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Where is there an end of it?

Alex Brown's weblog

Rethinking OOXML Validation, Part 1

04 November 2010 Alex (8)
ODF Plugfest Venue
Brussels ODF Plugfest venue

At the recent ODF Plugfest in Brussels, I was very interested to hear Jos van den Oever of KOffice present on how ODF’s alternative “flat” document format could be used to drive browser based rendering of ODF documents. ODF defines two methods of serializing documents: one uses multiple files in a “Zip” archive, the aforementioned “flat” format combines everything into a single XML file. Seeing this approach in action gelled with some thoughts I’d been having on how better to validate OOXML documents using standards-based XML tools …

Unlike ODF, OOXML has no “flat” file format – its files are OPC packages built on top of Zip archives. However, some interesting work has already been done in this area by Microsoft’s Eric White in such as blog posts as The Flat OPC Format, which points out that Microsoft Word™ (alone among the Office™ suite members [UPDATE: Word and PowerPoint can do this]) can already save in an unofficial flat format which can be processed with standards-based XML tools like XSLT processors.

Rather than having to rely on Word, or stick only to word processing documents, I thought it would be interesting to explore ways in which any OOXML document could be flattened and processed using standards-based processors. Ideally one would then also write a tool that did the opposite so that to work with OOXML content the steps would be first to flatten it, then to do the processing, and then to re-structify it into an OPC package.

Back to XProc

I have already written a number of blog posts on office document validation, and have used a variety of technical approaches to get the validation done. Most of my recent effort has been on developing the Office-o-tron, a hand-crafted Java application which functions primarily by unpacking archives to the file system before operating on their individual components. Earlier efforts using XProc has foundered on the difficulty of working with files inside a Zip archive — in particular because I was using the non-standard JAR URI scheme which, it turns out, is not capable of addressing items with certain names (e.g. “Object 1”) that one typically finds inside ODF documents.

However, armed with knowledge gained from developing Office-o-tron, and looking again at Zip handling extension functions of the Calabash XProc processor, made me think there was a way XProc could be used to get the job done. Here’s how …

Inspecting an OPC package

OOXML documents are built using the Open Packaging Convention (OPC, or ISO/IEC 29500-2), a generic means of building file formats within Zip archives which also happens to underpin the XPS format. OPC’s chief virtue – that it is very generic – is offset by much (probably too much) complexity in pursuit of this goal. Before we can know what we’ve got in an OPC package, and how to process it, some work needs to be done.

Fortunately, the essence of what we need consists of two pieces of information: a file inside the Zip guaranteed to be called “[Content_Types].xml”, and a manifest of the content of the package. XProc can get both of these pieces of information for us:

<?xml version="1.0"?>
<p:pipeline name="consolidate-officedoc"
  xmlns:p="http://www.w3.org/ns/xproc"
  xmlns:c="http://www.w3.org/ns/xproc-step"
  xmlns:cx="http://xmlcalabash.com/ns/extensions"
  xmlns:xo="http://xmlopen.org/officecert"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="1.0">

  <p:import href="extensions.xpl"/>

  <!-- specifies the document to be processed -->
  <p:option name="package-sysid" required="true"/>


  <!--
  
  Given the system identifer $package-sysid of an OOXML document,
  this pipeline returns a document whose root element is archive-info
  which contains two children: the [Content_Types].xml resource
  contained in the root of the archive, and a zipfile element
  created per the unzip step at:
  
  http://xmlcalabash.com/extension/steps/library-1.0.xpl
  
  -->
  <p:pipeline type="xo:archive-info">

    <p:option name="package-sysid" required="true"/>

    <cx:unzip name="content-types" file="[Content_Types].xml">
      <p:with-option name="href" select="$package-sysid"/>
    </cx:unzip>

    <cx:unzip name="archive-content">
      <p:with-option name="href" select="$package-sysid"/>
    </cx:unzip>

    <p:sink/>

    <p:wrap-sequence wrapper="archive-info">
      <p:input port="source">
        <p:pipe step="content-types" port="result"/>
        <p:pipe step="archive-content" port="result"/>
      </p:input>
    </p:wrap-sequence>

  </p:pipeline>

  <!-- get the type information and content of the package -->
  <xo:archive-info>
    <p:with-option name="package-sysid" select="$package-sysid"/>
  </xo:archive-info>

  <!-- etc -->

Executing this pipeline on a typical “HelloWorld.docx” file gives us an XML document which consists of a composite of our two vital pieces of information, as follows:

<archive-info>
  <Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">
    <Override PartName="/word/comments.xml"
      ContentType="application/vnd.openxmlformats-officedocument.wordprocessingml.comments+xml"/>
    <Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/>
    <Default Extension="xml" ContentType="application/xml"/>
    <Override PartName="/word/document.xml"
      ContentType="application/vnd.openxmlformats-officedocument.wordprocessingml.document.main+xml"/>
    <Override PartName="/word/styles.xml"
      ContentType="application/vnd.openxmlformats-officedocument.wordprocessingml.styles+xml"/>
    <Override PartName="/docProps/app.xml"
      ContentType="application/vnd.openxmlformats-officedocument.extended-properties+xml"/>
    <Override PartName="/word/settings.xml"
      ContentType="application/vnd.openxmlformats-officedocument.wordprocessingml.settings+xml"/>
    <Override PartName="/word/theme/theme1.xml"
      ContentType="application/vnd.openxmlformats-officedocument.theme+xml"/>
    <Override PartName="/word/fontTable.xml"
      ContentType="application/vnd.openxmlformats-officedocument.wordprocessingml.fontTable+xml"/>
    <Override PartName="/word/webSettings.xml"
      ContentType="application/vnd.openxmlformats-officedocument.wordprocessingml.webSettings+xml"/>
    <Override PartName="/docProps/core.xml"
      ContentType="application/vnd.openxmlformats-package.core-properties+xml"/>
  </Types>
  <c:zipfile href="file:/C:/work/officecert/hello.docx">
    <c:file compressed-size="368" size="712" name="docProps/app.xml" date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="375" size="747" name="docProps/core.xml"
      date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="459" size="1004" name="word/comments.xml"
      date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="539" size="1218" name="word/document.xml"
      date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="407" size="1296" name="word/fontTable.xml"
      date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="651" size="1443" name="word/settings.xml"
      date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="1783" size="16891" name="word/styles.xml"
      date="2009-05-25T14:15:08.000+01:00"/>
    <c:file compressed-size="1686" size="6992" name="word/theme/theme1.xml"
      date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="187" size="260" name="word/webSettings.xml"
      date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="265" size="948" name="word/_rels/document.xml.rels"
      date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="372" size="1443" name="[Content_Types].xml"
      date="1980-01-01T00:00:00.000Z"/>
    <c:file compressed-size="243" size="590" name="_rels/.rels" date="1980-01-01T00:00:00.000Z"/>
  </c:zipfile>
</archive-info>

The purpose of the information in the Types element is to tell us the MIME types of the contents of the package, either specifically (in Override elements), or indirectly by associating a MIME type with file extensions (in Default elements). What we are now going to do is add another step to our pipeline that resolves all this information so that we label each of the items in the Zip file with the MIME type that applies to it.

 <p:xslt>
    <p:input port="stylesheet">
      <p:inline>
        <xsl:stylesheet version="2.0"
          xmlns:opc="http://schemas.openxmlformats.org/package/2006/content-types">

          <xsl:variable name="ooxml-mappings" select="document('ooxml-map.xml')"/>

          <xsl:template match="/">
            <c:zipfile>
              <xsl:copy-of select="/archive-info/c:zipfile/@*"/>
              <xsl:apply-templates/>
            </c:zipfile>
          </xsl:template>

          <xsl:template match="c:file">
            <xsl:variable name="entry-name" select="@name"/>
            <xsl:variable name="toks" select="tokenize($entry-name,'\.')"/>
            <xsl:variable name="ext" select="$toks[count($toks)]"/>
            <c:file>
              <xsl:copy-of select="@name"/>
              <xsl:variable name="overriden-type"
                select="//opc:Override[ends-with(@PartName,$entry-name)]/@ContentType"/>
              <xsl:variable name="default-type"
                select="//opc:Default[ends-with(@Extension,$ext)]/@ContentType"/>
              <xsl:variable name="resolved-type"
                select="if(string-length($overriden-type)) then $overriden-type else $default-type"/>
              <xsl:attribute name="resolved-type" select="$resolved-type"/>
              <xsl:attribute name="schema"
                select="$ooxml-mappings//mapping[mime-type=$resolved-type]/schema-name"/>
              <expand name="{@name}"/>
            </c:file>
          </xsl:template>

        </xsl:stylesheet>
      </p:inline>
    </p:input>
  </p:xslt>

You’ll notice I am also using an XML document called “ooxml-map.xml” as part of this enrinchment process. This is a file which contains the (hard won) information about which document of which MIME types are governed by which schemas as published as part of the OOXML standard. That document is available online here.

The result of running this additional step is to give us an enriched manifest of the OPC package content:

<c:zipfile xmlns:c="http://www.w3.org/ns/xproc-step"
  xmlns:cx="http://xmlcalabash.com/ns/extensions"
  xmlns:xo="http://xmlopen.org/officecert"
  xmlns:opc="http://schemas.openxmlformats.org/package/2006/content-types"
  href="file:/C:/work/officecert/hello.docx">
  <c:file name="docProps/app.xml"
    resolved-type="application/vnd.openxmlformats-officedocument.extended-properties+xml"
    schema="shared-documentPropertiesExtended.xsd">
    <expand name="docProps/app.xml"/>
  </c:file>
  <c:file name="docProps/core.xml"
    resolved-type="application/vnd.openxmlformats-package.core-properties+xml"
    schema="opc-coreProperties.xsd">
    <expand name="docProps/core.xml"/>
  </c:file>
  <c:file name="word/comments.xml"
    resolved-type="application/vnd.openxmlformats-officedocument.wordprocessingml.comments+xml"
    schema="wml.xsd">
    <expand name="word/comments.xml"/>
  </c:file>
  <c:file name="word/document.xml"
    resolved-type="application/vnd.openxmlformats-officedocument.wordprocessingml.document.main+xml"
    schema="wml.xsd">
    <expand name="word/document.xml"/>
  </c:file>
  <c:file name="word/fontTable.xml"
    resolved-type="application/vnd.openxmlformats-officedocument.wordprocessingml.fontTable+xml"
    schema="wml.xsd">
    <expand name="word/fontTable.xml"/>
  </c:file>
  <c:file name="word/settings.xml"
    resolved-type="application/vnd.openxmlformats-officedocument.wordprocessingml.settings+xml"
    schema="wml.xsd">
    <expand name="word/settings.xml"/>
  </c:file>
  <c:file name="word/styles.xml"
    resolved-type="application/vnd.openxmlformats-officedocument.wordprocessingml.styles+xml"
    schema="wml.xsd">
    <expand name="word/styles.xml"/>
  </c:file>
  <c:file name="word/theme/theme1.xml"
    resolved-type="application/vnd.openxmlformats-officedocument.theme+xml"
    schema="dml-main.xsd">
    <expand name="word/theme/theme1.xml"/>
  </c:file>
  <c:file name="word/webSettings.xml"
    resolved-type="application/vnd.openxmlformats-officedocument.wordprocessingml.webSettings+xml"
    schema="wml.xsd">
    <expand name="word/webSettings.xml"/>
  </c:file>
  <c:file name="word/_rels/document.xml.rels"
    resolved-type="application/vnd.openxmlformats-package.relationships+xml"
    schema="">
    <expand name="word/_rels/document.xml.rels"/>
  </c:file>
  <c:file name="[Content_Types].xml" resolved-type="application/xml" schema="">
    <expand name="[Content_Types].xml"/>
  </c:file>
  <c:file name="_rels/.rels"
    resolved-type="application/vnd.openxmlformats-package.relationships+xml"
    schema="">
    <expand name="_rels/.rels"/>
  </c:file>
</c:zipfile>

Also notice that each of the items has been given a child element called expand – this is a placeholder for the documents which we are going to expand in situ to create our flat representation of the OPC package content. The pipeline step to achieve that expansion is quite straightforward:

  <p:viewport name="archive-content" match="c:file[contains(@resolved-type,'xml')]/expand">
    <p:variable name="filename" select="/*/@name"/>
    <cx:unzip>
      <p:with-option name="href" select="$package-sysid"/>
      <p:with-option name="file" select="$filename"/>
    </cx:unzip>
  </p:viewport>

At this point, we're only expanding the content that looks like it is XML – a fuller implementation would expand non-XML content and BASE64 encode it (perfectly doable with XProc).

The result of applying this process is a rather large document, with all the expand elements referring to XML documents replaced by that XML document content … in other words, a flat OPC file. With the additional metadata we have placed on the containing c:file elements, we have enough information to start performing schema validation. I will look at validation in more depth in the next part of this post …