Presents your 
XML E-NEWSLETTER for July 31, 2002

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REAP THE MOST BENEFIT FROM STRUCTURAL DESIGNS IN XML

One of the most common adjectives used to describe XML documents is
structured. But what is structure and how does it contribute to the 
overall
goal of XML documents? Let's examine how you can make the most of
structural designs in your XML documents.

GETTING ORGANIZED

Structure is often defined as organization. That really just means the
arrangement of items in the place where they go. But the question is,
"Where do these items go?" When we talk about XML documents and their
relative structure, we really mean the way that elements are grouped 
together
both in a sibling relationship and in a parent/child relationship. The
first part of getting organized with XML is understanding when to use 
which
relationship.

A LESSON IN PARENTING

Sibling elements in an XML document are those elements that share the
same parent element. It's possible to have an XML document that has no
sibling elements. For example:

<parent>
  <onlyChild>
    <onlyGrandChild/>
  </onlyChild>
</parent>

As you can see, the parent element has an only child, who in turn bears
an only grandchild. It's possible, although merely theoretically, to 
have
no child elements:

<sisterMary/>

As you can see, sisterMary is the root element and has neither child 
nor
sibling elements. It's important to remember that the rules of XML
specify that each XML document must contain at the very least a root 
element.

SIBLING RIVALRY

Now let's get back to our discussion of siblings. Each sibling shares
the same parent. Therefore, to have siblings, you start with a parent 
and
make two child elements below it:

<parent>
  <childOne/>
  <childTwo/>
</parent>

Both childOne and childTwo are child elements of the parent element.
Also, childOne and childTwo are sibling elements that share the same 
parent.

FAMILY LIFE CAN BE COMPLICATED

So far, our example is quite simple and makes sense (for the most 
part).
But let's complicate the issue a bit. What if our parent element
contains things other than child elements? Also, we only have a single 
parent.
Most kids come from two parents. Let's say that these parents also have
two dogs, a cat, and each have a car.

Describing these can be easy or difficult depending on how you 
structure
them in the XML document. Let's start with the major problem of having
two parents. XML doesn't allow an element to have two parents. Each
element (unless it's the root element) must have one, and only one, 
parent
element. In order to describe our family, we'll have to apply a 
different
technique.

CHANGE YOUR PERSPECTIVE TO SEE A NEW STRUCTURE

One way to think about the problem is to change our perspective. Rather
than the kids being children of the two parents, let's imagine all of
these people belonging to a family. Now, our XML jargon for child and 
parent
no longer applies to the parents and children in the family but to the
elements used to describe our family. To clarify, imagine that we have 
a
root element called family and the parents are children of the family
element:

<family>
  <parents>
    <mom/>
    <dad/>
  </parents>
</family>

The kids are no longer children elements of the parents but are part of
the family. They are children of the children element:

<family>
  <parents>
    <parent type="mom">
      <name>Jane</name>
      <car>Mercury Villager</car>
    </mom>
    <parent type="dad">
      <name>John</name>
      <car>Mazda Miata</car>
    </dad>
  </parents>
  <children>
    <child>
      <name>Johnny Jr.</name>
    </child>
    <child>
      <name>Sally</name>
    </child>
  </children>
  <pets>
    <pet type="dog">
      <name>Fido</name>
    </pet>
    <pet type="dog">
      <name>Spot</name>
    </pet>
    <pet type="cat">
      <name>Bartholomeow</name>
    </pet>
  </pets>
</family>

Notice that we've changed several aspects of our structure. Our parents
aren't just "mom" and "dad". But rather, they can be described
generically as parent elements, each with their own name and car. The 
same with the
children--they can be described with generic child elements belonging 
to
the children element.

In a basic sense, our family is composed of a parent element, a 
children
element, and a pet element. Each of these elements aggregates the set 
of
similar elements and creates a simple but powerful structure that can 
be
applied to many actual family structures.

Brian Schaffner is a senior consultant for Fujitsu Consulting. He
provides architecture, design, and development support for Fujitsu's 
Telcom360
group.

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