Virtual Filesystems Are Virtual Office Documents

Translated filesystems can be accessed through the xsltfs:// scheme. This filesystem can be interacted with using the libferris clients or exposed using Filesystem in Userspace (FUSE) through the Linux kernel.

As libferris allows you to see an XML file as a filesystem, the XML file shown in Listing 1 will be used as the input filesystem.

<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
<root>
  <file1 size="200"/>
  <file3>filesystems inside XML?</file3>
  <file7 myattr="foo" >Something blue</file7>
</root>

The XSL file shown in Listing 2 will create our translated filesystem from the input filesystem. It is important to keep in mind that although the input filesystem in this case is generated from an XML file, it could just as easily be data from a mounted LDAP server. The XSL will create two elements under the document root element. The file3 element will have the original contents of the virtual “file” for file3 in the input filesystem. The file7 will have the attribute myattr as its contents.

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet
  
  version="1.0"
  >
  <xsl:output method="xml"/>

  <xsl:template match="/">
    <root>
      <xsl:apply-templates/>
      </root>
  </xsl:template>

  <xsl:template match="file3">
    <context original-url="{@url}" name="file3">
        <xsl:value-of select="@content"/>
    </context>
  </xsl:template>

  <xsl:template match="file7">
    <context original-url="{@url}" name="file7">
        <xsl:value-of select="@myattr"/>
    </context>
  </xsl:template>
</xsl:stylesheet>

The translated filesystem can be used just like any other filesystem with the command-line utilities ferrisls, fcat, ferriscp and so on. The xsltfs:// URL scheme in libferris lives above most other URL schemes and allows you to materialize a filesystem at any point by supplying an XSL transform to apply. The location of the XSL files themselves is determined based on an xsltfs path you set in libferris. The use of an xsltfs path avoids embedding full stylesheet paths into xsltfs:// URLs. As the stylesheets are specified using a CGI-like syntax, avoiding the use of the / character means that there is no ambiguity for filenames in xsltfs://.

You can apply a stylesheet at any point in your virtual filesystem. The result of applying a stylesheet to the example.xml filesystem will become the contents of a directory rooted at the example.xml?stylesheet=example.xsl virtual directory.

Without any use of / in the xsltfs:// parameters, the filename and parameters together are used to specify the name of a virtual directory that xsltfs:// makes on demand. Because there is no unambiguity, you then can navigate directly into the translated filesystem rooted at this virtual directory. This is shown in the examples below.

Part of a filesystem is shown in Listing 3 to make things clearer. I have applied the foo.xsl to the example.xml file using the special CGI-like syntax to name a virtual directory. libferris creates this virtual directory for me to allow direct navigation into the translated filesystem. The rootElement is the root of the translated filesystem; in XML terms, it is the document root of the result of applying the foo.xsl stylesheet to the filesystem rooted at example.xml. Filesystems live inside the context subdirectory of xsltfs:// to allow other parameters and expansion to be done in xsltfs:// at a later time.

xsltfs://
  context
    file
      tmp
        example.xml
        example.xml?stylesheet=foo.xsl
          rootElement
            myFoo1
            myBar2

The xsltfs path can be set using the XSLT stylesheets page of the ferris-capplet-general configuration tool. In addition to setting the XSLT path with ferris-capplet-general, you can use the LIBFERRIS_XSLTFS_SHEETS_URL environment variable to pass in the path explicitly where your forward and reverse stylesheets are located. This makes using xsltfs with the FUSE module from shell scripts quite simple, as you do not need to install your stylesheet files. Stylesheets can be stored in any filesystem libferris can reach.

For the purposes of this example, I have the files and stylesheets stored in file://tmp/example. If I am running my examples from the example directory, it is sufficient to put . into my XSLT path—see the example in Listing 4.