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EWF/library/protocol/uri_template/doc/uritemplate-draft-gregorio-trunk.xml
Jocelyn Fiat 08bec49da4 added doc/spec for uri template
2011-07-25 12:38:16 +02:00

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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
<!ENTITY rfc2119 SYSTEM 'bibxml/reference.RFC.2119.xml'>
<!ENTITY rfc3986 SYSTEM 'bibxml/reference.RFC.3986.xml'>
<!ENTITY rfc3987 SYSTEM 'bibxml/reference.RFC.3987.xml'>
<!ENTITY rfc5234 SYSTEM 'bibxml/reference.RFC.5234.xml'>
<!ENTITY rfc2978 SYSTEM 'bibxml/reference.RFC.2978.xml'>
<!ENTITY rfc3629 SYSTEM 'bibxml/reference.RFC.3629.xml'>
<!ENTITY ASCII SYSTEM 'bibxml/_reference.ANSI.X3-4.1986.xml'>
<!ENTITY UNIV4 SYSTEM 'bibxml/univ4.xml'>
<!ENTITY UTR15 SYSTEM 'bibxml/utr15.xml'>
]>
<!--<?xml-stylesheet type='text/xsl' href='rfc2629.xslt'/ ?>-->
<?rfc toc="yes" ?>
<?rfc symrefs="yes" ?>
<?rfc sortrefs="yes"?>
<?rfc iprnotified="yes" ?>
<?rfc strict="yes" ?>
<?rfc compact="yes" ?>
<?rfc comments="yes" ?>
<?rfc inline="yes" ?>
<?rfc linkmailto="no" ?>
<?rfc tocdepth="3" ?>
<!--
1. Update the docName
2. Update the date
3. Update the Revision History.
-->
<rfc category="std" ipr="pre5378Trust200902" docName="draft-gregorio-uritemplate-06">
<front>
<title>URI Template</title>
<author initials='J.C.' surname="Gregorio" fullname='Joe Gregorio'>
<organization>Google</organization>
<address>
<email>joe@bitworking.org</email>
<uri>http://bitworking.org/</uri>
</address>
</author>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address>
<email>fielding@gbiv.com</email>
<uri>http://roy.gbiv.com/</uri>
</address>
</author>
<author initials='M.H.' surname="Hadley" fullname='Marc Hadley'>
<organization>Oracle</organization>
<address>
<email>Marc.Hadley@oracle.com</email>
<uri>http://oracle.com/</uri>
</address>
</author>
<author initials='M.N.' surname="Nottingham" fullname='Mark Nottingham'>
<organization></organization>
<address>
<email>mnot@pobox.com</email>
<uri>http://mnot.net/</uri>
</address>
</author>
<author initials='D.O.' surname="Orchard" fullname='David Orchard'>
<organization></organization>
<address>
<uri>http://www.pacificspirit.com/</uri>
</address>
</author>
<date day="20" month="Jul" year="2011"/>
<abstract>
<t>
A URI Template is a compact sequence of characters
for describing a range of Uniform Resource Identifiers
through variable expansion.
This specification defines the URI Template
syntax and the process for expanding a URI Template into a
URI reference, along with guidelines for the
use of URI Templates on the Internet.
</t>
</abstract>
<note title="Editorial Note (to be removed by RFC Editor)">
<t>
To provide feedback on this Internet-Draft, join the
<eref target="http://lists.w3.org/Archives/Public/uri/">W3C URI
mailing list (http://lists.w3.org/Archives/Public/uri/)</eref>.
</t>
</note>
</front>
<middle>
<section title="Introduction" anchor="intro">
<section title="Overview" anchor="overview">
<t>
A Uniform Resource Identifier (URI) <xref target="RFC3986"/>
is often used to identify a specific resource within a common
space of similar resources. For example, personal web spaces
are often delegated using a common pattern, such as
</t>
<figure><artwork>
http://example.com/~fred/
http://example.com/~mark/
</artwork></figure>
<t>
or a set of dictionary entries might be grouped in a hierarchy
by the first letter of the term, as in
</t>
<figure><artwork>
http://example.com/dictionary/c/cat
http://example.com/dictionary/d/dog
</artwork></figure>
<t>
or a service interface might be invoked with various user input
in a common pattern, as in
</t>
<figure><artwork>
http://example.com/search?q=cat&amp;lang=en
http://example.com/search?q=chien&amp;lang=fr
</artwork></figure>
<t>
URI Templates provide a mechanism for abstracting a space of
resource identifiers such that the variable parts can be easily
identified and described. URI templates can have many uses,
including discovery of available services, configuring resource
mappings, defining computed links, specifying interfaces, and
other forms of programmatic interaction with resources.
For example, the above resources could be described by the
following URI templates:
</t>
<figure><artwork>
http://example.com/~{username}/
http://example.com/dictionary/{term:1}/{term}
http://example.com/search{?q,lang}
</artwork></figure>
<t>
We define the following terms:
<list style="symbols">
<t>expression -
The text between '{' and '}', including the enclosing braces,
as defined in <xref target="syntax"/>.</t>
<t>expansion -
The string result obtained from a template expression after
processing it according to its expression type, list of variable
names, and value modifiers, as defined in <xref target="expansion"/>.</t>
<t>template processor -
A program or library that, given a URI Template and a set of
variables with values, transforms the template string into a
URI-reference by parsing the template for expressions and
substituting each one with its corresponding expansion.</t>
</list>
</t>
<t>
A URI Template provides both a structural description of a URI
space and, when variable values are provided, a simple instruction
on how to construct a URI corresponding to those values.
A URI Template is transformed into a URI-reference by replacing each
delimited expression with its value as defined by the
expression type and the values of variables named within the
expression. The expression types range from simple string
expansion to multiple key=value lists. The expansions
are based on the URI generic syntax, allowing an implementation
to process any URI Template without knowing the scheme-specific
requirements of every possible resulting URI.
</t>
<t>
For example, the following URI Template includes a
form-style parameter expression, as indicated by the "?" operator
appearing before the variable names.
</t>
<figure><artwork>
http://www.example.com/foo{?query,number}
</artwork></figure>
<t>
Each template expression describes, in a machine-readable manner,
how a URI is to be constructed. In this example,
the expansion process for templates beginning with the
question-mark ("?") operator follows the same pattern
as form-style interfaces on the World Wide Web.
</t>
<figure><artwork>
http://www.example.com/foo{?query,number}
\_____________/
|
|
For each defined variable in [ 'query', 'number' ],
substitute "?" if it is the first substitution or "&amp;"
thereafter, followed by the variable name, '=', and the
variable's value.
</artwork></figure>
<t>
If the variables have the values
</t>
<figure><artwork>
query := "mycelium"
number := 100
</artwork></figure>
<t>
then the expansion of the above URI Template is
</t>
<figure><artwork>
http://www.example.com/foo?query=mycelium&amp;number=100
</artwork></figure>
<t>
Alternatively, if 'query' is undefined, then the expansion
would be
</t>
<figure><artwork>
http://www.example.com/foo?number=100
</artwork></figure>
<t>
or if both variables are undefined, then it would be
</t>
<figure><artwork>
http://www.example.com/foo
</artwork></figure>
<t>
A URI Template may be provided in absolute form, as in the examples
above, or in relative form. A template MUST be expanded before the
resulting reference can be resolved from relative to absolute form.
</t>
<t>
Although the URI syntax is used for the result, the template
string is allowed to contain the broader set of characters
that can be found in IRI references <xref target="RFC3987"/>.
A URI Template is therefore also an IRI template, and the result
of template processing can be transformed to an IRI by following
the process defined in Section 3.2 of <xref target="RFC3987"/>.
</t>
</section>
<section title="Levels and Expression Types" anchor="types">
<t>
URI Templates are similar to a macro language with a fixed set of
macro definitions: the expression type determines the expansion
process. The default expression type is simple string expansion, wherein
a single named variable is replaced by its value as a string after UTF-8
encoding the characters and then pct-encoding any octets that are
not in the unreserved set.
</t>
<t>
Since most template processors implemented prior to this specification
have only implemented the default expression type, we refer to these as
Level 1 templates.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| Level 1 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| empty := "" |
| undef := null |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | Simple string expansion (Sec 3.2.2) |
| | |
| | {var} value |
| | {hello} Hello%20World%21 |
| | O{empty}X OX |
| | O{undef}X OX |
`-----------------------------------------------------------------'
</artwork></figure>
<t>
Level 2 templates add the plus ("+") operator for expansion of
values that are allowed to include reserved characters.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| Level 2 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| empty := "" |
| undef := null |
| path := "/foo/bar" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| + | Reserved string expansion (Sec 3.2.3) |
| | |
| | {+var} value |
| | {+hello} Hello%20World! |
| | O{+empty}X OX |
| | O{+undef}X OX |
| | {+path}/here /foo/bar/here |
| | here?ref={+path} here?ref=/foo/bar |
| | up{+path}{var}/here up/foo/barvalue/here |
| | |
`-----------------------------------------------------------------'
</artwork></figure>
<t>
Level 3 templates add more complex operators for lists of
comma-separated values, dot-prefixed labels, slash-prefixed path
segments, semicolon-prefixed path parameters, and the forms-style
construction of a query syntax consisting of key=value pairs that
are separated by an ampersand character.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| Level 3 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| empty := "" |
| undef := null |
| path := "/foo/bar" |
| x := "1024" |
| y := "768" |
| |
|-----------------------------------------------------------------|
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with multiple variables (Sec 3.2.2) |
| | |
| | {x,y} 1024,768 |
| | {x,hello,y} 1024,Hello%20World%21,768 |
| | ?{x,empty} ?1024, |
| | ?{x,undef} ?1024 |
| | ?{undef,y} ?768 |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with multiple variables (Sec 3.2.3) |
| | |
| | {+x,hello,y} 1024,Hello%20World!,768 |
| | {+path,x}/here /foo/bar,1024/here |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.4) |
| | |
| | X{.var} X.value |
| | X{.empty} X. |
| | X{.undef} X |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.5) |
| | |
| | {/var} /value |
| | {/var,empty} /value/ |
| | {/var,undef} /value |
| | {/var,x}/here /value/1024/here |
| | |
|-----+-----------------------------------------------------------|
| ; | Path-style parameters, semicolon-prefixed (Sec 3.2.6) |
| | |
| | {;x,y} ;x=1024;y=768 |
| | {;x,y,empty} ;x=1024;y=768;empty |
| | {;x,y,undef} ;x=1024;y=768 |
| | |
|-----+-----------------------------------------------------------|
| ? | Form-style query, ampersand-separated (Sec 3.2.7) |
| | |
| | {?x,y} ?x=1024&amp;y=768 |
| | {?x,y,empty} ?x=1024&amp;y=768&amp;empty= |
| | {?x,y,undef} ?x=1024&amp;y=768 |
| | |
`-----------------------------------------------------------------'
</artwork></figure>
<t>
Finally, Level 4 templates add the ability to specify value modifiers
as a suffix to the variable name. The prefix modifier (":") indicates
that only a limited number of characters from the beginning of the
value are used by the expansion. The explode ("*") modifier tells
the expansion process to treat the value as a multivalued structure ---
a list of values or key=value pairs -- rather than as a single string.
</t>
<figure><artwork>
.-----------------------------------------------------------------.
| Level 4 examples, with variables having values of |
| |
| var := "value" |
| hello := "Hello World!" |
| path := "/foo/bar" |
| list := [ "red", "green", "blue" ] |
| keys := [("semi",";"),("dot","."),("comma",",")] |
| empty_keys := [] |
| |
| Op Expression Expansion |
|-----------------------------------------------------------------|
| | String expansion with value modifiers (Sec 3.2.2) |
| | |
| | {var:3} val |
| | {var:30} value |
| | {list} red,green,blue |
| | {list*} red,green,blue |
| | {keys} semi,%3B,dot,.,comma,%2C |
| | {keys*} semi=%3B,dot=.,comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| + | Reserved expansion with value modifiers (Sec 3.2.3) |
| | |
| | {+path:6}/here /foo/b/here |
| | {+list} red,green,blue |
| | {+list*} red,green,blue |
| | {+keys} semi,;,dot,.,comma,, |
| | {+keys*} semi=;,dot=.,comma=, |
| | |
|-----+-----------------------------------------------------------|
| . | Label expansion, dot-prefixed (Sec 3.2.4) |
| | |
| | X{.var:3} X.val |
| | X{.list} X.red,green,blue |
| | X{.list*} X.red.green.blue |
| | X{.keys} X.semi,%3B,dot,.,comma,%2C |
| | X{.keys*} X.semi=%3B.dot=..comma=%2C |
| | X{.empty_keys} X |
| | X{.empty_keys*} X |
| | |
|-----+-----------------------------------------------------------|
| / | Path segments, slash-prefixed (Sec 3.2.5) |
| | |
| | {/var:1,var} /v/value |
| | {/list} /red,green,blue |
| | {/list*} /red/green/blue |
| | {/list*,path:4} /red/green/blue/%2Ffoo |
| | {/keys} /semi,%3B,dot,.,comma,%2C |
| | {/keys*} /semi=%3B/dot=./comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| ; | Path-style parameters, semicolon-prefixed (Sec 3.2.6) |
| | |
| | {;hello:5} ;hello=Hello |
| | {;list} ;list=red,green,blue |
| | {;list*} ;red;green;blue |
| | {;keys} ;keys=semi,%3B,dot,.,comma,%2C |
| | {;keys*} ;semi=%3B;dot=.;comma=%2C |
| | |
|-----+-----------------------------------------------------------|
| ? | Form-style query, ampersand-separated (Sec 3.2.7) |
| | |
| | {?var:3} ?var=val |
| | {?list} ?list=red,green,blue |
| | {?list*} ?list=red&amp;list=green&amp;list=blue |
| | {?keys} ?keys=semi,%3B,dot,.,comma,%2C |
| | {?keys*} ?semi=%3B&amp;dot=.&amp;comma=%2C |
| | |
`-----------------------------------------------------------------'
</artwork></figure>
</section>
<section title="Design Considerations" anchor="design">
<t>
Mechanisms similar to URI Templates have been defined within
several specifications, including WSDL, WADL and OpenSearch.
This specification extends and formally defines the syntax so
that URI Templates can be used consistently across multiple
Internet applications and within Internet message fields,
while at the same time retaining compatibility with those
earlier definitions.
</t>
<t>
The URI Template syntax has been designed to carefully balance
the need for a powerful expansion mechanism with the need for
ease of implementation. The syntax is designed to be trivial
to parse while at the same time providing enough flexibility to
express many common template scenarios. Implementations are able
to parse the template and perform the expansions in a single pass.
</t>
<t>
Templates are simple and readable when used with common
examples because the single-character operators match the URI
generic syntax delimiters. The operator's associated delimiter
(";", "?", "/", and ".") is omitted when none of the listed variables
are defined. Likewise, the expansion process for ";"
(path-style parameters) will omit the "=" when the variable value
is empty, whereas the process for "?" (form-style parameters)
will not omit the "=" when the value is empty. Multiple variables
and list values have their values joined with "," if there is no
predefined joining mechanism for the operator. Only one operator,
plus ("+"), will substitute unencoded reserved characters found
inside the variable values; the other operators will pct-encode
reserved characters found in the variable values prior to expansion.
</t>
<t>
The most common cases for URI spaces can be described with
Level 1 template expressions. If we were only concerned with
URI generation, then the template syntax could be limited to
just simple variable expansion, since more complex forms could
be generated by changing the variable values. However, URI
Templates have the additional goal of describing the layout of
identifiers in terms of preexisting data values. The template
syntax therefore includes operators that reflect how
resource identifiers are commonly allocated. Likewise, since
prefix substrings are often used to partition large spaces of
resources, modifiers on variable values provide a way
to specify both the substring and the full value string
with a single variable name.
</t>
</section>
<section title="Limitations" anchor="limitations">
<t>
Since a URI Template describes a superset of the identifiers,
there is no implication that every possible expansion for
each delimited variable expression corresponds to a URI of an
existing resource. Our expectation is that an application
constructing URIs according to the template will be provided
with an appropriate set of values for the variables being
substituted and will be able to cope with any errors that might
occur when the resulting URI is used for name resolution or access.
</t>
<t>
URI Templates are not URIs: they do not identify
an abstract or physical resource, they are not parsed as URIs,
and should not be used in places where a URI would be expected
unless the template expressions will be expanded by a template
processor prior to use. Distinct field, element, or attribute
names should be used to differentiate protocol elements that
carry a URI Template from those that expect a URI reference.
</t>
<t>
Some URI Templates can be used in reverse for the purpose of
variable matching: comparing the template to a fully formed
URI in order to extract the variable parts from that URI and
assign them to the named variables. Variable matching only works
well if the template expressions are delimited by the beginning or
end of the URI or by characters that cannot be part of the
expansion, such as reserved characters surrounding a simple
string expression. In general, regular expression languages
are better suited for variable matching.
</t>
</section>
<section title="Notational Conventions" anchor="notation">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be
interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of <xref target="RFC5234"/>. The following ABNF rules
are imported from the normative references <xref target="RFC5234"/>,
<xref target="RFC3986"/>, and <xref target="RFC3987"/>.
</t>
<figure><artwork type="abnf">
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
pct-encoded = "%" HEXDIG HEXDIG
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&amp;" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
/ %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
/ %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
/ %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
/ %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
/ %xD0000-DFFFD / %xE1000-EFFFD
iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
</artwork></figure>
</section>
<section title='Character Encoding and Unicode Normalization'
anchor="unicode-normal">
<t>
This specification uses the terms "character" and "coded
character set" in accordance with the definitions provided
in <xref target="RFC2978"/>, and "character encoding" in
place of what <xref target="RFC2978"/> refers to as a "charset".
</t>
<t>
The ABNF notation defines its terminal values to be
non-negative integers (codepoints) that are a superset of the
US-ASCII coded character set <xref target="ASCII"/>. This
specification defines terminal values as codepoints within the
Unicode coded character set <xref target="UNIV4"/>.
</t>
<t>
In spite of the syntax and template expansion process being defined
in terms of Unicode codepoints, it should be understood that
templates occur in practice as a sequence of characters in
whatever form or encoding is suitable for the context in which
they occur, whether that be octets embedded in a network protocol
element or paint applied to the side of a bus.
This specification does not mandate any particular character encoding
for mapping between URI Template characters and the octets used to
store or transmit those characters.
When a URI Template appears in a protocol element, the character
encoding is defined by that protocol; without such a definition,
a URI Template is assumed to be in the same character encoding as
the surrounding text. It is only during the process of template
expansion that a string of characters in a URI Template is REQUIRED
to be processed as a sequence of Unicode codepoints.
</t>
<t>
The Unicode Standard <xref target="UNIV4"/> defines various
equivalences between sequences of characters for various purposes.
Unicode Standard Annex #15 <xref target="UTR15"/> defines various
Normalization Forms for these equivalences. The normalization form
determines how to consistently encode equivalent strings.
In theory, all URI processing implementations, including template
processors, should use the same normalization form for generating
a URI reference. In practice, they do not. If a value has been
provided by the same server as the resource, then it can be assumed
that the string is already in the form expected by that server.
If a value is provided by a user, such as via a data-entry dialog,
then the string SHOULD be normalized as Normalization Form C
(NFC: Canonical Decomposition, followed by Canonical Composition)
prior to being used in expansions by a template processor.
</t>
<t>
Likewise, when non-ASCII data that represents readable strings is
pct-encoded for use in a URI reference, a template processor MUST
first encode the string as UTF-8 <xref target="RFC3629"/> and then
pct-encode any octets that are not allowed in a URI reference.
</t>
</section>
</section>
<section title="Syntax" anchor="syntax">
<t>
A URI Template is a string of printable Unicode characters
that contains zero or more embedded variable expressions, each
expression being delimited by a matching pair of braces ('{', '}').
</t>
<figure><artwork type="abnf">
URI-Template = *( literals / expression )
</artwork></figure>
<t>
Although templates (and template processor implementations) are
described above in terms of four gradual levels, we define the
URI-Template syntax in terms of the ABNF for Level 4. A template
processor limited to lower level templates MAY exclude the ABNF
rules applicable only to higher levels. However, it is RECOMMENDED
that all parsers implement the full syntax such that unsupported
levels can be properly identified as such to the end user.
</t>
<section title="Literals" anchor="literals">
<t>
The characters outside of expressions in a URI Template string
are intended to be copied literally to the URI-reference if the
character is allowed in a URI (reserved / unreserved / pct-encoded)
or, if not allowed, copied to the URI-reference in its UTF-8
pct-encoded form.
</t>
<figure><artwork type="abnf">
literals = %x21 / %x23-24 / %x26 / %x28-3B / %x3D / %x3F-5B
/ %x5D-5F / %x61-7A / %x7E / ucschar / iprivate
/ pct-encoded
; any Unicode character except: CTL, SP,
; DQUOTE, "'", "%" (aside from pct-encoded),
; "&lt;", "&gt;", "\", "^", "`", "{", "|", "}"
</artwork></figure>
</section>
<section title="Expressions" anchor="expressions">
<t>
Template expressions are the parameterized parts of a URI Template.
Each expression contains an optional operator, which defines the
expression type and its corresponding expansion process, followed by a
comma-separated list of variable specifiers (variable names and
optional value modifiers). If no operator is provided, the expression
defaults to simple variable expansion of unreserved values.
</t>
<figure><artwork type="abnf">
expression = "{" [ operator ] variable-list "}"
operator = "+" / "." / "/" / ";" / "?" / op-reserve
op-reserve = "=" / "," / "!" / "@" / "|"
; reserved for local use: "$" / "(" / ")"
</artwork></figure>
<t>
</t>
<t>
The operator characters have been chosen to reflect each of their roles
as reserved characters in the URI generic syntax. The operators defined
by this specification include:
plus ("+") for substituting values that may contain reserved characters;
dot (".") for substituting values as a sequence of name labels prefixed
by ".";
slash ("/") for substituting values as a sequence of path segments
prefixed by "/";
semicolon (";") for substituting key=value pairs as path parameters
prefixed by ";"; and,
question-mark ("?") for substituting a query component beginning
with "?" and consisting of key=value pairs separated by "&amp;".
These operators will be described in detail in <xref target="expansion"/>.
</t>
<t>
The operator characters equals ("="), comma (","), exclamation ("!"),
at-sign ("@"), and pipe ("|") are reserved for future extensions.
</t>
<t>
The expression syntax specifically excludes use of the dollar ("$")
and parentheses ["(" and ")"] characters so that they remain
available for local language extensions outside the scope of this
specification.
</t>
</section>
<section title="Variables" anchor="variables">
<t>
After the operator (if any), each expression contains a list of
one or more comma-separated variable specifiers (varspec).
The variable names serve multiple purposes: documentation for
what kinds of values are expected, identifiers for associating
values within a template processor, and the string to use
for each key on key=value expansions.
</t>
<figure><artwork type="abnf">
variable-list = varspec *( "," varspec )
varspec = varname [ modifier ]
varname = varchar *( varchar / "." )
varchar = ALPHA / DIGIT / "_" / ucschar / iprivate
/ pct-encoded
</artwork></figure>
<t>
A varname MAY contain one or more pct-encoded triplets. If so,
these triplets MUST be decoded to octets prior to mapping the
varname to a variable name. If the expression type calls for the
variable name to be included in the expansion result, then the
template processor MAY use either the original pct-encoded varname
or the string corresponding to a minimally encoded version of
the mapped variable name. I.e., a varname containing unnecessarily
pct-encoded characters might be expanded to use those characters
directly instead.
</t>
<t>
An expression MAY reference variables that are unknown
to the template processor or whose value is set to a special
"undefined" value, such as undef or null. Such undefined
variables are given special treatment by the expansion process.
</t>
<t>
A variable value that is a string of length zero is not considered
undefined; it has the defined value of an empty string.
</t>
<t>
A variable may have a composite or structured value, such as a
list of values, an associative array of (key, value) pairs, or
a structure of components defined by some separate schema.
Such value types are not directly indicated by the template
syntax, but do have an impact on the expansion process. A
composite or structured value with zero member values is
considered undefined.
</t>
<t>
If a variable appears more than once in an expression or
within multiple expressions of a URI Template, the value of
that variable MUST remain static throughout the expansion
process (i.e., the variable must have the same value for the
purpose of calculating each expansion).
</t>
</section>
<section title="Value Modifiers" anchor="modifiers">
<t>
Each of the variables in a Level 4 template expression can have
a modifier indicating either that its expansion is limited to a
prefix of the variable's value string or that its expansion is
exploded into components based on an external type or schema
associated with that variable.
</t>
<figure><artwork type="abnf">
modifier = prefix / explode
</artwork></figure>
<section title="Prefix Values" anchor="prefix-values">
<t>
A prefix modifier indicates that the variable expansion is limited
to a prefix of the variable's value string. Prefix modifiers are
often used to partition an identifier space hierarchically, as is
common in reference indices and hash-based storage. It also serves
to limit the expanded value to a maximum number of characters.
</t>
<figure><artwork type="abnf">
prefix = ":" max-length
max-length = %x31-39 *DIGIT ; positive integer
</artwork></figure>
<t>
The max-length is a positive integer that refers to a maximum number
of characters from the beginning of the variable's value as a
Unicode string.
Note that this numbering is in characters, not octets, in order to
avoid splitting between the octets of a multi-octet UTF-8 encoded
character or within a pct-encoded triplet.
If the max-length is greater than the length of the variable's
value, then the entire value string is used.
</t>
<t>
For example,
</t>
<figure><artwork>
Given the variable assignments
var := "value"
semi := ";"
Example Template Expansion
{var} value
{var:20} value
{var:3} val
{semi} %3B
{semi:2} %3B
</artwork></figure>
</section>
<section title="Component Values" anchor="component-values">
<t>
An explode modifier ("*") indicates that the
variable represents a composite value that may be substituted
in full or partial forms, depending on the variable's type or
schema. Since URI Templates do not contain an indication of
type or schema, this is assumed to be determined by context.
An example context is a mark-up element or header field that
contains one attribute that is a template and one or more other
attributes that define the schema applicable to variables found
in the template. Likewise, a typed programming language might
differentiate variables as strings, lists, associative arrays,
or structures.
</t>
<figure><artwork type="abnf">
explode = "*"
</artwork></figure>
<t>
Explode modifiers improve brevity in the URI Template syntax.
For example, a resource that provides a geographic map for a
given street address might accept a hundred permutations on
fields for address input, including partial addresses (e.g.,
just the city or postal code). Such a resource could be
described as a template with each and every address component
listed in order, or with a far more simple template that makes
use of an explode modifier, as in
</t>
<figure><artwork>
/mapper{?address*}
</artwork></figure>
<t>
along with some context that defines what the variable named
"address" can include, such as by reference to some other
standard for addressing (e.g., UPU S42 or AS/NZS 4819:2003).
A recipient aware of the schema can then provide appropriate
expansions, such as:
</t>
<figure><artwork>
/mapper?city=Newport%20Beach&amp;state=CA
</artwork></figure>
<t>
If an explode modifier is present, the expansion process for that
variable, as defined in <xref target="expansion"/>, is dependent on
both the operator being used and the type or schema of the value
being substituted.
</t>
</section>
</section>
</section>
<section title="Expansion" anchor="expansion">
<t>
The process of URI Template expansion is to scan the template
string from beginning to end, copying literal characters as-is
and replacing each expression with the result of applying the
expression's operator to the value of each variable named in
the expression. Each variable's value MUST be formed prior to
template expansion.
</t>
<t>
If a template processor encounters an error outside of an expression,
such as a character sequence that does not match the &lt;URI-Template&gt;
grammar, then processing of the template SHOULD cease, the URI-reference
result SHOULD be undefined, and the location and type of error SHOULD be
indicated to the invoking application. If an error is encountered inside
an expression, such as an operator or value modifier that it does not
recognize or cannot support, then the expression SHOULD be copied to
the result unexpanded, processing of the remainder of the template
SHOULD continue, and the location and type of error SHOULD be indicated
to the invoking application. In this latter case, the result returned
will not be a valid URI reference; it will be an incompletely expanded
template string that is only intended for diagnostic use.
</t>
<section title='Literal Expansion' anchor="literal-expansion">
<t>
If the literal character is allowed anywhere in the URI
syntax (unreserved / reserved / pct-encoded ), then it is copied
directly to the result string. Otherwise, the pct-encoded equivalent
of the literal character is copied to the result string by
encoding the character in UTF-8 (a sequence of octets) and
then encoding each octet as a pct-encoded triplet.
</t>
</section>
<section title='Expression Expansion' anchor="expression-expansion">
<t>
Each expression is indicated by an opening brace ("{") character
and continues until the next closing brace ("}"). The expression
is expanded by determining the expression type and then following
that type's expansion process for each comma-separated varspec
in the expression. Level 1 templates are limited to the default
operator (simple string value expansion) and a single variable
per expression. Level 2 templates are limited to a single
varspec per expression.
</t>
<t>
The expression type is determined by looking at the first
character after the opening brace. If the character is an
operator, then remember the expression type associated with
that operator for later expansion decisions and skip to the
next character for the varspec list. If the first character
is not an operator, then the expression type is simple
expansion and the first character is the beginning of the
varspec list.
</t>
<section title='Undefined Variable Expansion' anchor="undef-expansion">
<t>
Regardless of the expression type, a variable that is undefined has
no value. A variable defined as a list or structure of component
values is considered undefined if the list contains zero members or
all of the structure's components are undefined.
</t>
<t>
If all of the variables in an expression are undefined, then the
expression's expansion is the empty string.
</t>
</section>
<section title='Simple String Expansion: {var}' anchor="op-simple">
<t>
Simple string expansion is the default expression type when no
operator is given. For each defined variable in the
variable-list, modify its value as indicated by the optional
modifiers (if any), encode the value as UTF-8, pct-encode any
octets that are not in the unreserved set, and then append
the encoded value to the result string. If more than one
value is appended, separate each value with a comma (",").
</t>
<figure><preamble>For example,</preamble><artwork>
foo := "fred"
"{foo}" -&gt; "fred"
"{foo,foo}" -&gt; "fred,fred"
"{bar,foo}" -&gt; "fred"
"{bar|wilma}" -&gt; "wilma"
</artwork></figure>
<t>
Level 1 and 2 templates are limited to single variable expressions
without modifiers or value structures.
Level 3 templates allow a list of variables.
Level 4 templates add compound variable types and value modifiers,
as follows:
</t>
<t>
For a variable defined as a single value string, the explode
modifier has no effect. The prefix modifier limits the
expansion to the first max-length characters of that single
value. If the value contains pct-encoded triplets,
multibyte UTF-8, or both, care must be taken to avoid
splitting the value in mid-character: count each Unicode
codepoint as one character.
</t>
<t>
For a variable defined as a list of values, when no value
modifier is present or the explode modifier is given, the
variable's string expansion consists of a concatenation
of the individual values with each value separated by a
comma (","). A prefix modifier has no effect.
</t>
<t>
A variable defined as an associative array is expanded as a list
of alternating key and value pairs, excluding any keys for which
the corresponding value is undefined, when no value modifier is
present. If the explode modifier is given, then the keys with
defined values are expanded as "key=value" pairs instead of
"key,value". A prefix modifier has no effect.
</t>
</section>
<section title='Reserved expansion: {+var}' anchor="op-plus">
<t>
Reserved expansion is identical to simple expansion except that the
substituted values may contain characters in the reserved set.
The reserved expansion operator "+" is defined for Level 2
templates (and above).
</t>
<figure><preamble>For example,</preamble><artwork>
foo := "That's right!"
"{foo}" -&gt; "That%27s%20right%21"
"{+foo}" -&gt; "That%27s%20right!"
base := "http://example.com/home/"
"{base}index" -&gt; "http%3A%2F%2Fexample.com%2Fhome%2Findex"
"{+base}index" -&gt; "http://example.com/home/index"
</artwork></figure>
<t>
The same expansion process is followed as in <xref target="op-simple"/>
except that, instead of "pct-encode any octets that are not in
the unreserved set", we pct-encode any octets that are not in
either the reserved or unreserved sets.
</t>
</section>
<section title="Label expansion with dot-prefix: {.var}" anchor="op-dot">
<t>
The dot (".") operator indicates that the expression type is label
expansion, which can be useful for describing URI spaces with
varying domain names or path selectors (e.g., filename extensions).
</t>
<figure><artwork>
For each variable in the variable-list that has a defined value:
1) modify its value as indicated by any optional modifiers;
2) encode the value as UTF-8;
3) pct-encode any octets that are not in the unreserved set;
4) append "." to the result string; and,
5) append the encoded value to the result string.
</artwork></figure>
<t>
Since "." is not in the reserved set, a value that contains
a "." has the effect of adding multiple labels.
</t>
<figure><preamble>For example,</preamble><artwork>
foo := "fred"
"{.foo}" -&gt; ".fred"
"{.foo,foo}" -&gt; ".fred.fred"
"{.bar,foo}" -&gt; ".fred"
"{.bar|wilma}" -&gt; ".wilma"
</artwork></figure>
<t>
Label expansion only applies to Level 3 and Level 4 templates.
Level 4 templates add compound variable types and value modifiers,
as follows:
</t>
<t>
For a variable defined as a single value string, the explode
modifier has no effect. The prefix modifier limits the
expansion to the first max-length characters of that single
value. If the value contains pct-encoded triplets,
multibyte UTF-8, or both, care must be taken to avoid
splitting the value in mid-character: count each Unicode
codepoint as one character.
</t>
<t>
For a variable defined as a list of values, when no value
modifier is present or the explode modifier is given, the
variable's string expansion consists of a concatenation
of the individual defined values with each value prepended
by a dot ("."). A prefix modifier has no effect.
</t>
<t>
When no value modifier is present, a variable defined as an
associative array is expanded by appending the (key, value)
pairs as alternating labels (i.e., ".key.value"), but
excluding any keys for which the corresponding value is
undefined. If the explode modifier is given, then the keys with
defined values are expanded as ".key=value" instead of
".key.value". A prefix modifier has no effect.
</t>
</section>
<section title='Path segment expansion: {/var}' anchor="op-slash">
<t>
The slash ("/") operator indicates that the expression type is
hierarchical path segment expansion, which can be useful for
describing URI path hierarchies.
</t>
<figure><artwork>
For each variable in the variable-list with a defined value:
1) modify its value as indicated by any optional modifiers;
2) encode the value as UTF-8;
3) pct-encode any octets that are not in the unreserved set;
4) append "/" to the result string; and,
5) append the encoded value to the result string.
</artwork></figure>
<t>
Note that the expansion process for path segment expansion is
identical to that of label expansion aside from the substitution
of "/" instead of ".".
</t>
<figure><preamble>For example,</preamble><artwork>
foo := "fred"
"{/foo}" -&gt; "/fred"
"{/foo,foo}" -&gt; "/fred/fred"
"{/bar,foo}" -&gt; "/fred"
"{/bar|wilma}" -&gt; "/wilma"
</artwork></figure>
<t>
Label expansion only applies to Level 3 and Level 4 templates.
Level 4 templates add compound variable types and value modifiers,
as follows:
</t>
<t>
For a variable defined as a single value string, the explode
modifier has no effect. The prefix modifier limits the
expansion to the first max-length characters of that single
value. If the value contains pct-encoded triplets,
multibyte UTF-8, or both, care must be taken to avoid
splitting the value in mid-character: count each Unicode
codepoint as one character.
</t>
<t>
For a variable defined as a list of values, when no value
modifier is present or the explode modifier is given, the
variable's string expansion consists of a concatenation
of the individual defined values with each value prepended
by a slash ("/"). A prefix modifier has no effect.
</t>
<t>
When no value modifier is present, a variable defined as an
associative array is expanded by appending the (key, value)
pairs as alternating segments (i.e., "/key/value"), but
excluding any keys for which the corresponding value is
undefined. If the explode modifier is given, then the keys with
defined values are expanded as "/key=value" instead of
"/key/value". A prefix modifier has no effect.
</t>
</section>
<section title='Path-style parameter expansion: {;var}' anchor="op-semicolon">
<t>
TBD.
</t>
</section>
<section title='Form-style query expansion: {?var}' anchor="op-question">
<t>
TBD.
</t>
</section>
</section>
</section>
<section title="Security Considerations">
<t>
A URI Template does not contain active or executable content. Other
security considerations are the same as those for URIs, as described
in section 7 of <xref target="RFC3986"/>.
</t>
</section>
<section title="IANA Considerations" anchor="iana">
<t>
No IANA actions are required by this document.
</t>
</section>
<section title="Acknowledgments" anchor="ack">
<t>
The following people made significant contributions to this
specification: Mike Burrows, Michaeljohn Clement, DeWitt Clinton,
John Cowan, James H. Manger, Marc Portier, and James Snell.
</t>
</section>
</middle>
<back>
<references title='Normative References'>
&ASCII;
&UNIV4;
&UTR15;
&rfc2119;
&rfc2978;
&rfc3986;
&rfc3987;
&rfc3629;
&rfc5234;
</references>
<section title="Example URI Template Parser" anchor="parser">
<t>
Parsing a valid URI Template expression does not require
building a parser from the given ABNF. Instead, the set of allowed
characters in each part of URI Template expression has been
chosen to avoid complex parsing, and breaking an expression into
its component parts can be achieved by a series of splits
of the character string.
</t>
<t>
TBD.
</t>
</section>
<section title="Revision History (to be removed by RFC Editor)" anchor="revisions">
<t>06 - Removed template defaults for undefined variables.
Added paragraph on limitations of variable matching templates.
Replaced "offset" with "max-length" in definition of prefix.
Noted that IRI literals may need to be pct-encoded for the URI.
Removed NFKC requirement -- state that NFC should be applied only
for user-supplied data-entry values.
Removed pct-decode normalization process for entire template, since
that is a lot more than necessary -- just normalize the variable
names before mapping.
Fixed a lot of last-minute paste-o's in the examples.
Fixed Level 4 examples for unexploded keys to include the keys=
in the expansion.
</t>
<t>05 - Introduced levels to differentiate between legacy, partial,
and full implementations of URI Templates.
Changed the default indicator to pipe ("|") to allow the defaults
to contain the equals character and thus remove the need for complex
defaulting for the different variable types.
Removed suffix, remainder, and labelled value expansion because
there didn't seem much interest in them.
Clarified that templates and values are processed as sequences of
Unicode codepoints rather than prematurely encoded as UTF-8, since
that is easier to explain and more consistent with common language
routines for processing Unicode strings.
</t>
<t>04 - Changed the operator syntax to a single character that is
analogous to its reserved role within the URI generic syntax,
resulting in templates that are far more readable for the common
cases. Added value modifiers for prefix and suffix expansion.
Added explode modifiers to allow expansion of complex variables
and lists according to (external) variable types or schema.
Replaced use of "expansion" with "expression", since expansion
is traditionally used to refer to the result after expanding
a macro (not the macro itself). Made applicable to any hypertext
reference string, such that the process for template expansion
also includes transforming the surrounding string into a proper
URI-reference rather than assuming it is already in absolute URI form.
Rewrote the text accordingly.</t>
<t>03 - Added more examples. Introduced error conditions and defined
their handling. Changed listjoin to list. Changed -append to -suffix,
and allowed -prefix and -suffix to accept list variables. Clarified
the handling of unicode.</t>
<t>02 - Added operators and came up with coherent
percent-encoding and reserved character story.
Added large examples section which is extracted
and tested against the implementation.
</t>
<t>01
</t>
<t>00 - Initial Revision.
</t>
</section>
</back>
</rfc>
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