XML: eXtensible Markup Language

Developed after HTML (in the late 1990's), partly in response to HTML's quirkiness

Goals:

• Universal format for exchanging both documents and structured data (e.g., records in a database or computer data structures).
• Many languages in one: each XML document can define its own set of tags.

Sample XML xml <?xml version="1.0" encoding="utf-8"?> <person> <name>Ironman</name> <age>49</age> <state>New York</state> <occupation>Engineer</occupation> </person>

Format:

• Information in <...> is markup, all other information is raw text.
• Header line: identifies this as an XML document, indicates character encoding and XML version.
• Body of document consists of a hierarchical collection of elements, starting with a single outer element (<person>).
• XML does not require any particular element structure or tag names; each document can have its own schema.
• Every element must have an explicit start and end (but can use <foo /> as shorthand for <foo></foo>).
• Attributes can be contained per tag, such as <person name="Ironman" age="49" state="New York" height="49">
• Tags can be repeated and nested & Cannot use < or > directly in a document; these are reserved for tags.
  • Use entities instead: < and >
  • This means & is a special character also: use &
  • Also, need " to include a quote in an attribute.
  • Many other entities are defined by XML for convenience.
• XML has a few other features we won't cover here, such as namespaces for organizing tag names.
• Two optional mechanisms available to enforce a particular structure on an XML document:
  • DTD (Document Type Definition): an XML document that describes permissible structure for a class of XML documents ("... each <person> element must contain <name> and <age> children ...").
  • XML Schema: newer than DTD's, designed to get around some shortcomings of DTD's; also more complex.
  • There exist programs that will read a DTD or XML Schema file and validate an XML file against it.

Benefits:

• Textual format, supports Unicode and UTF-8 for internationalization.
• Simple, clean syntax.
• Can be used for a variety of different purposes.
• Heavily used for data exchange in Web-based applications, and for storing application data.
• The common syntax has allowed a large collection of tools to be developed, most of which will work on any XML document (e.g., validators).
• XML documents can be read by humans when necessary; the tags make them almost self-documenting.

Weaknesses:

• Verbose.
• Not as fast to generate or parse as other formats (but fairly efficient parsers have become available**.
• At the beginning people hoped XML would instantly allow any application that understands XML to communicate with any other application that understands XML, but this hasn't come to pass: if 2 applications use different XML schemas then they can't interact in a meaningful way.