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Now you might have noticed that the name of this video is "Well-formed-XML".
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So well-formed XML is
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actually the most flexible XML.
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An XML document or
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an XML stream is considered
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well formed if it adheres
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to the basic structural requirements of XML.
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And there aren't many.
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Just that we have a single
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root element, as we discussed
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before, a single bookstore in this
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case; that all of
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our tags are matching, we don't
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have open tags without closed
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tags; and our tags
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are properly nested, so we don't have interweaving of elements.
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And finally, within each
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element if we have attribute names, they're unique.
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And that's about it.
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That's all we require for a
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XML document, or
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a set of XML data to be considered well-formed.
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And for many applications, that's all we're concerned about.
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In order to test whether a
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document is well-formed, and specifically
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to access the components of
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the document in a program,
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we have what's called an XML parser.
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So, we'll take an XML
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document here, and we'll
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feed it to an XML parser,
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and the parser will check the
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basic structure of the document,
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just to make sure that everything is okay.
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If the document doesn't appear to
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these three requirements up here,
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the parser will just send an error saying it's not well-formed.
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If the document does adhere
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to the structure, then what comes out is parsed XML.
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And, there's various standards
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for how we show parsed XML.
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One is called the document object
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model, or DOM; it's a
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programmatic interface for sort
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of traversing the tree that's implied by XML.
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Another popular one is SAX.
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That's a more of a stream model for XML.
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So these are the ways in
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which a program would access the
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parsed XML when it comes out of the parser.
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So one issue that comes up,
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because the XML data is used
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frequently on the internet, is
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how we display XML.
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So one way to display XML is just
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as we see it here, but very
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often we want to format the
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data that's in an XML
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document or an XML string
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in a more intuitive way.
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And actually there's a nice setup for doing that.
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What we can do is use a
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rule-based language to take
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the XML and translate it automatically
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to HTML, which we can then render in a browser.
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A couple of popular languages
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are cascading style sheets known
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as CSS or the extensible
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style sheet language known as XSL.
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We're going to look a little bit
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with XSL on a later video
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in the context of query in XML.
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We won't be covering CSS in this course.
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But let's just understand how these
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languages are used, what the basic structure is.
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So the idea is that
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we have an XML document
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and then we send it to
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an interpreter of CSS or
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XSL, but we also have to have
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the rules that we're going to use on that particular document.
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And the rules are going to do things
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like match patterns or add
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extra commands and once
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we send an XML document thorugh
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the interpreter we'll get an
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HTML document out and then we can render that document in the browser.
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Now, one thing I should mention is
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that we'll also check with the
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parser to make sure
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that the document is well formed
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as well before we translate it to HTML.
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To conclude, XML is a
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standard for data representation and exchange.
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It can also be thought of as a data model.
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Sort of a competitor to the
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relational model for structuring the data in one's application.
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It generally has a lot more
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flexibility than the relational
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model, which can be a plus and a minus, actually.
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In this video we covered the
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well formed XML, so, XML
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that adheres to basic structural requirements,
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in the next video we will
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cover valid XML, where we
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actually do introduce a kind of
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schema for XML.
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The last thing I want to mention, is
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that the formal specification for XML is quite enormous.
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There are a lot of bells and whistles.
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We're going to cover, in these
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videos, the most important components
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for understanding anything XML.