XRL XML Reference Linker

XRL is an XML pull runtime for combining XML as a recursive series of pulled include evaluations and URI translations. It consists of a set of accessors that provide a mechanism for flexible creation and serving of static or dynamic content.

It consists of three distinct parts:

1. mapper which maps an external uri space to an internal uri space. This allows an abitrary URI space such as a website map to be mapped into resources that are stored or generated in a different structure.
2. xrl which provides a convenient template filling and link resolution operations as well as some convenient low level document manipulation functions. It has the concept of templates and content.
3. mapper-export which exports all external links in a links document to an external location. This is useful for deploying content on a static web server.

It is implemented using the following accessors: mapper, xrl and mapper-export.

active:xrl

XRL is an accessor to perform inclusion of named links, resolution of named links and substitution of fragments.

Its purpose is to provide a single accessor solution to dynamic content publishing and solve the problems of recursive template filling, resolution of named external links and specification of title/header/footer fields.

Interface

The active URI syntax for xrl is as follows...

active:xrl +links@link_uri +template@xrl:template_link_name [+content@xrl:content_link_name] [+ other arguments ...]

The links and template arguments are mandatory, content is an optional argument described below, other arguments may be passed on to sub-requests based upon the args specified by a link.

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 (described below). The engine uses the xrl:include URI to resolve a link in the links document. The link provides an internal resource URI which is requested and substitued at the include point.

The processing order is as follows:

1. process all <xrl:include> elements in document order
2. repeat step one until there are no more <xrl:include> elements
3. resolve all external links @xrl:resolve
4. perform all xpath evaluations xrl:include[@xpath] and xrl:eval[@xpath]

Link Document

The link document has the following structure.

Link Syntax

@basepath is the basepath from which all external URIs will be resolved.

name is the link name that will be exposed as the uri xrl:link_name.

ext is an external uri which may be relative to @basepath, if no ext is defined then the link is purely for internal use.

int is an internal URI to be resolved in the private address space of the module making the XRL request.

args defines hints as to which arguments to pass to request for the uri if they are available.

URIs may use the xrl: scheme in which case they will be recursively evaluated.

Template

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

tag	description
<xrl:include xmlns:xrl="http://1060.org/xrl" href="uri" />	If the uri is xrl: resolve the named link to the internal URI. Then request the URI with additional args appended, finally substitute content into template.
<xrl:include xmlns:xrl="http://1060.org/xrl" href="uri">  <some-fragment /> </xrl:include>	Same as above but the fragment inside xrl:include tag will be appended to the internal URI request as the param argument.
<a xmlns:xrl="http://1060.org/xrl" href="xrl:name" xrl:resolve="href">click</a>	Resolve the attribute named in the xrl:resolve, if it is a uri in the xrl: scheme then find the named link and substitute the external URI into the referenced attribute.
<td xmlns:xrl="http://1060.org/xrl" style="background:url('[[xrl:image]]')" xrl:resolve="style">hello</td>	Same as xrl:resolve above but substitute all [[xrl:****]] in the xrl:resolve attribute with the external URI resolved from the link.
<style xmlns:xrl="http://1060.org/xrl" type="text/css" xrl:resolve="text"> body { background: url([[xrl:image]]); } </style>	Same as xrl:resolve above but substitutes the text of current element.
<xrl:eval xmlns:xrl="http://1060.org/xrl" fallback="'not-found'" xpath="/html/h1/text()" />	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.
<xrl:eval xmlns:xrl="http://1060.org/xrl" attribute="href" xpath="/html/h1/text()" />	Evaluate the given xpath expression to text and add an attribute to the parent element with the given attribute name and evaluated text value.
<xrl:include xmlns:xrl="http://1060.org/xrl" xpath="/html/h1" />	Evaluate the given xpath expression to a fragment and substitute this tag with the fragment. Copy and pastes fragments around a document.
<xrl:include xmlns:xrl="http://1060.org/xrl" href="xrl:content" />	xrl:content is a special link uri to replace this tag with the content argument specified on the xrl request.

Example

See the XML tech tour xrl entry.

XRL Mapper

Mapper is an accessor to take a URI request and map it to a diferent URI based on the links document syntax specified above.

Interface

active:mapper+operand@request_uri+operator@links_uri [+ other arguments ...]

Operation

The operand URI of the incoming request is used to match the external uri of a link in the links document. The interal URI is then determined which can either be a direct lookup or via a chain of named xrl: links. Any args on the mapper request are passed on to internal request according to the values specified in the args element. Finally when the internal URI is known the mapper issues a request for that URI and returns the response.

If an operand path cannot be found the mapper attempts to resolve the link xrl:404 and passes a parameter doc of the form:

If the link xrl:404 cannot be found an exception is thrown by the accessor.

If an error occurs in requesting an internal uri the mapper attempts to resolve the link xrl:500 and passes a parameter doc of the form:

If the link xrl:500 cannot be found an exception is thrown by the accessor.

If an optional <args> element is found in the links doc for an internal request then the specified args are sent to that request.