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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="ID" rev="3.0">
  <title>Map-first pre-processing</title>
  <titlealts>
    <navtitle>Map-first pre-processing</navtitle>
  </titlealts>
  <shortdesc><ph id="map-first-preproc-desc">DITA-OT provides a map-first pre-processing option as an alternative to the
      default <codeph>preprocess</codeph> operation. The method, which was introduced in DITA-OT 2.5 as an experimental
      feature, has since been improved and is ready for use in production scenarios. Map-first pre-processing provides
      the same functionality as the default <codeph>preprocess</codeph>, but takes a different
    approach.</ph></shortdesc>
  <prolog>
    <metadata>
      <keywords>
        <indexterm>filters
          <indexterm>map-first pre-processing</indexterm></indexterm>
        <indexterm>map-first pre-processing</indexterm>
        <indexterm>pre-processing
          <indexterm>map first</indexterm></indexterm>
        <indexterm>DITA 1.0</indexterm>
        <indexterm>DITA 1.2</indexterm>
        <indexterm>DITA 1.3
          <indexterm>effect on pre-processing</indexterm></indexterm>
        <indexterm>pipelines
          <indexterm>map first</indexterm></indexterm>
        <indexterm>validate</indexterm>
        <indexterm>maps
          <index-see>DITA maps</index-see></indexterm>
        <indexterm>DITA maps
          <indexterm>validate</indexterm></indexterm>
      </keywords>
    </metadata>
  </prolog>
  <conbody>
    <p id="map-first-preproc-gain">Whereas the default pre-processing routine handles both maps and topics at the same
      time, often switching back and forth between map operations and topic operations, the map-first approach only
      begins processing topics after nearly all map processing is complete. This simplifies the processing logic and
      creates cleaner module responsibilities, which makes it easier to process only those topics that are actually
      referenced after filtering, for example, or to only process the map to validate the map structure.</p>
    <p>The current pre-processing architecture was established during the DITA 1.0 era when there were fewer DITA
      features that operated on the map level. Initially, the difference between processing modes was not that great.
      DITA 1.2 and 1.3 introduced many more map-level features, such as keys and key scopes, that make it difficult to
      reliably work with topics before all map features have been resolved. </p>
    <p>The original pre-processing operation already handles many map operations first, but this was not the original
      design and requires regular refactoring to handle edge cases. The new map-first pre-processing is designed with
      this model in mind, improving the overall processing flow and making it more formal about the map-first model. The
      new model also takes advantage of hashed topic file names in the temporary directory, which simplifies many
      processing steps, and is better able to handle topics referenced outside of the map directory (that case has
      resulted in a variety of issues with the original model).</p>
    <p>As of DITA-OT 4.2, the map-first pre-processing pipeline also supports additional subject scheme features.</p>
    <note id="map-first-preproc-note">The map-first pre-processing option is enabled by default in DITA-OT 3.0 for PDF
      and HTML Help. These formats were chosen because they generate a compiled result file, so temporarily hashed file
      names should all be invisible to the build. After further testing and feedback, the new option has been enabled
      for HTML5 output as of DITA-OT 4.2.</note>
    <section>
      <title>How to use map-first pre-processing</title>
      <p>To use (or test) map-first pre-processing, call the <codeph>preprocess2</codeph> Ant target in your custom
        transformation types instead of the <codeph>preprocess</codeph> target.</p>
      <p>For example, if you have a custom HTML5 transformation type named "myhtml", then you may have a plug-in
        extension that looks this:</p>
      <codeblock outputclass="language-xml">&lt;!-- Simple variant: set properties and call default HTML5 -->
&lt;target name="dita2myhtml" depends="myhtml.init,dita2html5"/>
</codeblock>
      <p>This type of extension is quite common, and is used to set default properties for your environment followed by
        a normal build to use those properties. As of DITA-OT 4.2, this approach will inherit the map-first
        pre-processing routine from the HTML5 transformation.</p>
      <p>In earlier versions, you’d need to replace <codeph>dita2html5</codeph> with the normal HTML5 steps, swapping
        out <codeph>preprocess</codeph> for <codeph>preprocess2</codeph>:</p>
      <codeblock
        outputclass="language-xml normalize-space show-line-numbers show-whitespace"
      >&lt;!-- Simple variant: set properties and call default HTML5 -->
&lt;target name="dita2myhtml" 
        depends="myhtml.init,
                 html5.init,
                 build-init,
                 <b>preprocess2,</b>
                 html5.topic,
                 html5.map,
                 html5.css"/></codeblock>
      <note>If you use this simple method for customized PDF or HTML Help builds, you will automatically be using
          <codeph>preprocess2</codeph>.</note>
      <p>Some custom transformation types already require you to repeat the default dependencies, in which case you
        should already call <codeph>preprocess</codeph> directly, as in the following:</p>
      <codeblock
        outputclass="language-xml normalize-space show-line-numbers show-whitespace"
      >&lt;!-- More complex variant: add processing steps to default HTML5 -->
&lt;target name="dita2myhtml"
        depends="myhtml.init,
                 build-init,
                 preprocess,
                 local-extensions-after-preprocess,
                 html5.topic,
                 html5.map,
                 html5.css"/></codeblock>
      <p>In such cases, the modification is much easier – simply add a <codeph>2</codeph> to the existing
          <codeph>preprocess</codeph> target.</p>
    </section>
    <section>
      <title>How to test in a production environment</title>
      <p>In some cases, you may be responsible for maintaining transformation types that are actually run by many people
        on your team or around a company. In this case, you likely need to maintain your existing transformation types
        based on the backwards-compatible <codeph>preprocess</codeph> modules, but also want to provide your colleagues
        with a way to test their own documents using <codeph>preprocess2</codeph>.</p>
      <p>There are several ways to do this. One fairly straightforward approach would be to create a new custom
        transformation type that is exactly the same, except for pre-processing. For example, if you have a local HTML
        variant called <codeph>myhtml</codeph> as above, instead of modifying that transformation directly, you could
        create a second transformation type called <codeph>myhtml-beta</codeph> that provides exactly the same support,
        but with the new map-first pre-processing:</p>
      <codeblock outputclass="language-xml normalize-space show-line-numbers show-whitespace"><i
        >&lt;!-- Original "myhtml" is not modified, used for production --></i>
&lt;target name="dita2myhtml5" depends="myhtml.init,dita2html5"/>

<i>&lt;!-- "myhtml-beta" used to test and provide feedback on preprocess2 --></i>
&lt;target name="dita2myhtml-beta" 
        depends="myhtml.init,
                 html5.init,
                 build-init,
                 <b>preprocess2,</b>
                 html5.topic,
                 html5.map,
                 html5.css"/></codeblock>
    </section>
    <section>
      <title>Known limitations</title>
      <p conkeyref="reusable-components/no-internal-preprocess2-ext"/>
    </section>
  </conbody>
</concept>
