XML information set

XML information set

XML Information Set (Infoset) is a World Wide Web Consortium (W3C) specification, dated February 4, 2004. An XML information set is an abstract model of the information stored in an XML document. The information set establishes a separation between data and information in a way that suits most common uses of XML. Several of the concrete XML data models are defined by referring to XML information set items and their properties.

Whereas an XML information set is an abstract model of the information stored in an XML document, an information item is an abstract representation of some component of an XML document. SOAP Version 1.2 makes use of this abstraction to define the information in a SOAP message without ever referring to XML Version 1.x. The SOAP HTTP binding specifically permits alternative media types that provide for, as a minimum, the transfer of the SOAP XML Infoset.

SOAP Message Transmission Optimization Mechanism describes SOAP 1.2 constructs in terms of information items whereas SOAP 1.1 is defined in terms of XML elements. MTOM enables SOAP bindings to optimize the transmission or wire format (or both) of a SOAP message by selectively encoding portions of the message while still presenting an XML information set to the SOAP application. The SOAP 1.2 attribute is now in the SOAP namespace. The XML information sets require the support of XML namespaces. The core XML recommendation does not require the support of XML namespaces; however namespaces are required to support the XML schema.

The XML information set does not require or favor a specific interface or class of interfaces. The XML information set specification presents the information set as a tree for the sake of clarity and simplicity, but there is no requirement that the XML information set be made available through a tree structure. Other types of interfaces, including but not limited to event-based and query-based interfaces, are also capable of providing information conforming to the information set. As long as the information in the information set is made available to XML applications in one way or another, the requirements of the XML information set are satisfied.

The XML information set provides a set of definitions to be used in other specifications that refer to the information in a well-formed XML document. For any given XML document, there are a number of corresponding information sets.

A unique minimal information set consisting of the core properties of the core items and nothing else.
A unique maximal information set consisting of all the core and all the peripheral items with all the peripheral properties, and one for every combination of present and absent peripheral items and properties in between. The in-between information sets must be fully consistent with the maximal information set.

Information set items

The XML information set is a description of the information that is available in a well-formed XML document, and it describes an abstract data model of an XML document in terms of a set of information set items. An information item is an abstract description of some part of an XML document, and each information item has a set of associated named properties. 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.

Guidelines for using information set items include:

There is no requirement for an XML document to be valid in order to have an information set.
An XML document has an information set if it satisfies the namespace constraints.
An XML document has an information set if it is well-formed
Only one document information item is permitted in the information set.
An information set for an XML document consists of two or more information items.
The information set for any well-formed XML document will contain at least the minimum information items: one document information item and one element information item.
Each information item has a set of associated properties, some of which are core and some of which are peripheral.

An information set can contain up to eleven different types of information items:

Document information item
Element information items

Attribute information items
Processing instruction information items
Unexpanded entity reference information items
Character information items
Comment information items
The Document Type Declaration (DTD) information item
Unparsed entity information items
Notation information items
Namespace information items
Note that the information set of the XML document might not be a complete list of all information items.
Certain kinds of invalidity affect the values assigned to some properties. Entities, notations, elements and attributes can be undeclared. We can have multiple declarations for notations and elements. Multiple declarations are valid for entities and attributes. An ID can be undefined or multiply defined. Such cases are noted where relevant in the information item definitions in the XML Information Set specification.

Syntax

The XML information set uses a square-bracket syntax, meaning the property names are shown in square brackets. For example, the document information item has the following properties:

Property Description
[children] An ordered list of child information items, in document order.
[document element] The element information item corresponding to the document element.
[notations] An unordered set of notation information items, one for each notation declared in the DTD. If any notation is multiply declared, this property has no value.
[unparsed entities] An unordered set of unparsed entity information items, one for each unparsed entity declared in the DTD.
[base URI] The base URI of the document entity.
[character encoding scheme] The name of the character encoding scheme in which the document entity is expressed.
[standalone] An indication of the stand-alone status of the document, either yes or no. This property is derived from the optional standalone document declaration in the XML declaration at the beginning of the document entity, and has no value if there is no standalone document declaration.
[version] A string representing the XML version of the document. This property is derived from the XML declaration optionally present at the beginning of the document entity, and has no value if there is no XML declaration.
[all declarations processed] This property is not strictly speaking part of the information set of the document. Rather it is an indication of whether the processor has read the complete DTD. Its value is a boolean. If it is false, then certain properties (indicated in their descriptions later in this section) might be unknown. If it is true, those properties are never unknown.

All information sets are understood to describe the XML document with all entity references already expanded; that is, represented by the information items corresponding to their replacement text. In the case an entity reference cannot be expanded, because an XML processor has not read its declaration or its value, explicit provision is made for representing such a reference in the information set.

Related:

XML-binary Optimized Packaging
Web services specifications and APIs
XML Information Set

Property	Description
[children]	An ordered list of child information items, in document order.
[document element]	The element information item corresponding to the document element.
[notations]	An unordered set of notation information items, one for each notation declared in the DTD. If any notation is multiply declared, this property has no value.
[unparsed entities]	An unordered set of unparsed entity information items, one for each unparsed entity declared in the DTD.
[base URI]	The base URI of the document entity.
[character encoding scheme]	The name of the character encoding scheme in which the document entity is expressed.
[standalone]	An indication of the stand-alone status of the document, either yes or no. This property is derived from the optional standalone document declaration in the XML declaration at the beginning of the document entity, and has no value if there is no standalone document declaration.
[version]	A string representing the XML version of the document. This property is derived from the XML declaration optionally present at the beginning of the document entity, and has no value if there is no XML declaration.
[all declarations processed]	This property is not strictly speaking part of the information set of the document. Rather it is an indication of whether the processor has read the complete DTD. Its value is a boolean. If it is false, then certain properties (indicated in their descriptions later in this section) might be unknown. If it is true, those properties are never unknown.