xpath query (filter) syntax problem?

tclotworthy · January 30th, 2011, 07:42 PM

Hello,
I am using a xml-based subscribe-notify framework called SES (OpenGIS Sensor Event Service). It has an xpath-based filtering capability to appropriately match a subscription with notifications.

Based on a subscription of:

Code:

 
<?xml version="1.0" encoding="UTF-8"?>
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:wsa="http://www.w3.org/2005/08/addressing" xmlns:wsnt="http://docs.oasis-open.org/wsn/b-2">
    <soap:Header>
        <wsa:To>http://localhost:8080/ses-main-1.0-SNAPSHOT/services/SesPortType</wsa:To>
        <wsa:Action>http://docs.oasis-open.org/wsn/bw-2/NotificationProducer/SubscribeRequest</wsa:Action>
        <wsa:MessageID>uuid:4e595160-185a-9b3c-3eb6-592c7c5b0c7a</wsa:MessageID>
        <wsa:From>
            <wsa:Address>http://www.w3.org/2005/08/addressing/role/anonymous</wsa:Address>
        </wsa:From>
    </soap:Header>
    <soap:Body>
        <wsnt:Subscribe>
            <wsnt:ConsumerReference>
                <wsa:Address>http://localhost:8082/</wsa:Address>
            </wsnt:ConsumerReference>
            <wsnt:Filter>
                <wsnt:MessageContent Dialect="http://www.w3.org/TR/1999/REC-xpath-19991116">//om:procedure@xlink:href='urn:eu:fp7:genesis:procedure:aircraft_A'</wsnt:MessageContent>
            </wsnt:Filter>
        </wsnt:Subscribe>
    </soap:Body>
</soap:Envelope>

where you see the filter (in bold: wsnt:Filter). I was thinking that xpath expression I defined would have resolved to "true" for a notification with the following structure:

Code:

 
<?xml version="1.0" encoding="UTF-8"?>
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:wsa="http://www.w3.org/2005/08/addressing" xmlns:wsnt="http://docs.oasis-open.org/wsn/b-2">
    <soap:Header>
        <wsa:To>http://localhost:8080/ses-main-1.0-SNAPSHOT/services/SesPortType</wsa:To>
        <wsa:Action>http://docs.oasis-open.org/wsn/bw-2/NotificationConsumer/Notify</wsa:Action>
        <wsa:MessageID>uuid:1b4d3025-f80a-a5b6-aa37-864c47fa1a7e</wsa:MessageID>
        <wsa:From>
            <wsa:Address>http://www.w3.org/2005/08/addressing/role/anonymous</wsa:Address>
        </wsa:From>
    </soap:Header>
    <soap:Body>
        <wsnt:Notify>
            <wsnt:NotificationMessage>
                <wsnt:Topic Dialect="http://docs.oasis-open.org/wsn/t-1/TopicExpression/Simple"> 
	IntegratedHealthIndex
                </wsnt:Topic>
                <wsnt:Message>
                    <om:Observation gml:id="o211" xmlns:om="http://www.opengis.net/om/1.0" xmlns:gml="http://www.opengis.net/gml" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:sa="http://www.opengis.net/sampling/1.0" xsi:schemaLocation="http://www.opengis.net/om/1.0 http://schemas.opengis.net/om/1.0.0/om.xsd http://www.opengis.net/sampling/1.0 http://schemas.opengis.net/sampling/1.0.0/sampling.xsd">
      <om:samplingTime>
        <gml:TimeInstant xsi:type="gml:TimeInstantType">
          <gml:timePosition>2009-05-30T01:00:00+02:00</gml:timePosition>
        </gml:TimeInstant>
      </om:samplingTime>
      <om:procedure xlink:href="urn:eu:fp7:genesis:procedure:aircraft_A"/>
      <om:observedProperty xlink:href="urn:eu:fp7:genesis:property:position"/>
      <om:featureOfInterest>
        <sa:SamplingPoint gml:id="Pos_A_01">
          <gml:name>Aircraft_B_Position</gml:name>
          <sa:sampledFeature xlink:href=""/>
          <sa:position>
            <gml:Point>
              <gml:pos srsName="urn:ogc:crs:epsg:4326">-119.853 40.817</gml:pos>
            </gml:Point>
          </sa:position>
        </sa:SamplingPoint>
      </om:featureOfInterest>
      <om:result>position</om:result>
    </om:Observation>
                </wsnt:Message>
            </wsnt:NotificationMessage>
        </wsnt:Notify>
    </soap:Body>
</soap:Envelope>

and forwarding the corresponding notification to the appropriate consumer.
Just as a sanity check, I can indeed get the notification forwarded appropriately when I make the filter wide-open using:

Code:

 <wsnt:Filter>
                <wsnt:MessageContent Dialect="http://www.w3.org/TR/1999/REC-xpath-19991116">*</wsnt:MessageContent>
            </wsnt:Filter>

however, my slightly-more-constrained filter is generating no forwarded notifications. I am hoping someone might see something obviously wrong with my xpath expression, or else be able to give me some tips. Any reply is appreciated. Thank you!

samjudson · January 31st, 2011, 03:15 AM

That isn't a valid xpath expression. You want the predicate to be in square brackets:

Code:

//om:procedure[@xlink:href='urn:eu:fp7:genesis:procedure:aircraft_A']

mhkay · January 31st, 2011, 05:04 AM

Check the SES documentation to see how it handles namespace prefixes in XPath expressions. Your expression has a prefix "om" that isn't bound to any namespace in the context of the expression. It might take the namespace bindings from the source document, in which case you're OK - or it might not.

tclotworthy · January 31st, 2011, 09:11 AM

thanks for replies thus far. I tried just now:

Code:

//om:procedure[@xlink:href='urn:eu:fp7:genesis:procedure:aircraft_A']

but I still am receiving no response.

Also, based upon what mhkay said about namespace prefixes, I was wondering whether a simpler expression like:

Code:

<wsnt:Filter>
                <wsnt:MessageContent Dialect="http://www.w3.org/TR/1999/REC-xpath-19991116">//@xlink:href='urn:eu:fp7:genesis:procedure:aircraft_A'</wsnt:MessageContent>
            </wsnt:Filter>

but this did not work either. Grateful for any other ideas! Thanks!

tclotworthy · February 1st, 2011, 11:57 AM

yes it was the fact that the xpath processor didn't have the bindings for the om and xlink namespaces. All works now. Thanks!