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What about XML schemas?  What are they?

A schema is a collection of validation rules that are designed to constrain and manage data values in XML.  The schema is programmed into your XML file, using the regular XML syntax… though you will have to learn a bit of schema vocabulary, there are no additional languages that you need to use.

Schemas help you to perform some functions within your file, including functions involving integers, dates, and times.  This can be quite useful, as many of the alternatives to schemas are unable to handle these specific forms of data.  On the down side, though, there is much less support for schemas available than there is for some other XML technologies… the schema is newer than some of the others, and while more support is created for schemas every day it still has a way to go before it catches up with its predecessors.

Ok, so what is XML-FO?

XML-FO is a special type of namespace that is used by XSL to format objects.  (Get it? FO?  Format Object?  Yeah, that’s what I thought, too.)  XML-FO is used to help control the generation of XML documents by XML processors, and is responsible for creating custom layouts and other XML effects.

Of course, XML-FO is only half of the story… XSL uses it along with XSLT to create transitions, format pages, and transform XML pages into other languages.  XSLT (which stands for “XSL Transitions”) is used to connect the various elements in a meaningful way, whereas XML-FO is used to place the elements dynamically on the screen.

An example of XML-FO in action would look like this, defining the layout of three different pages (cats, cat_friends, and cat_enemies):

      <?xml version="1.0"?>

            <fo:root>

      <fo:layout-master-set>

      <fo:simple-page-master master-name="cats"

             page-height="10cm"

            page-width="10cm"

            margin-top="0.2cm"

            margin-bottom="0.2cm"

            margin-left="0.2cm"

            margin-right="0.2cm">

      </fo:simple-page-master>

      <fo:simple-page-master master-name="cat_friends"

            page-height="11cm"

            page-width="11cm"

            margin-left="0.2cm"

            margin-right="0.2cm"

            margin-top="0.2cm"

            margin-bottom="0.2cm">

      </fo:simple-page-master>

      <fo:simple-page-master master-name="cat_enemies"

            page-height="12cm"

            page-width="12cm"

            margin-left="0.2cm"

            margin-right="0.2cm"

            margin-top="0.2cm"

            margin-bottom="0.2cm">

      </fo:simple-page-master>

             </fo:layout-master-set>

Notice how each page defines the height, width, and margins so that the XML document will know the exact parameters that it’s working inside of.