Network Working Group J. Gregorio, Ed.
Internet-Draft Google
Intended status: Standards Track M. Hadley, Ed.
Expires: September 27, 2008 Sun Microsystems
M. Nottingham, Ed.
D. Orchard
BEA Systems, Inc.
Mar 26, 2008
URI Template
draft-gregorio-uritemplate-03
Status of this Memo
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Abstract
A URI Template is a compact sequence of characters used for the
construction of URIs. This specification defines the URI Template
syntax and the process for expanding a URI Template into a URI, along
with guidelines and security considerations for the use of URI
Templates on the Internet. The URI Template syntax allows for the
construction of strings that are a superset of URIs, allowing an
implementation to process any URI Template without knowing the
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scheme-specific requirements of every possible resulting URI.
Editorial Note
To provide feedback on this Internet-Draft, join the W3C URI mailing
list (http://lists.w3.org/Archives/Public/uri/) [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Design Considerations . . . . . . . . . . . . . . . . . . 4
1.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Notational Conventions . . . . . . . . . . . . . . . . . . 4
2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. URI Template . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Variables . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Template Expansions . . . . . . . . . . . . . . . . . . . 6
4.3. Error Handling . . . . . . . . . . . . . . . . . . . . . . 6
4.4. URI Template Substitution . . . . . . . . . . . . . . . . 7
4.4.1. ('var') substitution . . . . . . . . . . . . . . . . . 8
4.4.2. The 'opt' operator . . . . . . . . . . . . . . . . . . 8
4.4.3. The 'neg' operator . . . . . . . . . . . . . . . . . . 8
4.4.4. The 'prefix' operator . . . . . . . . . . . . . . . . 9
4.4.5. The 'suffix' operator . . . . . . . . . . . . . . . . 9
4.4.6. The 'join' operator . . . . . . . . . . . . . . . . . 9
4.4.7. The 'list' operator . . . . . . . . . . . . . . . . . 10
4.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Appendix A - Parsing URI Template Expansions . . . . . . . . . 13
8. Normative References . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 14
Appendix B. Revision History . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
A URI Template provides a simple and extensible format for URI
construction. A URI Template is a string that contains embedded
expansions, text marked off in matching braces ('{', '}'), that
denotes a part of the string that is to be substituted by a template
processor to produce a URI. A URI Template is transformed into a URI
by substituting the expansions with their calculated value.
Several specifications have defined URI Templates with varying levels
of formality, such as WSDL, WADL and OpenSearch. This specification
is derived from these concepts, giving a rigorous definition to such
templates.
This specification uses the terms "character" and "coded character
set" in accordance with the definitions provided in [RFC2978], and
"character encoding" in place of what [RFC2978] refers to as a
"charset".
1.1. Overview
A URI Template allows a structural description of URIs while allowing
a consumer of the template to construct a final URI by providing the
values of the expansion variables. For example, given the following
URI Template:
http://www.example.com/users/{userid}
And the following variable value
userid := "fred"
The expansion of the URI Template is:
http://www.example.com/users/fred
Here is an example that constructs a query from multiple variables:
http://www.example.com/?{-join|&|query,number}
And the following variables
query := "mycelium"
number := 100
The expansion of the URI Template is:
http://www.example.com/?query=mycelium&number=100
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The template expansion describes in a machine readable manner how the
URI is to be constructed.
http://www.example.com/?{-join|&|query,number}
\____________________/
|
|
Join 'var=value' for each variable
in ['query', 'number'] with '&'.
1.2. Design Considerations
The URI Template syntax has been designed to carefully balance the
need for a powerful substitution mechanism with ease of
implementation and security. The syntax is designed to be easy to
parse while at the same time providing enough flexibility to express
many common templating scenarios.
Another consideration was to keep the syntax and processing in-line
with the pre-existing templating schemes present in OpenSearch, WSDL
and WADL.
The final design consideration was control over the placement of
reserved characters in the URI generated from a URI Template. The
reserved characters in a URI Template can only appear in the non-
expansion text, or in the argument to an operator, both locations are
dictated by the URI Template author. Given the percent-encoding
rules for variable values this means that the source of all
structure, i.e reserved characters, in a URI generated from a URI
Template is decided by the URI Template author.
1.3. Applicability
While URI Templates use a notation that is similar to some URI path
matching notations in web frameworks, URI Templates were not designed
for that use case, nor are they appropriate for that purpose. URI
Templates are not URIs, they do not identify an abstract or physical
resource, they are not to be treated like URIs, nor should not be
used in places where a URI would be expected.
1.4. Notational Conventions
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234], including the following core ABNF syntax rules
defined by that specification: ALPHA (letters) and DIGIT (decimal
digits). See [RFC3986] for the definitions of the URI-reference,
percent-encoded, reserved, and unreserved rules.
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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 [RFC2119].
2. Characters
A URI Template is a sequence of characters, and has the same issues
as URIs with regard to codepoints and character sets. That is, URI
Template characters are frequently encoded as octets for transport or
presentation. This specification does not mandate any particular
character encoding for mapping between URI characters and the octets
used to store or transmit those characters. When a URI appears in a
protocol element, the character encoding is defined by that protocol;
without such a definition, a URI is assumed to be in the same
character encoding as the surrounding text.
The ABNF notation defines its terminal values to be non-negative
integers (codepoints) based on the US-ASCII coded character set
[ASCII]. Because a URI is a sequence of characters, we must invert
that relation in order to understand the URI syntax. Therefore, the
integer values used by the ABNF must be mapped back to their
corresponding characters via US-ASCII in order to complete the syntax
rules.
3. Terminology
o template processor - A program or library that converts a URI
Template into a URI.
o template expansion - The text between '{' and '}', including the
enclosing brackets.
4. URI Template
A URI Template is a sequence of characters that contains any number
of embedded template expansions, see Section 4.2. Each expansion
references one or more variables whose values are used when
determining the substition value for an expansion. A URI Template
becomes a URI when all the template expansions are substituted with
their values (see Section 4.4). The generated URI will be a URI-
reference, i.e. either an absolute URI or a relative reference.
4.1. Variables
Every variable is either a Unicode string or a list of Unicode
strings.
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A template expansion MAY reference variables that are unknown to the
template processor. Those variables are 'undefined' and template
expansion takes into consideration 'undefined' variables.
Conversely, every variable that he template processor knows about is
considered 'defined'.
A variable that contains a string of length zero MUST NOT be
considered 'undefined' by the template processor. A list variable
that contains no members, that is of zero length, MUST NOT be
considered 'undefined' by the template processor.
Beyond the scope of this specification is the allowable programming
constructs that can be used for a list variable. For example, a
Python implementation might allow only built-in list types, or it may
allow any iterable to be used as the source for a list variable.
Some variables may be supplied with default values. The default
value must comde from ( unreserved / pct-encoded ). Note that there
is no notation for supplying default values to list variables.
A variable may appear in more than one expansion in a URI Template.
The value used for that variable MUST remain the same for every
template expansion when converting a URI Template into a URI.
4.2. Template Expansions
Template expansions are the parameterized components of a URI
Template. A template expansion MUST match the 'expansion' rule.
op = 1*ALPHA
arg = *(reserved / unreserved / pct-encoded)
var = varname [ "=" vardefault ]
vars = var [ *("," var) ]
varname = (ALPHA / DIGIT)*(ALPHA / DIGIT / "." / "_" / "-" )
vardefault = *(unreserved / pct-encoded)
operator = "-" op "|" arg "|" vars
expansion = "{" ( var / operator ) "}"
4.3. Error Handling
During template substitution error conditions may arise. The exact
circumstances for those errors are described in Section 4.4. When an
error occurs the template processor MUST NOT return a URI. It is
language specific and beyond the scope of this document how the
template processor signals that an error has occured and that a URI
will not be generated from the template.
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4.4. URI Template Substitution
Template substitution is the process of turning a URI Template into a
URI given definitions for the variables used in the template.
Substitution replaces each expansion with its calculated value. A
template processor take two inputs, a URI Template and a set of
variables, and returns a URI-reference.
Before substitution the template processor MUST convert every
variable value into a sequence of characters in ( unreserved / pct-
encoded ). The template processor does that using the following
algorithm: The template processor normalizes the string using NFKC,
converts it to UTF-8 [RFC3629], and then every octet of the UTF-8
string that falls outside of ( unreserved ) MUST be percent-encoded,
as per [RFC3986], section 2.1. For variables that are lists, the
above algorithm is applied to each value in the list.
The Unicode Standard [UNIV4] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences, in particular Normalization Form KC (NFKC,
Compatibility Decomposition, followed by Canonical Composition).
Since different Normalized Forms unicode strings will have different
UTF-8 represenations the only way to guarantee that template
processors will produce the same URI is to require a common
Normalized Form.
Requiring that all characters outside of ( unreserved ) be percent
encoded means that the only characters outside of ( unreserved ) that
will appear in the generated URI-reference will come from outside the
template expansions in the URI Template or from the argument of a
template expansion. This means that the designer of the URI Template
determines the placement of reserved characters in the resulting URI,
and thus the structure of the resulting generated URI-reference.
If the expansion is an operator then the substitution value is
determined by the given operator. Each operator works only on the
variables that are defined within their expansion.
The result of substitution MUST match the URI-reference rule and
SHOULD also match any known rules for the scheme of the resulting
URI.
If a template processor encounters an operator that it does not
understand then it MUST fail and MUST NOT produce a URI from the URI
Template. The list of operators that a template processor knows is
not constrained by this specification, that is, later specifications
may add new operators.
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Every expansion consists of either a variable ('var') or an operator
expression ('operator'), and the rules for how to expand each of
these is given below. For every expansion a template MUST have at
least one variable name in the template expansion. It is an error if
no variables are supplied. All of the variables supplied to a
template expansion MAY be undefined and the expansion rules below
specify how to process the template expansion in that situation.
4.4.1. ('var') substitution
In a variable ('var') expansion, if the variable is defined then
substitute the value of the variable, otherwise substitute the
default value. If no default value is given then substitute with the
empty string.
Example:
foo := "fred"
"{foo}" -> "fred"
"{bar=wilma}" -> "wilma"
"{baz}" -> ""
4.4.2. The 'opt' operator
If each variable is undefined or an empty list then substitute the
empty string, otherwise substitute the value of 'arg'.
Example:
foo := "fred"
"{-opt|fred@example.org|foo}" -> "fred@example.org"
"{-opt|fred@example.org|bar}" -> ""
4.4.3. The 'neg' operator
If each variable is undefined or an empty list then substitute the
value of arg, otherwise substitute the empty string.
Example:
foo := "fred"
"{-neg|fred@example.org|foo}" -> ""
"{-neg|fred@example.org|bar}" -> "fred@example.org"
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4.4.4. The 'prefix' operator
The prefix operator MUST only have one variable in its expansion.
More than one variable is an error condition. If the variable is
undefined or an empty list then substitute the empty string. If the
variable is a defined non-list then substitute the value of arg
preceeded by the value of the variable. If the variable is a defined
list then substitute the concatenation of every list value preceeded
by the arg.
Example:
foo := "fred"
bar := ["fee", "fi", "fo", "fum"]
baz := []
"{-prefix|/|foo}" -> "/fred"
"{-prefix|/|bar}" -> "/fee/fi/fo/fum"
"{-prefix|/|baz}" -> ""
"{-prefix|/|qux}" -> ""
4.4.5. The 'suffix' operator
The prefix operator MUST only have one variable in its expansion.
More than one variable is an error condition. If the variable is
undefined or an empty list then substitute the empty string. If the
variable is a defined non-list then substitute the value of arg
followed by the value of the variable. If the variable is a defined
list then substitute the concatenation of every list value followed
by the arg.
Example:
foo := "fred"
bar := ["fee", "fi", "fo", "fum"]
baz := []
"{-suffix|/|foo}" -> "fred/"
"{-suffix|/|bar}" -> "fee/fi/fo/fum/"
"{-suffix|/|baz}" -> ""
"{-suffix|/|qux}" -> ""
4.4.6. The 'join' operator
Supplying a list variable to the join operator is an error. For each
variable that is defined and non-empty create a keyvalue string that
is the concatenation of the variable name, "=", and the variable
value. Concatenate more than one keyvalue string with intervening
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values of arg to create the substitution value. The order of
variables MUST be preserved during substitution.
Example:
foo := "fred"
bar := "barney"
baz := ""
"{-join|&|foo,bar,baz,qux}" -> "foo=fred&bar=barney&baz="
"{-join|&|bar}" -> "bar=barney"
"{-join|&|qux}" -> ""
4.4.7. The 'list' operator
The listjoin operator MUST have only one variable in its expansion
and that variable must be a list. More than one variable is an
error. If the list is non-empty then substitute the concatenation of
all the list members with intervening values of arg. If the list is
empty or the variable is undefined them substitute the empty string.
Example:
foo := ["fred", "barney", "wilma"]
bar := ["a", "", "c"]
baz := ["betty"]
qux := []
"{-list|/|foo}" -> "fred/barney/wilma"
"{-list|/|bar}" -> "a//c"
"{-list|/|baz}" -> "betty"
"{-list|/|qux}" -> ""
"{-list|/|corge}" -> ""
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4.5. Examples
Given the following template variable names and values:
+---------+----------------------------------+
| Name | Value |
+---------+----------------------------------+
| foo | \u03d3 |
| bar | fred |
| baz | 10,20,30 |
| qux | ["10","20","30"] |
| corge | [] |
| grault | |
| garply | a/b/c |
| waldo | ben & jerrys |
| fred | ["fred", "", "wilma"] |
| plugh | ["\u017F\u0307", "\u0073\u0307"] |
| 1-a_b.c | 200 |
+---------+----------------------------------+
Table 1
The variable 'foo' is the unicode character GREEK UPSILON WITH ACUTE
AND HOOK SYMBOL. This character was chosen because it is one of only
three characters that has a different normal form for each of the
four normalization forms (NFC, NFD, NFKC, NFKD). The name 'xyzzy'
has not been defined, the value of 'grault' is the empty string. The
variables qux, corge, fred, and plugh are lists.
The following URI Templates will be expanded as shown:
----
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http://example.org/?q={bar}
http://example.org/?q=fred
/{xyzzy}
/
http://example.org/?{-join|&|foo,bar,xyzzy,baz}
http://example.org/?foo=%CE%8E&bar=fred&baz=10%2C20%2C30
http://example.org/?d={-list|,|qux}
http://example.org/?d=10,20,30
http://example.org/?d={-list|&d=|qux}
http://example.org/?d=10&d=20&d=30
http://example.org/{bar}{bar}/{garply}
http://example.org/fredfred/a%2Fb%2Fc
http://example.org/{bar}{-prefix|/|fred}
http://example.org/fred/fred//wilma
{-neg|:|corge}{-suffix|:|plugh}
:%E1%B9%A1:%E1%B9%A1:
../{waldo}/
../ben%20%26%20jerrys/
telnet:192.0.2.16{-opt|:80|grault}
telnet:192.0.2.16:80
:{1-a_b.c}:
:200:
----
5. Security Considerations
A URI Template does not contain active or executable content. Other
security considerations are the same as those for URIs, see section 7
of RFC3986.
6. IANA Considerations
In common with RFC3986, URI scheme names form a registered namespace
that is managed by IANA according to the procedures defined in
[RFC4395]. No IANA actions are required by this document.
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7. Appendix A - Parsing URI Template Expansions
Parsing a valid URI Template expansion does not require building a
parser from the given ABNF. Instead, the set of allowed characters
in each part of URI Template expansion has been chosen to avoid
complex parsing, and breaking an expansion into its component parts
can be achieved by a series of splits of the character string.
Here is example Python code that parses a URI Template expansion and
returns the operator, argument, and variables as a tuple. The
variables are returned as a dictionary of variable names mapped to
their default values. If no default is given then the name maps to
None.
def parse_expansion(expansion):
if "|" in expansion:
(op, arg, vars_) = expansion.split("|")
op = op[1:]
else:
(op, arg, vars_) = (None, None, expansion)
vars_ = vars_.split(",")
variables = {}
for var in vars_:
if "=" in var:
(varname, vardefault) = var.split("=")
else:
(varname, vardefault) = (var, None)
variables[varname] = vardefault
return (op, arg, variables)
And here is an example of the parse_expansion() function being used.
>>> parse_expansion("-join|&|a,b,c=1")
('join', '&', {'a': None, 'c': '1', 'b': None})
>>> parse_expansion("c=1")
(None, None, {'c': '1'})
8. Normative References
[ASCII] American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 115,
RFC 4395, February 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[UNIV4] The Unicode Consortium, "The Unicode Standard, Version
4.0.1, defined by: The Unicode Standard, Version 4.0
(Reading, MA, Addison-Wesley, 2003. ISBN 0-321-18578-1),
as amended by Unicode 4.0.1
(http://www.unicode.org/versions/Unicode4.0.1/)",
March 2004.
[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex # 15, April 2003.
[1] <http://lists.w3.org/Archives/Public/uri/>
Appendix A. Contributors
The following people made significant contributions to this
specification: Michaeljohn Clement, DeWitt Clinton, John Cowan, James
H. Manger, and James Snell.
Appendix B. Revision History
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.
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.
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01
00 - Initial Revision.
Authors' Addresses
Joe Gregorio (editor)
Google
Email: joe@bitworking.org
URI: http://bitworking.org/
Marc Hadley (editor)
Sun Microsystems
Email: Marc.Hadley@sun.com
URI: http://sun.com/
Mark Nottingham (editor)
Email: mnot@pobox.com
URI: http://mnot.net/
David Orchard
BEA Systems, Inc.
Email: dorchard@bea.com
URI: http://bea.com/
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Full Copyright Statement
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