Description

XML is a text-based data format, standardized in 1996, intended to be both human- and machine-readable.

It can be used specifically as a markup language, similarly to HTML (which shares the same ancestor, SGML), to annotate a document, or as a way to represent any data structure.

Example of an XML document:

<Flight>
	<Departure>
		<AirportCode>GVA</AirportCode>
		<Date>2019-06-29</Date>
	<Departure>
	<Arrival>
		<AirportCode>YUL</AirportCode>
	</Arrival>
</Flight>

Functionalities and tools

XML is ubiquitous and, as such, can be used in most programming languages. It is also accompanied by a number of tools and languages.

• XML Schema (XSD): definition of the structure an XML document must have (used to define acceptable messages for API requests, and to document the structure of responses); this way an XML file can be validated against an XSD
• XSLT: language defining the transformation of an XML document into another XML one, or various formats such as PDF and PNG
• XPath and XQuery: query languages to retrieve information from an XML file

Applicability to NDC & ONE Order

The NDC and ONE Order standards consist of XML schemas (XSDs), each one defining the structure of an XML message, e.g. AirShoppingRQ (request for flight offers) and AirShoppingRS (response containing flight offers).

Key references

• Study of Data Formats (alongside Web Services Paradigms and GraphQL)
• W3C: XML Technology
• w3schools: XML Tutorial (and DOM, XPath, XQuery, XSLT, XSD, etc.)
• NDC schemas