Scatter/gather pattern [was: New XPipe Presentation Available]

["Roger L. Costello" <costello@mitre.org>]

Sean, you have several slides in your XPipe presentation on the
scatter/gather pattern.  Very interesting!  I would like to understand
this better, as I can see many benefits from having an intimate
knowledge of this pattern.  

I work best with a concrete example, so let's consider this
BookCatalogue XML document:

<?xml version="1.0"?>
<BookCatalogue>
        <Book>
                <Title>My Life and Times</Title>
                <Author>Paul McCartney</Author>
                <Date>July, 1998</Date>
                <ISBN>94303-12021-43892</ISBN>
                <Publisher>McMillin Publishing</Publisher>
        </Book>
        <Book>
                <Title>Illusions The Adventures of a 
                       Reluctant Messiah</Title>
                <Author>Richard Bach</Author>
                <Date>1977</Date>
                <ISBN>0-440-34319-4</ISBN>
                <Publisher>Dell Publishing Co.</Publisher>
        </Book>
        <Book>
                <Title>The First and Last Freedom</Title>
                <Author>J. Krishnamurti</Author>
                <Date>1954</Date>
                <ISBN>0-06-064831-7</ISBN>
                <Publisher>Harper &amp; Row</Publisher>
        </Book>
</BookCatalogue>

On slide 52 you say, "For any given task t to be performed on documents
conforming to schema s, there is a fragment expression that can be used
to chop any document into n pieces on which t can be performed
independently."

1. I am not sure what you mean by "fragment expression"?  I am guessing
that it refers to "how we slice up the XML document".  Correct?  For the
above instance document I would guess that the "fragment expression"
would correspond to an XPath expression such as: BookCatalogue/Book,
i.e., break up the document into 3 Book fragments.  Right?

You follow with this statement: "These points are called fulcra and are
a function of (t,s)."

2. Why is the fulcra a function of the schema, s?  I don't see how the
"slicing-up strategy" depends on the schema.  In the above XML document
I don't even have a schema.  Any fragments that I might create aren't
depending on a schema.  

On slide 55 you say: "For data-oriented XML, the fulcra ... may be
independent of t."

3. I read this as saying that "the task to be performed is indendent of
how we slice up an XML document."  I am struggling to see how this could
be true.  It seems to me that if we want to perform parallel processing
on an XML document, the task to be performed will heavily influence how
we slice up an instance document.  No?

4. I am not real clear on the difference between document-oriented
versus data-oriented (perhaps someone could explain the differences?),
but I believe that the above XML document would be considered
data-oriented. Yes?  

I can see how a better knowledge of this pattern could help design
better schemas.  /Roger