns:null="http://www.opengis.org/fes/2.0">
                <null:PropertyIsEqualTo>${gemarkungsname}<null:ValueReference>gemarkungsname</fes:ValueReference>
                </fes:PropertyIsEqualTo>
            </fes:Filter>
        </wfs:Query>
    </wfs:QueryExpressionText>
</wfs:StoredQueryDescription>

I wonder, why the null is saved instead of fes in my Stored Query in the data directory? I can edit the XML file. I can change here null to fes and everything works fine.

Then I had a look into the capabilites with geoserver/wfs?service=wfs&version=2.0.0&request=GetCapabilities. I guess I'm making a mistake in the syntax. Here the relevant part:

<fes:Filter_Capabilities>
    <fes:Conformance>
        <fes:Constraint name="ImplementsQuery">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsAdHocQuery">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsFunctions">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsResourceId">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsMinStandardFilter">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsStandardFilter">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsMinSpatialFilter">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsSpatialFilter">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsMinTemporalFilter">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsTemporalFilter">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsVersionNav">
            <ows:NoValues/>
            <ows:DefaultValue>FALSE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsSorting">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsExtendedOperators">
            <ows:NoValues/>
            <ows:DefaultValue>FALSE</ows:DefaultValue>
        </fes:Constraint>
        <fes:Constraint name="ImplementsMinimumXPath">
            <ows:NoValues/>
            <ows:DefaultValue>TRUE</ows:DefaultValue>
        </fes:Constraint>
    </fes:Conformance>
    <fes:Id_Capabilities>
        <fes:ResourceIdentifier name="fes:ResourceId"/>
    </fes:Id_Capabilities>
    <fes:Scalar_Capabilities>
        <fes:LogicalOperators/>
        <fes:ComparisonOperators>
            <fes:ComparisonOperator name="PropertyIsLessThan"/>
            <fes:ComparisonOperator name="PropertyIsGreaterThan"/>
            <fes:ComparisonOperator name="PropertyIsLessThanOrEqualTo"/>
            <fes:ComparisonOperator name="PropertyIsGreaterThanOrEqualTo"/>
            <fes:ComparisonOperator name="PropertyIsEqualTo"/>
            <fes:ComparisonOperator name="PropertyIsNotEqualTo"/>
            <fes:ComparisonOperator name="PropertyIsLike"/>
            <fes:ComparisonOperator name="PropertyIsBetween"/>
            <fes:ComparisonOperator name="PropertyIsNull"/>
            <fes:ComparisonOperator name="PropertyIsNil"/>
        </fes:ComparisonOperators>
    </fes:Scalar_Capabilities>

WFS and Geoserver - what exactly is FES and what is the correct syntax?

Answer 1

FES is the shorthand used for the Filter Encoding Specification and is defined by xmlns:fes="http://www.opengis.net/fes/2.0". The fes bit is just an arbitrary string and could be anything.

The fact it keeps becoming NULL when stored looks like a bug, so you should ask on the GeoServer users list and then probably open a ticket about it.