Beginner question - What is a schema?

[Eric Bohlman]

On Mon, 24 Jan 2000, Doug Forman wrote:
> Can anybody give me a good definition of what is a schema, and how it
> applies to xml?
>  
> My understanding is that it is a logical template for a heirarchical
> database; and any xml document is based on some schema.

Insert ', such as' right after 'template' and you'd be correct.  A schema
is simply an (abstract) specification for how things relate to each other
and what can go where.  It may be implicit, e.g. defined by how some code
processes a particular type of XML document, or it may be explicit, such
as a DTD or a document written in the instance syntax proposed in the
W3C's XML-Schema draft.

An explicit, machine-readable schema is necessary if you want an XML
processor to be able to tell you whether a document is valid according to
its rules, or to provide default values for certain kinds of omitted data.
In many applications, this isn't necessary, but it's almost always useful
to have some formalized schema, even if it exists only on paper, as
documentation.



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