WD-xml-link-970630WD-xml-link-970630
This is a W3C Working Draft for review by W3C members and other
interested parties. It is a draft document and may be updated,
replaced or obsoleted by other documents at any time. It is
inappropriate to use W3C Working Drafts as reference material or to
cite them as other than "work in progress". A list of current W3C
working drafts can be found at http://www.w3.org/pub/WWW/TR.
Note: Since working drafts are subject to frequent
change, you are advised to reference the above URL, rather than the
URLs for working drafts themselves.
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.
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:
resourcelinking
elementlocatorlabeltraversalmulti-directional linkin-line linkout-of-line linkThe 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 international ballot 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 may be either in-line or
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:
RoleROLE
attribute is used to provide both link and resource
roles.
ResourceHREF attribute, as
described below.LabelTITLE attribute.
This specification does not require that applications make any particular use
of the label.BehaviorSHOW 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.
In/Out-of-LineINLINE attribute may be used to communicate whether the
linking element is in-line or not.
This specification places no constraints on the contents
of linking elements; for this reason, in the sample declarations given below,
they are given declared content of ANY.
Of course, since any element may be recognized as a linking element based
on use of the XML-LINK attribute, to be valid, a linking element
must conform to the constraints expressed in its governing DTD.
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
types 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 element 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.
The INLINE attribute can take the values
TRUE and FALSE.
The value TRUE, which is the default, means that all of the
content of the linking element, except those of
its children which are part of the linking element syntax described in this
specification, is to be considered a resource of the link.
When the link is in-line, the CONTENT-ROLE and
CONTENT-TITLE attributes may be used to provide the label and
role information for this "content" resource.
If INLINE is FALSE, it is not an error to provide
the CONTENT-TITLE or CONTENT-ROLE attribute, but
they have no effect.
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:
EMBEDREPLACENEWThe ACTUATE attribute is used to express a policy as
to when traversal of a link should occur.
It may take one of two values:
AUTOUSERThe 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 element 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, 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:
CHILDDESCENDANTANCESTORPREVIOUSNEXTPRECEDINGFOLLOWINGFurther 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:
| ElType | ||||||||||||||||
|
The ElType 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>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 attribute
whose value is DE
occurring within the element with an ID
attribute whose value is 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*)> |