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<artheader>
<title>OLink Semantics in the DocBook DSSSL Stylesheets</title>
<author><firstname>Norman</firstname><surname>Walsh</surname></author><pubdate>
10 Sep 1998</pubdate>
<abstract>
<para><sgmltag>OLink</sgmltag> allows you to construct cross-document links
in SGML. This document describes the <sgmltag>OLink</sgmltag> semantics supported
by the DocBook stylesheets.</para>
</abstract>
</artheader>
<sect1 id="olink-understanding">
<title>Understanding OLink</title>
<para>The semantics of <sgmltag>OLink</sgmltag> are effected by both the <sgmltag>
OLink</sgmltag> tag and the <sgmltag>ModeSpec</sgmltag> tag.</para>
<sect2>
<title><sgmltag>OLink</sgmltag></title>
<para><sgmltag>OLink</sgmltag> is a wrapper around the text which forms the
head of the link. It has four relevant attributes:</para>
<variablelist>
<varlistentry><term><sgmltag class="attribute">TargetDocEnt</sgmltag></term>
<listitem>
<para>This <literal>ENTITY</literal> attribute points to the document which
is or contains the link target.</para>
</listitem>
</varlistentry>
<varlistentry><term><sgmltag class="attribute">LocalInfo</sgmltag></term>
<listitem>
<para>Contains the ID of the target in the document specified by <sgmltag
class="attribute">TargetDocEnt</sgmltag>. <sgmltag class="attribute">LocalInfo
</sgmltag> is the equivalent of <sgmltag class="attribute">LinkEnd</sgmltag>
on the other linking elements.</para>
</listitem>
</varlistentry>
<varlistentry><term><sgmltag class="attribute">LinkMode</sgmltag></term>
<listitem>
<para>Points to a <sgmltag>ModeSpec</sgmltag> element which further modifies
the semantics of the link.</para>
</listitem>
</varlistentry>
<varlistentry><term><sgmltag class="attribute">Type</sgmltag></term>
<listitem>
<para>Identifies the link type. The DocBook stylesheets include special processing
for <sgmltag class="attribute">Type</sgmltag>=<literal>href</literal>.</para>
<para>If the link type is &ldquo;href&rdquo;, then the HREF attribute from
the <link linkend="olink-general">summary document</link> is used directly,
without redirection through a CGI script.<?Pub Caret></para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
<title><sgmltag>ModeSpec</sgmltag></title>
<para>An <sgmltag>OLink</sgmltag> element can refer to a <sgmltag>ModeSpec
</sgmltag> element to further modify the semantics of the link. Two aspects
of the <sgmltag>ModeSpec</sgmltag> element are relevant:</para>
<variablelist>
<varlistentry><term><sgmltag class="attribute">XRefLabel</sgmltag></term>
<listitem>
<para>If the content of <sgmltag>OLink</sgmltag> is empty, it is possible
to use <sgmltag class="attribute">XRefLabel</sgmltag> to control the format
of generated text.</para>
</listitem>
</varlistentry>
<varlistentry><term><sgmltag>ModeSpec</sgmltag> Content</term>
<listitem>
<para>If the content of the <sgmltag>ModeSpec</sgmltag> element is not empty,
it is used by the HTML stylesheet in the contruction of the HTML link &ldquo;href&rdquo;.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="olink-html">
<title>OLinking in HTML</title>
<para>Constructing cross-document links with entities in the authoring system
is all well and good, but how does it work on the web? The heart of the matter
is mapping from the SGML/XML entity in the <sgmltag class="attribute">TargetDocEnt
</sgmltag> to the base URL on the web, and there are two options: early binding
and late binding.</para>
<sect2>
<title>Early Binding</title>
<para>To use early binding, you must know the mapping from entities to URLs
in advance. The base URL is supplied in the content of the <sgmltag>ModeSpec
</sgmltag> element and that is what the processor uses to construct the HTML
&ldquo;href&rdquo;. For example, given the document in <xref linkend="exam1">,
the stylesheet would generate something like this for the link:</para>
<screen>&lt;A CLASS="OLINK" HREF="http://nwalsh.com/otherdocs/myotherdoc.htm#idval">This
is an OLink&lt;/a></screen>
<para>Note that the <sgmltag class="attribute">LocalInfo</sgmltag> is used
as the fragment identifier.</para>
<example id="exam1">
<title>An Early Binding Sample Document</title>
<screen>&lt;!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook V3.1//EN" [
&lt;!ENTITY otherdoc PUBLIC "-//Norman Walsh//DOCUMENT My Other Document V1.0//EN">
]>
&lt;chapter>
&lt;docinfo>
&lt;modespec id=otherms>http://nwalsh.com/otherdocs/myotherdoc.htm&lt;/modespec>
&lt;/docinfo>
&lt;title>Test Document&lt;/title>
&lt;para>
&lt;olink targetdocent=otherdoc linkmode="otherms" localinfo="idval">This is
an OLink&lt;/olink>.
&lt;/para>
&lt;/chapter></screen>
</example>
</sect2>
<sect2>
<title>Late Binding</title>
<para>Late binding delays the resolution of entity to URL mapping until the
link is followed. It does this by requiring that a web server process resolve
the link. &ldquo;Out of the box,&rdquo; the DocBook stylesheets assume that
a cgi-bin script on the server called <filename>/cgi-bin/olink</filename>
is responsible for the resolution.</para>
<para>Given the document in <xref linkend="exam2">, late binding would produce
a link something like this:</para>
<screen>&lt;A CLASS="OLINK" HREF="/cgi-bin/olink?pubid=-//Norman%20Walsh
     //DOCUMENT%20My%20Other%20Document%20V1.0//EN&amp;fragid=idval">
This is an OLink&lt;/a></screen>
<para>(without the line break in the middle of the public identifier, of course).
</para>
<para>Again, note that the <sgmltag class="attribute">LocalInfo</sgmltag>
is used as the fragment identifier.</para>
<example id="exam2">
<title>A Sample Document</title>
<screen>&lt;!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook V3.1//EN" [
&lt;!ENTITY otherdoc PUBLIC "-//Norman Walsh//DOCUMENT My Other Document V1.0//EN">
]>
&lt;chapter>
&lt;docinfo>
&lt;/docinfo>
&lt;title>Test Document&lt;/title>
&lt;para>
&lt;olink targetdocent=otherdoc localinfo="idval">This is an OLink&lt;/olink>.
&lt;/para>
&lt;/chapter></screen>
</example>
<para>If a public identifier is used to declare the entity, then it is passed
to the link resolution script; otherwise the system identifier is passed.
</para>
</sect2>
</sect1>
<sect1>
<title>OLink Generated Text</title>
<para>Even when links span across documents, it's useful to have the stylesheet
generate appropriate cross-reference text. This greatly reduces the &ldquo;fragility&rdquo;
of the links at the expense of some complexity in the <sgmltag>OLink</sgmltag>
processing system.</para>
<para>If you supply content in an <sgmltag>OLink</sgmltag>, that content is
always used and generated text processing does not apply.</para>
<para>The format of generated text is controlled by the <sgmltag class="attribute">
XRefLabel</sgmltag> attribute on the <sgmltag>ModeSpec</sgmltag> pointed to
by the <sgmltag>OLink</sgmltag>. The text of the <sgmltag class="attribute">
XRefLabel</sgmltag> attribute is used for the generated text, with the following
substitutions:</para>
<variablelist>
<varlistentry><term><literal>%g</literal></term>
<listitem>
<para>Is replaced by the &ldquo;name&rdquo; of the link target. This is generally
the &ldquo;human readable&rdquo; name of the target element. For example,
if the target is a <sgmltag>Figure</sgmltag>, <literal>%g</literal> would
be &ldquo;Figure&rdquo;; if the target is a <sgmltag>Sect3</sgmltag>, <literal>
%g</literal> would be &ldquo;Section&rdquo;.</para>
</listitem>
</varlistentry>
<varlistentry><term><literal>%n</literal></term>
<listitem>
<para>Is replaced by the label (number) of the link target.</para>
</listitem>
</varlistentry>
<varlistentry><term><literal>%t</literal></term>
<listitem>
<para>Is replaced by the title of the link target.</para>
</listitem>
</varlistentry>
</variablelist>
<para>Suppose, for example, that the link target is the second section in
the first chapter of a book and that it has the title &ldquo;My Test Title&rdquo;.
If the <sgmltag class="attribute">XRefLabel</sgmltag> attribute contains the
text &ldquo;<literal>see %g %n, %t</literal>&rdquo;, then the generated content
would be &ldquo;see Section 1.3, <emphasis>My Test Title</emphasis>&rdquo;.
</para>
</sect1>
<sect1>
<title>Really Simple OLinks</title>
<para>If the author provides content in the <literal>OLink</literal> element,
that's the content that is used and no extra processing is required. The sections
that follow each deal with variations in the level of processing required
for generated text in <sgmltag>OLink</sgmltag>s.</para>
</sect1>
<sect1 id="olink-simple">
<title>Simple OLinks</title>
<para>Simple <sgmltag>OLink</sgmltag>s avoid some of the gory machinery required
to handle <link linkend="olink-general">general</link> <sgmltag>OLink</sgmltag>s
at the expense of most semantic variation.</para>
<para>The simple semantics come into play when an <sgmltag>OLink</sgmltag>
has the following form:</para>
<synopsis>&lt;olink targetdocent="entity">&lt;/olink></synopsis>
<para>In particular, note that there is no <sgmltag class="attribute">LinkMode
</sgmltag>. The generated text in this case is derived entirely from the public
and system identifiers by the <function>(olink-resource-title)</function>
function. By default, this is simply the title of the document derived from
the description field in the public identifier. The first and last &ldquo;words&rdquo;
of the public identifier description field are trimmed off, leaving what is
presumably just the document title. For example, given &ldquo;<literal>-//Norman
Walsh//DOCUMENT My Document Title V1.0//EN</literal>&rdquo;, the derived title
would be &ldquo;<emphasis>My Document Title</emphasis>&rdquo;.</para>
</sect1>
<sect1 id="olink-general">
<title>General OLinks</title>
<para>The generated text for general <sgmltag>OLink</sgmltag>s comes from
the <sgmltag class="attribute">XRefLabel</sgmltag> attribute on the relevant <sgmltag>
ModeSpec</sgmltag>. The hard part is locating the appropriate replacement
text: the name, label, and title of the element pointed to by the combination
of the <sgmltag class="attribute">TargetDocEnt</sgmltag> and <sgmltag class="attribute">
LocalInfo</sgmltag>.</para>
<para>One way to do this would be to load the <sgmltag class="attribute">
TargetDocEnt</sgmltag>, find the element with the ID mentioned in <sgmltag
class="attribute">LocalInfo</sgmltag> and extract the data directly. I chose
something else because I see two significant problems with this approach:
</para>
<orderedlist>
<listitem><para>Loading and parsing potentially large documents potentially
many times appears to have the potential for significant performance problems.
</para>
</listitem>
<listitem><para>It would not be possible to form <sgmltag>OLink</sgmltag>
references to documents written in DTDs other than DocBook.</para>
</listitem>
</orderedlist>
<para>Instead of loading the actual target document, the stylesheets load
a summary of that document's content. For DocBook documents, this summary
can be generated by another DSSSL stylesheet, <filename>olink.dsl</filename>,
supplied with the DocBook Stylesheet distribution. The first few lines of
the summary for this document is shown in looks like this:<xref linkend="exam3">.
</para>
<example id="exam3">
<title>OLink Summary Document</title>
<programlisting>&lt;!DOCTYPE div PUBLIC "-//Norman Walsh//DTD DocBook OLink Summary V1.0//EN">
&lt;div type="article" name="Article">
  &lt;ttl>OLink Semantics in the DocBook DSSSL Stylesheets&lt;/ttl>
  &lt;div type="sect1" name="Section" id="OLINK-UDERSTANDING" label="1">
    &lt;ttl>Understanding OLink&lt;/ttl>
    &lt;div type="sect2" name="Section" label="1.1">
      &lt;ttl>OLink&lt;/ttl>
    &lt;/div>
...</programlisting>
</example>
<para>The basic organization of the summary document is a nested series of <sgmltag>
div</sgmltag>s and <sgmltag>obj</sgmltag>s with titles (<sgmltag>ttl</sgmltag>s).
Attributes on these elements provide the IDs, labels, and names of the elements.
The GI of the element is also provided. (If you find objects that you think
are missing from the summary, please let me know).</para>
<para>The stylesheets locate this document by resolving the system identifier
of the target document and replacing the SGML or XML extension with <literal>
%olink-outline-ext%</literal> (<literal>.olink</literal>, by default).</para>
</sect1>
<sect1>
<title>Intra-document OLinks</title>
<para>If the <sgmltag class="attribute">TargetDocEnt</sgmltag> attribute is
missing, it seems reasonable to assume that the link refers to the current
document. This actually offers a feature, you can customize the generated
cross reference text for a particular reference.</para>
<para>When the <sgmltag>OLink</sgmltag> is an intra-document link, it is possible
to use additional %-substitutions in the cross reference template. See <function>
(auto-xref)</function> in <sgmltag>dbcommon.dsl</sgmltag> for more details.
</para>
</sect1>
<sect1>
<sect1info>&modespecs;
</sect1info>
<title>Examples</title>
<para>The examples that follow are all links into the document shown in <xref
linkend="fig1">.</para>
<figure id="fig1">
<title>Example Target Document</title>
<programlisting><inlinegraphic entityref="olinktarget" format="linespecific"></inlinegraphic></programlisting>
</figure>
<para> The <sgmltag>ModeSpec</sgmltag>s in this document are shown in <xref
linkend="fig2">.</para>
<figure id="fig2">
<title>Example Target Document</title>
<programlisting><inlinegraphic entityref="modespecs.include" format="linespecific"></inlinegraphic></programlisting>
</figure>
<example>
<title>An OLink with Content</title>
<para>Source: <literal>&lt;olink targetdocent=olinktarget>some text&lt;/olink>
</literal></para>
<para>Result: <olink targetdocent="olinktarget">some text</olink></para>
</example>
<example>
<title>A Simple OLink</title>
<para>Source: <literal>&lt;olink targetdocent=olinktarget>&lt;/olink></literal></para>
<para>Result: <olink targetdocent="olinktarget"></olink></para>
</example>
<example>
<title>An OLink to a Document</title>
<para>Source: <literal>&lt;olink targetdocent=olinktarget linkmode=ms1>&lt;/olink>
</literal></para>
<para>Result: <olink targetdocent="olinktarget" linkmode="ms1"></olink></para>
</example>
<example>
<title>An OLink to a Section</title>
<para>Source: <literal>&lt;olink targetdocent=olinktarget linkmode=ms2 localinfo=a1s2>&lt;/olink>
</literal></para>
<para>Result: <olink targetdocent="olinktarget" linkmode="ms2" localinfo="a1s2"></olink></para>
</example>
<example>
<title>An Intra-Document OLink</title>
<para>Source: <literal>&lt;olink linkmode=ms1 localinfo="olink-understanding">&lt;/olink>
</literal></para>
<para>Result: <olink linkmode="ms1" localinfo="olink-understanding"></olink></para>
</example>
<example>
<title>An XRef</title>
<para>Source: <literal>&lt;xref linkend="olink-understanding"></literal></para>
<para>Result: <xref linkend="olink-understanding"></para>
</example>
</sect1>
</article>
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