XML and beyond

1
XML and beyond

• XML
• Parsing XML
• Schemas
• GML

2
XML

• structured document
• markup (metadata)
• content (data)
• programmers point of view
• nodes
• node has a name and type
• node may have attributes
• node may have a parent, siblings (previous and
  next) and children
• node may have a value

3
Parsing XML

• Two main methods
• parsing into a DOM (Document Object Model,
  w3.org/DOM)
• which is a tree of nodes
• event-based parsing (SAX, Simple API for XML,
  sax.sourceforge.net)
• which reports events to the application through
  callback functions

4
Validity of XML 1

• Validating input is often a pain
• is this a number?
• what about security?
• what is univeristy?
• well-formed XML
• markup is ok
• characters, tag names, attributes, elements
  close, etc
• contents is ok
• character set

5
Validity of XML 2

• The benefit of a known structure
• application knows what to look for
• DTD, XML Schema, RELAX NG, SOX, Schematron
• structure, context, data types, ...
• application (its programmer) knows what to look
  for and what to expect

6
Shared ontologies
copy is sent
XML
XML
creates
interprets
XSD
receiver
uses
sender
uses
7
Namespaces 1

• attribute xmlnssome_prefixsome_URI in the
  document element
• shows the document creators intent that elements
  whose names are of form some_prefixSome_name are
  created according to certain rules
• what the rules are, are assumed to be clear from
  the URI
• further hints may be given with an attribute
  xsischemaLocationsome_URI name_of_the_schema
  (for example)
• BTW Compare to namespaces in Perl

8
Namespaces 2

• if everything works out ok,
• the creator can use a validating parser to make
  sure she is sending a valid document and
• the receiver can check the received document is
  ok for further processing
• Remember the schema is in any case hard-coded
  into the applications using the schema

9
GML

• A namespace with and associated XML schema
  document
• the schema is written in XML Schema
• a draft ISO standard
• an OpenGIS Recommendation Paper
• now in version 3.1
• Note the usual xmlnsgmlhttp//www.opengis.net/
  gml does not tell which version is used
• May be used
• as one namespace in an XML document
• as a basis for higher level schemas