Boolean Result for Filter Statement

96 Copyright © 2010 Open Geospatial Consortium, Inc.

9.1.3 Resolve Content Given By Reference

The GML model allow values of feature properties to be given by a reference a URL in an xlink:href property rather than inline of the XML instance. The idea is that a system that receives such an XML instance would automatically resolve the reference to get the value and insert it into the inline content. An Event Service should automatically retrieve event properties given by reference if they are queried in the filter statements of a subscription. This can happen on demand. Problems can occur if the value cannot be resolved e.g. if wrong connection timeouts have been chosen or if the value itself is complex and has properties given by reference. Such deep reference structures can slow down an Event Service. The effects and possible performance issues of using event properties given by reference in event filtering should be investigated in more detail. When using xlink references encoded according to the GML specification, data providers that create event instances should be aware that applications receiving the events may not preserve the namespace-prefix binding for the XPath expression used in the reference. Consider the following example: xlink:href=http:www.aixm.aeroschema5.1dnotamexample elementdnotam:Event[gml:identifier=aPreviousId] It shows an xlink with a URI reference that the GML standard OGC 07-036 calls an element scheme based XPointer . The absolute URI http:www.aixm.aeroschema5.1dnotamexample is followed by the fragment identifier and then the XPointer. Although this behavior is not recommended by the W3C, an application that receives an event e.g. an XML router application could change the prefixes in the attributes and elements of an XML instance but not in the content of them. In the above example, the dnotam and gml prefix would then no longer bind to the correct namespaces. A service could no longer resolve the correct element from the given URI. Therefore, it is more safe to use an xpointer scheme based XPointer see OGC 07-036 – the example would then look like this: xlink:href=http:www.aixm.aeroschema5.1dnotamexample xmlnsdnotam=http:www.aixm.aeroschema5.1dnotam xmlnsgml= http:www.opengis.netgml3.2 xpointerdnotam:Event[gml:identifier=aPreviousId] Here, the namespace-prefix binding is preserved in the reference itself.

9.1.4 Reference System Transformation

When a filter statement is created to be used in subscriptions, it often includes filter operand values. In simple applications, the reference system of these operands e.g. a