[Mirrored from: http://www.w3.org/pub/WWW/TR/WD-xml-link-970406.html]
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This work is part of the
W3C SGML Activity
(for current status, see http://www.w3.org/pub/WWW/MarkUp/SGML/Activity).
This document specifies a simple set of constructs that may be inserted into
XML documents to describe links between objects. It is a goal to use the
power of XML to create a structure that can describe both the simple
unidirectional hyperlinks of today's HTML, as well as more sophisticated
multi-ended, typed, self-describing links.
April 6, 1997
This draft is intended for public discussion.
1. Introduction
1.1 Origin and Goals
1.2 Relationship to Existing
Standards
1.3 Unfinished Work
1.3.1
Structured Labels
1.3.2
Element/Attribute Remapping
1.4 Terminology
1.5 Notation
1.6 Types of link types
2. Link Recognition
2.1 Operational Issues Concerning
Link Recognition
3. Linking Elements
3.1 Information Associated With
Links
3.2 Simple Links
3.3 Extended link
4. Link Behavior
4.1 The SHOW Axis
4.2 The ACTUATE Axis
5. Addressing
5.1 Locator Syntax in
General
5.2 Locator Syntax for XML
Resources
5.3 TEI Extended Pointers
5.3.1
XPointer Structure
5.3.1.1
The ROOT Keyword
5.3.1.2
The HERE Keyword
5.3.1.3
The DITTO Keyword
5.3.1.4
Addressing by ID Attribute
5.3.2
Location Terms - The Keyword Phase
5.3.3
Location Terms - The Steps Phase
5.3.4
Interaction of Keywords and Steps
5.3.4.1
The DESCENDANT Keyword
5.3.4.2
The ANCESTOR Keyword
5.3.4.3
The PREVIOUS Keyword
5.3.4.4
The NEXT Keyword
5.3.4.5
The PRECEDING Keyword
5.3.4.6
The FOLLOWING Keyword
6. Extended Link Groups
A. References
This document specifies a set of constructs which may be inserted in XML documents to describe links between objects. A link, as the term is used here, is a relationship which is asserted to exist between two or more data objects or portions of data objects. This specification is concerned with the syntax used to assert link existence and describe link characteristics. Implicit (unasserted) relationships, for example that of one word to the next, or that of a word in a text to its entry in an on-line dictionary, are outside its scope. Explicitly asserted links do not constitute the only useful kind of link, but this specification is intended neither to provide machinery for every possible kind of link nor to preclude the use of such machinery.
The existence of links is asserted by the presence of elements contained in XML documents. They may or may not reside at the locations of, or in the same documents with, the objects which they serve to connect.
This specification aims to provide an effective, compact structure for representing links that can be in documents or external to them, that can have multiple typed locators, indirection, and precise specification of resource locations in XML and SGML data. It also aims to represent the abstract structure and significance of links, leaving formatting issues to stylesheets or other mechanisms as far as practical.
Three standards have been especially influential:
Many other linking systems have also informed this design, including Dexter, MicroCosm, and InterMedia.
The simple labelling mechanism described in this draft is insufficiently flexible to cope with internationalization or the use of multimedia in link labels. A future version will provide a mechanism for the use of structured link labels.
The technique whereby linking elements are recognized based on the use of special attributes is described with insufficient thoroughness. Furthermore, the idea of allowing the attributes of linking elements, as well as their element types, to be remapped to user-chosen values, is under serious consideration.
The following basic terms apply in this document.
The formal grammar for locators is given using a simple Extended Backus-Naur Form (EBNF) location, as described in Part 1.
There is an extensive literature on link typology. Some well-known axes are:
The existence of a link is asserted by a
linking element. These must be recognized reliably
by software in order to provide appropriate display and behavior. XML linking
elements are recognized based on the use of a designated
attribute
named XML-LINK. Possible values are SIMPLE,
EXTENDED, LOCATOR, GROUP, and
DOCUMENT, signalling in each case that the element in whose
start-tag the attribute appears is to be treated as an element of the indicated
type, as described in this specification.
An example of such a link:
<A XML-LINK="SIMPLE" HREF="http://www.w3.org/">The W3C</A> |
There are two distinct mechanisms that may be used to associate the
XML-LINK attribute with a linking element. The simplest is to
provide it explicitly, as in the example above. However, this practice is
verbose, and would be not only cumbersome but wasteful of network bandwidth
in the case where there are large numbers of linking elements. Fortunately,
XML's facilities for declaring
default
attribute values can be used to address this problem. For example, suppose
one wished to declare the "A" element to be an XML
SIMPLE link. The following would accomplish this:
<!ATTLIST A XML-LINK CDATA #FIXED "SIMPLE"> |
Such a declaration may be placed in either the external or internal subsets of the Document Type Declaration. Placing it in both subsets would be the obvious thing to do for convenient network operation. So doing, at the time of creation of this specification, would cause the document to fail to be valid. Note that the successful completion of the current work on a technical corrigendum to ISO 8879 that is in the process of submission to ISO/IEC JTC1/SC18/WG8 would resolve this problem and allow this practice in valid documents. However, for interoperability, the declaration should not be placed in both subsets.
This specification defines two types of linking
elements. First, a simple link, which is always
in-line and one-directional, very like the HTML
<A> element. Second, a much more general extended link
(EXTENDED) which is out-of-line
and may be used for multi-directional links, links
into read-only data, and so on.
This specification describes a variety of information that may be (and in some cases is required to be) associated with linking elements:
Role
ROLE
attribute is used
to provide both link and resource roles.
Resource
HREF attribute, as described
below.
Label
TITLE attribute. This specification does not require that
applications make any particular use of the label.
Behavior
SHOW and ACTUATE attributes may be used for
an author to communicate general policies concerning the traversal behavior
of the link; this specification defines a small set of policies for this
purpose. The BEHAVIOR attribute may be used to communicate detailed
instructions for traversal behavior; this specification does not constrain
the contents, format, or meaning of this attribute.
Simple links are very much like HTML <A> or TEI <XREF> elements, but with more general reference capabilities. A simple link may contain only one locator; thus there is no necessity for a separate child element, and the locator attributes are attached directly to the linking element.
Following is a declaration for an XML simple link; note that the element
type need not be SIMPLE, since the linking element will be
recognized based on the value of the XML-LINK attribute:
<!ELEMENT SIMPLE ANY> |
A extended link can involve any number of resources, and need not be co-located with any of them. An application may be expected to provide traversal among all of them (subject to semantic constraints outside the scope of this paper). The key issue with extended links is how to manage and find them, since they do not necessarily co-occur with any of their resources, and often are located in completely separate documents. This process is discussed under Extended Link Groups below.
A extended link's locators are contained in
child
elements of the linking element, each with its own set of attributes.
Once again, in the following declaration, the linking element type need not
be EXTENDED and LOCATOR:
<!ELEMENT EXTENDED (#PCDATA | LOCATOR)*> |
Note that many of the attributes may be provided for both the parent linking element and the child locator element. If any such attribute is provided in the linking attribute but not in a locator element, the value provided in the linking element is to be used in processing the locator element. In other words, the attributes provided in the linking element may serve as defaults for the (possibly many) locator elements.
This specification provides a mechanism for the authors of linking elements
to signal their intentions as to the timing and effects of traversal. Such
intentions can be expressed along two axes, labeled SHOW and
ACTUATE. These are used to express policies rather
than mechanisms; programs which are processing links in XML documents
are free to devise their own mechanisms, best suited to the user environment
and processing mode, to implement the requested policies.
In many cases, there will be a requirement for much finer control over the
details of traversal behavior; existing hypertext software typically provides
such control. Such fine control of link traversal is outside the scope of
this specification; however, the BEHAVIOR attribute is provided
as a standard place for authors to provide, and in which programs should
look for, such detailed behavioral instructions.
The SHOW
attribute
is used to express a policy as to the context in which a resource that is
traversed to should be displayed or processed. It may take one of three values:
EMBED
REPLACE
NEW
The ACTUATE attribute is used to express a policy as to when
traversal of a link should occur. It may take one of two values:
AUTO
USER
The locator value for a resource is provided in the HREF attribute. HREF may have at one point stood for "Hypertext reference"; the name is adopted for compatibility with existing practice.
In general, a locator contains a URL, as described in
RFC 1738.
As that and related RFCs state, the URL may be followed by "?"
and a query, or by "#" and a fragment
identifier, with the query interpreted by the host providing the indicated
resource, and the interpretation of the fragment specifier dependent on the
data type of the indicated resource. Thus, when a locator in an XML linking
element identifies a resource that is not an
XML
document (for example, an HTML or PDF document), this specification does
not constrain the syntax or semantics of the query nor of the fragment specifier.
However, this specification does provide, starting in the next section, a complete description of query and fragment identifier syntax and semantics in the case when the indicated resource is an XML document.
When a locator identifies a resource that is an XML document, the locator value may contain either or both a URL and a TEI Extended Pointer (hereinafter XPointer). Special syntax may be used to request the use of particular processing models in accessing the locator's resource. This is designed to reflect the realities of network operation, where it may or may not be desirable to exercise fine control over the distribution of work between local and remote processors.
| Locator | ||||||||||||||||||||
|
In this discussion, the term designated resource refers to the resource participating in the link which the locator serves to locate. The following rules apply:
ID(Name)"; i.e. the sub-resource is the element in the containing
resource that has an XML
ID
attribute whose value
matches
the Name. This shorthand is to encourage use of the robust ID
addressing mode.
#", this signals an intent that the containing
resource is to be fetched as a whole from the host that provides it, and
that the XPointer processing to extract the sub-resource is to be performed
on the client, that is to say on the same system where the linking element
is recognized and processed.
?XML-XPTR=", this signals an intent that
the entire locator is to be transmitted to the host providing the resource,
and that the host should perform the XPointer processing to extract the
sub-resource, and that only the sub-resource should be transmitted to the
client.
|", no intent is signaled as to what processing
model is to be used to go about accessing the designated resource.
XML uses a locator syntax derived from that for TEI extended pointers. These operate in a straightforward way on the parse tree which is defined by the elements of an XML document.
The basic form of such a locator is a series of location terms, each of which
specifies a location, either absolute or (more frequently) relative to the
prior one. Each term has a name, such as ID, CHILD,
ANCESTOR, and so on, and can be qualified by parameters such
as an instance number, element types, or attributes. For example, the locator
string CHILD(2,CHAP)(4,SEC)(3) refers to the 3rd child of the
4th SEC within the 2nd CHAP within the referenced document.
Note: Formally, these may be specified as operating on groves as defined in DSSSL, using the grove plan (set of structural information) specified in HyTime. Every construct in such locators has a corresponding expression in DSSSL's SDQL query language, and most also have direct equivalents in the HyTime location module.
The syntax for TEI Extended Pointers has been adjusted in order to allow them to be packaged naturally with URLs without requiring URL-escaping of space characters:
..". These define the beginning and end of a span which constitutes
the resource. This merely combines the capability of the TEI
FROM and TO attributes into the locator syntax.
At the heart of the XPointer is the location term. Location terms are designed to work in sequences. Each location term works in the context of a location source, a location or element in the document. The location term itself provides instructions which, based on the location source context, navigate to another location in the document, establishing the location source for the next location term.
The location source for the first location term in a locator is, by default, the root element of the XML document which is the containing resource.
The result of of evaluating a location term may be (in this version of the specification, always is) an element.
A locator can contain either one or two series of location terms; if there
are two, they are separated by the string "..". If it contains
one, its designated resource is the element
or location selected by that series. if it contains two, its
designated resource is all of the text from
the location, or start of the element, selected by the first series, through
to the location, or the end of the element, selected by the second series:
| XPointer | ||||||||||||||||||||
|
If the first or second series of location terms are preceded by
ROOT, this means that the location source for the first location
term of the series is the
root
element of the containing resource. This is the default behavior, thus
the presence of the ROOT keyword has no effect on the interpretation
of the locator; it exists in the interests of clarity and comprehensibility
of the language design.
If the first or second series of location terms are preceded by
HERE, this means that the location source for the first location
term of the series is the linking element containing
the locator, rather than the default root element. This allows extended pointers
to select items such as "the paragraph immediately preceding the one within
which this pointer occurs". It is an error to use HERE in a
locator where a URL is also provided and identifies a resource different
from the document which contains the linking element.
If the second series is preceded by DITTO, this means that the
location source for the first location term is the location source specified
by the entire first series, in order to facilitate relative specification
of a span.
If the first or second series of location terms are preceded by
ID(Name), this means that the location source for the first
location term is element in the containing resource which has an attribute
of type ID with a value matching the given Name.
For example, the location specification
ID(a27) |
chooses the necessarily unique element of the containing resource which has
an attribute declared to be of type ID, whose value is a27.
Each location term specifies a two-stage selection process. The first selection phase is identified by keyword:
| Location Term | ||||||||
|
The keyword selects zero or more elements, relative to the location source:
CHILD
DESCENDANT
ANCESTOR
PREVIOUS
NEXT
PRECEDING
FOLLOWING
The locations or elements selected by the keyword are referred to as the candidate locations.
Further discussion of the detailed interpretation of each of these keywords appears in a later section.
The second phase of a location term's selection process uses a Steps expression:
| Steps | ||||||||
|
Each Step expression is used in order to select elements or locations from the candidate locations, generating a new set of candidate candidates.
| Instance | ||||
|
When the value of instance is the number N, it selects
the Nth of the candidate locations.
If the special value ALL is given, then all the candidate
locations are selected. Negative numbers count from the last candidate location
to the first; numbers out of range constitute an error.
Candidates can be selected by element type as well as number:
| Element | ||||||||||||||||
|
The Element gives an XML element type; only elements of the type indicated will be selected from among the candidate locations. For example, the location term
CHILD(3,DIV1)(4,DIV2)(29,P) |
selects the 29th paragraph of the fourth sub-division of the third major division of the location source.
The XPointer
DESCENDANT(-1,EXAMPLE) |
selects the last example in the document.
Selection by type is strongly recommended because it makes links more perspicuous and more robust. It is perspicuous because humans typically refer to things by type: as "the second section", "the third paragraph", etc. It is robust because it increases the chance of detecting breakage if (due to document editing) the target originally pointed at no longer exists.
The type may be specified by Name or using the values
"*CDATA" or "*". If the type is specified as
"*", any element type is matched; this means that the following
are synonymous:
CHILD(2,*) |
If the type is specified as "*CDATA", the location term selects
only untagged sub-portions of an element with
mixed
content.
The location term
CHILD(3,*CDATA) |
thus chooses the third span of character data directly contained by the current location source. If the location source is a paragraph containing
<Q> and
</Q>
<NOTE> and
</NOTE>
then CHILD(3,*CDATA) selects sentence C, while
CHILD(3) selects sentence B.
Candidates can be selected based on attribute name and value:
| Attribute | ||||||||||||||||||||||||||||||||||
|
The Attr and Val are used to provide attribute names and values to use in selecting among candidates.
If specified within quotation marks, the attribute-value parameter is case-sensitive; otherwise not.
As with generic identifiers, attribute names may be specified as
"*" in location terms in the (unlikely) event that an attribute
value constitutes a constraint regardless of what attribute name it is a
value for.
For example, the location term
CHILD(1,*,TARGET,*) |
selects the first child of the location source for which the attribute
TARGET has a value.
For example, the location specification
CHILD(1,*,N,2)(1,*,N,1) |
chooses an element using the N attribute. Beginning at the location
source, the first child (whatever element type it is) with an N
attribute having the value 2 is chosen; then that element's
first child element having the value 1 for the same attribute
is chosen.
The location specification
CHILD(1,FS,RESP,*IMPLIED) |
selects the first child of the location source which is an FS
element for which the RESP attribute has been left unspecified.
The location specification
ID(a23)DESCENDANT(2,TERM,LANG,DE) |
selects the second TERM element with a LANG of
DE occurring within the element with an ID of
A23. The search for matching elements occurs in the same order
as the XML data stream (depth-first, left-to-right).
If an instance number is negative, the search is depth-first right-to-left, in which the right-most, deepest matching element is numbered -1, etc. The location specification
DESCENDANT(-1,NOTE) |
thus chooses the last NOTE element in the document, that is,
the one with the rightmost start-tag.
The ANCESTOR location term selects an element from among the
direct ancestors of the location source. The parameters are as for
CHILD. However, the ANCESTOR keyword selects elements
from the list of containing elements or "ancestors" of the location source,
counting upwards from the parent of the location source (which is ancestor
number 1) to the root of the document instance (which is ancestor number
-1).
For example, the location term
ANCESTOR(1,*,N,1)(1,DIV) |
first chooses the smallest element properly containing the location source
and having attribute N with value 1; and then the
smallest DIV element properly containing it.
The PREVIOUS keyword selects an element or character-data string
from among those which precede the location source within the same parent
element. We speak of the elements and character-data strings contained by
the same
parent
element as siblings; those which precede a given element or string in
the document are its elder siblings; those which follow it are its younger
siblings.
The instance number in the location value of a PREVIOUS term
designates the nth elder sibling of the location source, counting from most
recent to less recent.
ID(a23)PREVIOUS(1) |
thus designates the element immediately preceding the element with an
ID of a23. Negative instance numbers also designate
elder siblings, but counting from the eldest left sibling to the youngest.
If the location source has at least one elder sibling, then the location
term
PREVIOUS(-1) |
designates the very eldest sibling and is synonymous with
ANCESTOR(1)CHILD(1) |
The value ALL may be used to select the entire range of elder
siblings of an element:
ID(a23)PREVIOUS(ALL) |
thus designates the set of elements preceding the element with an
ID of a23 and contained by the same parent.
The keyword NEXT behaves like PREVIOUS, but selects
from the younger siblings of the location source, not the elder siblings.
The location ladder
ID(a23)NEXT(1) |
thus designates the element or string immediately following the element which
has an ID of A23. Negative instance numbers designate
younger siblings counting from the youngest sibling toward the location source.
If the location source has at least one younger sibling, then the location
term
NEXT(-1) |
designates its youngest sibling.
The PRECEDING keyword selects an element or character-data string
from among those which precede the location source, without being limited
to the same containing element. The set of elements and strings which may
be selected is the set of all elements and strings in the entire document
which occur or begin before the location source. (For purposes of the keywords
PRECEDING and FOLLOWING, elements are interpreted
as occurring where they start.) The PRECEDING keyword thus resembles
PREVIOUS but differs in searching a larger set of strings and
elements; its result is not guaranteed to be a subset of its location source.
The instance number in the location value of a preceding term
designates the nth element or character-data string preceding the location
source, counting from most recent to less recent. The location ladder
ID(a23)PRECEDING(5) |
thus designates the fifth element or string before the element with an
ID of a23. Negative instance numbers also designate
preceding elements or strings, counting from the eldest to the youngest.
The location source must have at least as many elder siblings as the absolute
value of the instance number; otherwise, the PRECEDING term
fails. The value ALL may be used to select the entire portion
of the document preceding the beginning of the location source.
The keyword FOLLOWING behaves like PRECEDING, but
selects from the portion of the document following the location source, not
preceding it.
XML describes the syntax of link elements embedded in documents. Many applications, when processing a document, may wish to process not only the links embedded in that document, but links in other documents which point into it. For example, it may be desirable to highlight the resources of such links to make the linkage network's existence apparent. In other words, it may be appropriate to process a group of interlinked documents, rather than a single document.
In these cases, the Extended Link Group element
may be used to store a list of links to other documents that together constitute
an interlinked document group. Each such document is identified using the
HREF attribute of an Extended Link Document element,
which is a child element of the GROUP. The value of the HREF attribute is
a locator, with the same interpretation as described
above.
These elements, just as with EXTENDED, SIMPLE, or LOCATOR elements, are
recognized by the use of the XML-LINK attribute with the values
GROUP or DOCUMENT.
Here are declarations for the GROUP and DOCUMENT elements:
<!ELEMENT GROUP (DOCUMENT*)> |