pt:sources

The sources property is used to set the active source label for an element and its descendants. Its expression is used directly as a key to a string -> source label mapping provided by the product processor to PT on instantiation. The resolved active source label acts as the evaluation context of the XPath expressions of other properties, and as the source of fetches. The sources property must therefore be declared on - or on an ancestor of - any element that declares any other PT properties.

Given how the notion of an active source is defined, it is currently not possible to assign property expressions declared on the same element to different source label evaluation contexts. Note, however, that as detailed in a later section the template element is accessible by various means within XPath expressions, regardless of which source is active for an element.

Source groups

A sources key can map to a sequence of source labels - a "source group". In this case, the implied intent is to duplicate the subtree starting from the given element, producing siblings that are each assigned a single label from the source group as their active source. This behaviour is especially useful when fetching the same pluralisable class from multiple source products, e.g. in the case of generating derived composite products where the aggregation of certain metadata entries from each source product might be desired.

The following example shows the source group functionality used to construct a reference to each input product in the group mapped by the all key.

<emrsp_rm:Processing_Inputs pt:fetch="false()">
    <emrsp_rm:Processing_Input_Identification pt:sources="all"> <!-- one entry each for all source products -->
        <emrsp_rm:type>PDS product</emrsp_rm:type>
        <emrsp_rm:Operational_Reference>
            <lid_reference/>
            <emrsp_rm:operational_vid/>
        </emrsp_rm:Operational_Reference>
    </emrsp_rm:Processing_Input_Identification>
</emrsp_rm:Processing_Inputs>

Notice that the cardinality of emrsp_rm:Processing_Input_Identification is determined at runtime by the number of source labels assigned to the all group by the product processor. It is therefore trivial to support derived output products that need to accommodate a variable number of input products.