main()
{   // generate infoPage using page-template
    req = context.createSubRequest("active:xrl");
    req.addArgument("template","page-template.xml");
    req.addArgument("operator","myLinks.xml");
    req.addArgument("content","infoPage.xml");
    result=context.issueSubRequest(req);
}
    

Purpose

The xrl accessor is used to generate an XML document by a combination of pulling in external fragments generated by service invocations, looking up and resolving links and applying xpath evaluations. Its features reflect those needed for the dynamic generation of web content.

For more details of XRL runtime see the guide.

By default xrl is strict and throws exceptions if includes cannot be resolved or other errors are encountered. Additional modes allow for tolerant behaviour and setting of output mimetype:

• xrl-tolerant is tolerant of missing includes
• xrl-html produces text/html mimetyped result and is strict
• xrl-html-tolerant produces text/html mimetyped result and tolerates missing includes - good for production system

If no links document is specified in the operator a default URI of ffcpl:/etc/XRLLinks.xml is used. If this cannot be found the accessor will fail.

Operation

The template document is the starting point for a recursive inclusion of linked XML resources. The XRL engine traverses the template finding xrl:includes and evaluating other xrl: operations. Links are resolved according to the links document (which is described in the XRL Guide.) The processing elements are processed in the following order:

1. process all <xrl:include> elements in document order
2. repeat step one until there are no more <xrl:include> elements (to find any xrl:includes that may have just been included)
3. resolve all external links @xrl:resolve
4. perform all xpath evaluations xrl:include[@xpath] and xrl:eval[@xpath]

Template Patterns

The following tag patterns in the namespace xmlns:xrl="http://1060.org/xrl" will be processed by the XRL accessor:

Include an arbitrary XML resource into template

The URI specified in the href attribute is requested as XML and inserted into the template document replacing this tag.

If the URI is has the xrl: scheme, resolve the named link and then request the URI with additional args (as specified in the links document) appended.

Include as above but requested with a literal XML fragment

The XML fragment inside the tag is attached as a literal argument with the name "param" to the request specified by the href attribute.

This is very useful for substituting formatted sections into a document where an external service provides the formatting and the content is provided by the template.

Resolve an attribute to the external URI of a link

The attribute specified by the xrl:resolve attribute is resolved to a link and the attributes value is substituted with the external URI of that link.

This is useful for dynamically resolving hyperlinks and image sources into HTML documents.

Resolve substrings of an attribute to the external URIs of a link

Same as xrl:resolve above but substitute all [[xrl:****]] in the xrl:resolve attribute with the external URI resolved from the link.

This is useful for placing image links into CSS style tags.

Resolve substrings of text to the external URIs of a link

Same as xrl:resolve above but substitutes the text of current element.

Replace text with the evaluation an xpath expression

Evaluate the given xpath expression for text and replace this tag with the text. If this xpath cannot be evaluated try to evaluate the optional fallback xpath expression. If this fails substitute a zero length string.

Add attribute with value of the evaluation an xpath expression

Evaluate the given xpath expression to text and add an attribute to the parent element with the given attribute name and evaluated text value.

Include a fragment from another part of the document

Evaluate the given xpath expression to a fragment and substitute this tag with the fragment. Copy and pastes fragments around a document.

Include the "content" into a template

xrl:content is a special link uri to replace this tag with the content argument specified on the xrl request.