View source for Entity UDI

== Introduction ==
This page defines a Higgins [[NodeIdDatatype]].

== Definition ==
* A [[NodeIdDatatype]] is a kind of URI [[Data Range]].
* A [[NodeIdDatatype]] MAY be absolute or relative:
** An absolute [[NodeIdDatatype]] MUST uniquely identify both a [[Context]] and a specific [[Node]] within that [[Context]].
** A relative [[NodeIdDatatype]] MUST uniquely identify a [[Node]] within a given [[Context]].
* A [[NodeIdDatatype]] MUST conform to the ABNF defined below.


== ABNF ==
The following [http://en.wikipedia.org/wiki/Augmented_Backus-Naur_form ABNF] defines the syntax of a Higgins [[NodeIdDatatype]]. Rules not defined in this ABNF are defined in either:
* The [[ContextIdDatatype]] definition, or
* The [http://wiki.oasis-open.org/xri/XriCd02/Xri2dot0Abnf ABNF for XRI 2.0] (which includes the IRI ABNF from [http://www.ietf.org/rfc/rfc3987.txt RFC 3987]).

 NodeIdDatatype     = AbsoluteNodeId / RelativeNodeId
 AbsoluteNodeId     = AbsoluteNodeURI / AbsoluteNodeXRI
 AbsoluteNodeURI    = ContextURI [ "#" ifragment ]
 AbsoluteNodeXRI    = ContextXRI [ "//" xri-segment ]
 RelativeNodeId     = ifragment / xri-segment

== Notes ==
* In an '''AbsoluteNodeId''', a '''RelativeNodeId''' is optional when the [[Context]] identified by the '''ContextURI''' or '''ContextXRI''' contains exactly one [[Node]].
* The '''AbsoluteNodeURI''' form is consistent with one of the two proposed forms of [http://www.w3.org/TR/2007/WD-cooluris-20071217/ Cool URIs] defined by the [http://www.w3.org/ W3C].
* The '''AbsoluteNodeXRI''' form is an [http://en.wikipedia.org/wiki/XRI XRI] as defined by [http://wiki.oasis-open.org/xri/XriCd02/Xri2dot0Abnf XRI Syntax 2.0].
* See [[Context Discovery]] for rules defining resolution of an instance of a [[NodeIdDatatype]].

== Relationship to OpenID ==
* An [http://openid.net/specs/openid-authentication-2_0.html OpenID 2.0] compliant URL or XRI may serve as an absolute [[NodeIdDatatype]]. See [[ContextIdDatatype]] for details.

== See Also ==
* [[ContextIdDatatype]]
* [[ContextId]]
* [[NodeId]]
* [[Higgins Global Graph]]

== Links ==
* [http://eclipse.org/higgins Higgins Home]