INTERNET-DRAFT J. Boynton
<draft-boynton-uol-03.txt> Produx House, Corp.
Expires six months from --> 5 August, 2001
Uniform Object Locator -- UOL
Status of This Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Abstract
A Uniform Object Locator (UOL) provides a general-purpose identifier
for "human" interaction with object oriented and relational data.
UOL is designed to meet the recommendations for URI queries laid out
in "Uniform Resource Identifiers (URI) Generic Syntax" [RFC2396].
This document defines syntax and semantics of UOL, including both
absolute and relative forms, and guidelines for their use; it
revises features, definitions, and examples, given in
"draft-boynton-uol-02" and updates the scheme to provide additional
functionality.
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Table of Contents
1 Introduction
1.1 Purpose
1.2 General Description
1.3 Terminology
2 UOL Syntactic Components
2.1 Path Components
2.1.1 Authority
2.1.2 Object Constructor
2.1.3 Object Name
2.2 Absolute and Relative Form
2.3 Reserved Markers
2.4 Example UOL
3 Object Elements
3.1 Objects
3.2 Containers
3.3 Undefined
4 Name Elements
4.1 Names
4.2 Index Names
4.3 Default Names
5 Reserved UOL Parameters
6 Relational Data
7 UOL, XPath, and XML
8 References
9 Author's Address (send comments)
1 Introduction
1.1 Purpose
A Uniform Object Locator (UOL) provides an intuitive, hierarchical,
"human-readable", identifier for defining the location and
relationship of data within object oriented and relational data
structures. UOL is intended for general purpose use as a command
line argument/ID for retrieval and storage of data. It also defines
elements that identify relative forms of UOL for simplified
concatenation and efficient serialization of UOL within a data
stream.
1.3 General Description
The UOL is constructed from a hierarchical list of elements in
similar fashion to a URL path component; with the '/' forward slash
as the delimiter for directory elements. A "//" (double slash) is
used to define an authority element.
UOL maps a data structure by providing two separate components that
define strings of related hierarchical elements. The two components
are separated by a '#' (crosshatch), and divide the UOL into
re-usable segments; an object constructor and object name. When
used together, the two segments form a unique all-purpose
identifier.
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By promoting re-use of the component strings, UOL intends to reduce
errors associated with data entry and interpretation. It also
allows both user and machine to efficiently and accurately derive
data relationships with out regard to location.
1.2 Terminology
For clarity, the specific role of elements within the UOL are
defined below.
element
A single, un-delimited, string of characters.
directory
A typical hierarchical string of elements delimited by a '/'
(forward slash).
attribute
A parameter element descending from an object or directory that
terminates a path.
schema
For this draft, schema is a subset of nodes, directories, or
attributes, defined by a schema, DTD(Document Type Definition),
or class definition.
object
A collection of directories and/or attributes defined by a
schema. An object can be visualized as a re-usable tree of nodes
descending from a single directory. In UOL, objects are
identified by the prefix "." and by a corresponding name element.
container
An object that servers as the parent component to one or more
named sub-elements. A container is identified by the prefix "..".
constructor
A complete collection of object and/or directory nodes followed
by a single attribute; where no attribute is given, the directory
path itself.
object name
A hierarchical collection of "." delimited elements that, when
used together, provide the full name for a UOL constructor.
name element
An individual element within an object name. Its value is mapped
to an object element within a constructor.
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2. UOL Syntactic Components
UOL characteristics are derived from widely implemented URL syntax
and semantics. A UOL consists of a scheme, authority, constructor
(path component) and name fragment. UOL may also be appended to a
URI as a query component; [RFC2396]. The UOL scheme, itself, does
not define a query component.
<scheme><authority><constructor>#<name>
Where used as a query to a URL, the authority and path component of
the URL comprise the authority for the UOL
<scheme><authority-path-component>?<constructor>#<name>
2.1 Path Components
Elements within a UOL form objects and containers by mapping key
names to directories and attributes within a path.
2.1.1 Authority
The authority component defines a top level root for resolving a uol
path to an absolute form. All elements within the uol are bound by
the context of the authority and may only resolve to elements that
descend from the authority.
2.1.2 Object Constructor
The constructor component maps the relationship of objects,
directories, and attributes through the use of a path metaphor. In
most cases, the constructor does not map the actual location of
data. Instead, a UOL parser uses the constructor as a pattern for
binding keys (name elements) to object elements within the
constructor. The "compiled" form is then used to retrieve or store
data.
2.1.3 Object Name
The name component is comprised of key elements that are mapped to
object elements within the constructor. Name elements are appended,
from left-to-right, corresponding to the object element each
identifies.
The constructor and name component are separated by a '#'
(crosshatch). Name elements are delimited by a "." (dot).
2.2 Absolute and Relative Forms
A UOL can be absolute or relative. An absolute UOL will resolve to,
or contain, a top level element preceded by two forward slashes
"//" (the authority).
For directory elements descending from an object or authority, UOLs
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beginning with "../", "./", or "/", are resolved in a manner
consistent with the behavior for URI path components in Section 5.2
[RFC2396]. The prefix, itself, is not considered an element of the
UOL path.
In addition to the components above, UOL provides two special
elements for resolving objects. ".~/" and "..~/" resolve a relative
path to the current object or container object, respectively.
During UOL parsing, the relative elements above are disambiguated
from object elements by the absence of intervening characters
between ".", "~", or "/".
The following elements are defined for resolving a partial UOL to:
"/" = AUTHORITY
"./" = the current DIRECTORY | OBJECT
"../" = the next higher DIRECTORY | OBJECT.
This element will not resolve above OBJECT | AUTHORITY.
".~/" = the current OBJECT.
Combinations of this element will resolve to higher OBJECT
nodes.
"..~/" = the current CONTAINER.
Combinations of this element will resolve to higher CONTAINER
nodes.
Identical prefixes may be combined such that successive attempts to
match the element will resolve to higher nodes within the UOL.
The last successful match will resolve the UOL to an absolute
form. If no match occurs, an application will indicate that no
such element exist.
Note: Semantics for combining un-like special elements are not
defined by UOL and considered unsafe.
2.3 Reserved Markers
The UOL scheme reserves the characters '=', '!', and '$' for
assigning special behavior to a UOL. The characters modify the
manner in which a UOL is treated by an application.
Each marker is reserved for use as a UOL prefix. The meaning for
each modifies the entire relative or absolute UOL segment to which
it is applied. Marked UOLs must not have white space between the
marker and the UOL it modifies.
link = "="
Identifies the UOL as a variable within a formula or data
cell. When encountered as a value to an argument, the
value referenced by the UOL is returned.
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comment = "!"
Identifies the UOL as a references to a "comment" value.
The comment provides specific details regarding the data
mapped by the UOL.
constant = "$"
Indicates that a UOL is mapped to a constant in-memory
location that cannot be moved, translated, or resolved to
another path.
Note: When used in conjunction with other markers, "="
must always appear first, then "!" then "$".
2.4 Example UOL
The following section provides examples for how object elements are
used in a UOL. The examples given are for a fictitious company with
branch offices in three cities LA, Dallas, and Tampa. Each office
has, three departments: sales, accounting, and personnel.
The example below shows a UOL referencing the total hours worked by
an employee named Jones, from the sales department of the LA branch
office.
"//company/..branch/..dept/.log/hours/total#LA.sales.Jones"
To following pseudo XML code illustrates the construction concepts
for the above UOL.
<branch name="LA">
<dept name="sales">
<log name="Jones" hours-total="value"/>
</dept>
</branch>
The next example uses the same constructor to reference the total
hours worked by an accounting employee at the Dallas branch.
"/..branch/..dept/.log/hours/total#Dallas.accounting.Smith"
Like the constructor, object names can also be re-used. In the
example below, a constructor is modified to retrieve several records
for a personnel associate in Tampa.
"/..branch/..dept/.log/hours/total#Tampa.personnel.Carter"
"/..branch/..dept/.log/hours/overtime#Tampa.personnel.Carter"
"/..branch/..dept/.log/hours/vacation#Tampa.personnel.Carter"
The next example shows absolute and relative forms of UOL. The UOLs
shown point to an attribute in the department object (.dept).
abs = "/..branch/.dept/administrator#Tampa.personnel"
rel = "..~/..~/.dept/administrator#Tampa.personnel"
rel = ".~/.~/administrator#personnel"
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Note that the second relative UOL inherits the branch name "Tampa".
Therefore, it does not need to provide it.
3. Object Elements
3.1 Objects
In the UOL scheme, an object is a directory element that declares
its sub-elements to represent a tree of objects, directories, or
attributes, defined by a schema, DTD(Document Type Definition), or
class definition. All UOL objects are mapped by name elements
appended to the name component of the UOL. An object element is
identified by the prefix "." (dot).
It is important to note that the values "./", "../", ".~/", and
"..~/" are reserved for resolving a relative UOL path to an absolute
form and are NOT considered object elements; Section 2.2.
object reference
"/..object/dir/attr#name"
relative reference
"../dir/attr"
3.2 Containers
An object element with a prefix of ".." (two dots) is interpreted as
a "container" for other objects. A container element extends an
object by sharing its name space as well as its name element. It
does not, however, reference the objects attributes. Instead, the
container diverts the UOL path to nested objects within its scope.
UOL allows objects and containers to be expressed within the same
name space because the elements are mutually exclusive (they cannot
be visible at the same time). The following relative UOLs
illustrate this concept by describing attributes and tags within an
HTML table.
ref to <table> - .table/border#table1="10"
ref to <td> - ..table/..tr/.td/align#table1.row1.td1="Center"
<table cellspacing=0 cellpadding=0 border=10>
<tr>
<td align="Center" valign="Top">Example Text</td>
</tr>
</table>
3.3 Undefined
To facilitate use of UOL in describing SGML documents and popular
sub-sets of SGML, such as HTML, UOL provides an all-purpose
undefined object. The object is primarily used to identify text
between mark-up tag.
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The undefined object is represented by a "~" (tilde). Within a UOL
constructor, it has the same characteristics as an attribute but,
because it is an object, it has an associated name element. The
name element assigned is always an index value corresponding to the
location of the element within its container.
html = <font size="+2">my text</font>
uol = /..font/~#fnt1.1="my text"
4 Name Elements
4.1 Names
Each object name space is assigned a unique name id. The name is
appended to the object name (if any) inherited from the parent
directory. The new object name is then inherited by all
sub-directories of the identified object element. Elements in the
object name are delimited by a "." (dot).
"/.object/sub_dir/field#name"
"/.parent_object/.child_object/field#parent_name.child_name"
4.2 Index Names
UOL allows name substitution with index values. The index name is
inclosed by "(" and ")" (open and closed parentheses). For any
list of object names, the integer index may be used in place of the
actual name. Where no list is given, the name element will be
parsed to the string value of the index number (i.e. "(9)" == "9").
It is envisioned that name elements containing parentheses may
contain platform dependant code such as functions, arithmetic
operators, and variables. Therefore, parsers should not attempt to
interpret the index value directly. Also, because such content is
likely to include reserved and unsafe characters, such code should
be converted to "x-form-url-encoded" strings. Characters outside of
the parentheses, but within the name element, should be removed.
4.3 Default Name
The "" (empty string) is reserved for naming default object
elements. Applications can use the default object for storing
schemas or default values. Where a named object appears with a
default object, the "." (dot) delimiter implies the default name.
default name (empty)
"//authority/..object1/..object2/attr#"
default name (leading)
"//authority/..object1/..object2/attr#.name2"
default name (trailing)
"//authority/..object1/..object2/attr#name1"
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5 Reserved UOL Parameters
Though UOL does not define a language for interacting with its
components, it does provide a limited convention for passing
parameters to a host application. Unlike other elements in a UOL,
parameter elements are case insensitive.
The parameters provide useful information on directory elements
(including objects) within the UOL constructor. The information
provided may be used by software for memory allocation, assigning
permissions, indexing, and for the creation of "in-memory" objects
based upon UOL patterns.
UOL parameters are delimited from the constructor by an "@"
(commercial at sign). Where present, "@" indicates both the start
of a parameter and the end of a directory. All characters between
"@" and the next "#", or the end of a UOL, are considered a
parameter element.
To simplify concatenation of a parameter element within a UOL, it is
permissible to append a parameter to a directory element without
removing the directory's trailing forward slash. However, a well
formed UOL should have the "/" removed.
"/.object@attlist#Name1" | "/.object/@attlist#Name1"
The following parameters are reserved for UOL:
@names = Used to obtain a line delimited string of name elements.
Each represent a "human readable" key extension in the current
object directory.
@schema = Used to obtain a line delimited list of UOL strings
representing a complete schema for sub-elements of the current
directory. The list provides partial UOL constructors that will
resolve correctly when combined with the current path. Each
element path is enumerated in turn. The object name fragment
(if any) is removed. This list is "read-only".
@attlist = a non-recursive list of elements contained in the current
directory.
@code = the class description or file for an object that will use
data from this directory.
@codebase = the path description or URL to a class described by
"@code"
@aka = "Also Known As" holds a single, alternate name for describing
an object element. Where present, "aka" should provide the most
common generic substitute for the actual name used.
@functions = Used to retrieve or update a line delimited list of
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functions that will interact with values associated with a UOL
directory. functions should be stated in a form consistent with
a "classid" URI; Section 13.3 [REC html 4.01].
6 Relational Data [Pending]
This section will discuss conventions for expressing fields within a
relational data base through the use of nested UOL strings.
7 UOL, XPath, and XML
User interaction with parsed XML data is one possible use for the
UOL scheme. However, UOL is NOT proposed for referencing attributes
from within XML schemas, DTDs, or payload. This functionality is
provided by XML Path Language which was specifically written for
this task. At the time this draft was updated, the W3C
recommendation for XML Path Language was [XPath10];
8 References
[RFC2396] T. Berners-Lee, R. Fielding, and L. Masinter.
"Uniform Resource Identifiers (URI) Generic Syntax".
IETF RFC 2396 August 1998.
[HTML401] D. Raggett, A. Le Hors, and I. Jacobs, Editors,
"HTML 4.01 Specification".
W3C Recommendation HTML401, 24 December 1999.
[XPath10] J. Clark, and S. DeRose, Editors,
"XML Path Language (XPath) Version 1.0".
W3C Recommendation XPath10, 16 November 1999.
9 Author's Address
Jon L. Boynton
Produx House, Corp.
19300 Nalle Rd.
North Ft.Myers, FL 33917
Phone 941 543 4491
Email Comments to jon@datamessenger.com
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