XML documents have a hierarchical structure and can conceptually be interpreted as a tree structure, called an XML tree.
XML documents must contain a root element (one that is the parent of all other elements). All elements in an XML document can contain sub elements, text and attributes. The tree represented by an XML document starts at the root element and branches to the lowest level of elements. Although there is no consensus on the terminology used on XML Trees, at least two standard terminologies have been released by the W3C:
XPath defines a syntax named XPath expressions that identifies one or more internal components (elements, attributes, etc.) of an XML document. XPath is widely used to accesses XML-structured data.
The XML Information Set, or XML infoset, describes an abstract data model for XML documents in terms of information items. It is often used in the specifications of XML languages, for its convenience in describing constraints on constructs those languages allow.
In mathematics, a tree is an undirected graph in which any two vertices are connected by exactly one simple path. Any connected graph without simple cycles is a tree. A tree data structure simulates a hierarchical tree structure with a set of linked nodes. A hierarchy consists of an order defined on a set. The term hierarchy is used to stress a hierarchical relation among the elements.
The XML specification defines an XML document as a well-formed text if it satisfies a list of syntax rules defined in the specification. This specification is long, however 2 key points relating to the tree structure of an XML document are:
These features resemble those of trees, in that there is a single root node, and an order to the elements. XML has appeared as a first-class data type in other languages. The JavaScript (E4X) extension explicitly defines two specific objects (XML and XMLList), which support XML document nodes and XML node lists as distinct objects and use a dot-notation specifying parent-child relationships. [1] These data structures represent XML documents as a tree structure.
An XML Tree represented graphically can be as simple as an ASCII chart or a more graphically complex hierarchy. For instance, the XML document and the ASCII tree have the same structure. XML Trees do not show the content in an Instance document, only the structure of the document. In this example Product is the Root Element of the tree and the two child nodes of Product are Name and Details. Details contains two child nodes, Description and Price. The tree command in Windows and *nix also produce a similar tree structure and path.
Product
├───Name
└───Details
└───Description
└───Price
<Product>
<Name>Widget</Name>
<Details>
<Description>
This Widget is the highest quality widget.
</Description>
<Price>5.50</Price>
</Details>
</Product>
XPath, the XML Path Language, is a query language for selecting nodes from an XML document. XPath defines a syntax named XPath expressions that can query an XML document for one or more internal components (elements, attributes, etc.). XPath is widely used in other core-XML specifications and in programming libraries for accessing XML-encoded data. [2]
The XPath Data Model is a long specification, and goes into many features unrelated to XML trees. Listed below are key terms from that specification and the XML specification.[3][4]
Within a given tree, document order satisfies the following constraints:[5]
XML Information Set (XML Infoset) describes an abstract data model of an XML document in terms of a set of information items. The definitions in the XML Information Set specification are meant to be used in other specifications that need to refer to the information in a well-formed XML document. The infoset makes it convenient to describe constraints on the XML constructs other XML languages allow. An XML document has an information set if it is well-formed and satisfies the namespace constraints. An information set can contain up to eleven different types of information items:
The XML Information Set is a long specification, and goes into many features unrelated to XML trees. Listed below are the most important terms relating to XML tree terminology:
"There is exactly one document information item in the information set, and all other information items are accessible from the properties of the document information item, either directly or indirectly through the properties of other information items. The document information item has the following properties:
There is an element information item for each element appearing in the XML document. One of the element information items is the value of the [document element] property of the document information item, corresponding to the root of the element tree, and all other element information items are accessible by recursively following its [children] property. An element information item has the following properties:
There is an attribute information item for each attribute (specified or defaulted) of each element in the document, including namespace declarations. The latter however appear as members of an element's [namespace attributes] property rather than its [attributes] property. Attributes declared in the DTD with no default value and not specified in the element's start tag are not represented by attribute information items. An attribute information item has the following properties: