Network Working Group M. Nottingham
Internet-Draft December 12, 2007
Intended status: Informational
Expires: June 14, 2008
FIQL: The Feed Item Query Language
draft-nottingham-atompub-fiql-00
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The Feed Item Query Language is a simple but flexible, URI-friendly
syntax for expressing filters across the entries in a Web feed. It
also specifies a mechanism to allow feeds to indicate what types of
queries are supported.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. FIQL Queries . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. FIQL Expressions . . . . . . . . . . . . . . . . . . . . . 4
3.2. FIQL Constraints . . . . . . . . . . . . . . . . . . . . . 4
3.2.1. Selectors . . . . . . . . . . . . . . . . . . . . . . 5
3.2.2. FIQL Comparison Types . . . . . . . . . . . . . . . . 5
4. Feed Queries and HTTP . . . . . . . . . . . . . . . . . . . . 9
5. Feed Extensions for Queries . . . . . . . . . . . . . . . . . 10
5.1. The fq:interface element . . . . . . . . . . . . . . . . . 10
5.2. The fq:index element . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Normative References . . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 13
Appendix B. Default Element Comparison Types . . . . . . . . . . 14
B.1. Atom 1.0 . . . . . . . . . . . . . . . . . . . . . . . . . 14
B.2. RSS 2.0 . . . . . . . . . . . . . . . . . . . . . . . . . 14
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
The Feed Item Query Language (FIQL, pronounced "fickle") is a simple
but flexible, URI-friendly syntax for expressing filters across the
entries in a syndicated feed. For example,
title==foo*;(updated=lt=-P1D,title==*bar)
will return all entries in a feed that meet the following criteria;
o have a title beginning with "foo", AND
o have been updated in the last day OR have a title ending with
"bar".
This specification defines an extensible syntax for FIQL queries (in
Section 3), explains their use in HTTP (Section 4), and defines feed
extensions for discovering and describing query interfaces
(Section 5).
2. Notational Conventions
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 BCP 14 [RFC2119], as
scoped to those conformance targets.
This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
to uniquely identify XML element names. It uses the following
namespace prefix for the indicated namespace URI;
"fq": "http://purl.org/syndication/query"
This specification uses terms from the XML Infoset
[W3C.REC-xml-infoset-20040204]. However, this specification uses a
shorthand; the phrase "Information Item" is omitted when naming
Element Information Items. Therefore, when this specification uses
the term "element," it is referring to an Element Information Item in
Infoset terms.
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC4234], including the DIGIT rule from the core.
Additionally, the unreserved, gen-delims, and pct-encoded rules are
included from [RFC3986], and QName from [W3C.REC-xml-names-19990114].
The complete syntax is collected in Appendix C.
Although they refer to Atom [RFC4287] normatively, the mechanisms
described herein can be used with similar syndication formats, such
as the various flavours of RSS.
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3. FIQL Queries
A FIQL query's input is a string of Unicode characters in the form of
an expression.
The output of an FIQL query is a feed that has the same head section
as the input feed, but that only contains entries that match the
given expression (that is, those that yield True).
3.1. FIQL Expressions
An FIQL expression is composed of one or more constraints, related to
each other with Boolean operators.
FIQL expressions yield Boolean values: True or False.
expression = [ "(" ]
( constraint / expression )
[ operator ( constraint / expression ) ]
[ ")" ]
operator = ";" / ","
o ";" is the Boolean AND operator; it yields True for a particular
entry if both operands evaluate to True, otherwise False.
o "," is the Boolean OR operator; it yields True if either operand
evaluates to True, otherwise False.
By default, the AND operator takes precedence (i.e., it is evaluated
before any OR operators are). However, a parenthesised expression
can be used to change precedence, yielding whatever the contained
expression yields.
3.2. FIQL Constraints
A FIQL constraint is composed of a selector, which identifies a
portion of an entry's content, and an optional comparison/argument
pair, which refines the constraint. When processed, a constraint
yields a Boolean value.
constraint = selector [ comparison argument ]
selector = 1*( unreserved / pct-encoded )
comparison = ( ( "=" 1*ALPHA ) / fiql-delim ) "="
argument = 1*arg-char
arg-char = unreserved / pct-encoded / fiql-delim / "="
fiql-delim = "!" / "$" / "'" / "*" / "+"
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3.2.1. Selectors
A selector identifies the portion of an entry that a constraint
applies to. When evaluated against an entry in the feed, it returns
one or more node-sets [W3C.REC-xpath-19991116] from the entry,
whether they be elements, attributes or textual content.
By default, a selector is treated as an XML QName
[W3C.REC-xml-names-19990114] which selects any and all child elements
of the entry element which share the same syntax (i.e., the same
prefix and localname; the namespace URI is not considered), along
with their descendant content if any.
For example, given the entry
<entry xmlns="http://www.w3.org/2005/Atom"
xmlns:ex="http://other.example.net/">
<title>Test</title>
<ex:foo>
<bar/>
</ex:foo>
<ex:foo>
<baz/>
</ex:foo>
</entry>
o "title" selects the text "Test".
o "ex:foo" selects both the "bar" and "baz" elements.
This behaviour can be overridden by by explicitly defining a selector
in the feed; see Section 5.
3.2.2. FIQL Comparison Types
Constraints with comparison operators are evaluated according to the
comparison type associated with the selector. This can be determined
by (in order of precedence):
1. An explicit type associated with it by the feed (see Section 5),
or
2. A default type defined by the specification of the selected
entry-level metadata element, or
3. A default type for the entry-level metadata element, as specified
in Appendix B, or
4. Falling back to the simple textual type (see Section 3.2.2.1).
A constraint with no comparison operator will yield True if the
selector matches any node.
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This specification defines a number of comparison types below, and
allows extension types to be identified with URIs.
3.2.2.1. Simple Textual Comparisons
Simple text comparisons allow for case insensitive, non-language-
specific searching of the textual content of selected nodes.
text-arg = [ "*" ] 1*arg-char [ "*" ]
Two comparison operators are applicable to simple text comparisons;
o "==" yields True if the string value (as per XPath) of any
selected node matches the argument; otherwise False.
o "!=" yields True if the string value of every selected node does
not match the argument; otherwise False.
A match occurs when the text in question is character-by-character
equivalent with the argument, after observing the following rules, in
order:
1. Both the argument and the selected node's content MUST have their
encodings (percent-encoding and entity encoding, respectively)
removed, to produce Unicode strings.
2. Leading and trailing white space MUST be stripped from the
selected node's string. White space within the selected node's
string MUST be collapsed to single space characters.
3. Both strings MUST have locale-independent case folding applied,
as specified in Section 5.18 of [unicode].
4. Both strings MUST have Normalization Form C [unicode-norm]
applied.
5. If the argument string begins or ends with an asterisk character
("*"), it acts as a wild card, matching any characters preceding
or following (respectively) that position.
For example, given the entry:
<entry>
<title>Hello world</title>
<description>Just starting.</description>
<author>
<name>Mark Nottingham </name>
</author>
<content type="xhtml">
This is <html:em>just</html:em> the start.
</content>
<!-- other stuff -->
</entry>
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"title==Hello%20World" yields True
"title!=Hello" yields True
"title==Hello*" yields True
"title==hello*" yields True
"author==Mark*" yields True
"author==*Nottingham yields True
"description==*start*" yields True
"description==*Just*" yields True
"description==Just%20starting." yields True
"content==*just%20the%20start*" yields True
"description=="*just" yields False
This comparison type can be identied with the URI
"http://purl.org/syndication/query/simple-text".
3.2.2.2. Date Comparisons
Date comparisons allow both relative and absolute comparison of date-
related values in the string-value of selected nodes.
date-arg = dateTime / duration ; as defined in XML Schema Datatypes
Four operators are relevant to date comparisons;
o "==" yields True if the point in time specified in the argument
matches that indicated by the string-value of the selected node;
otherwise False.
o "!=" yields True if the point in time specified in the argument
does not match that indicated by the string-value of the selected
node; otherwise False.
o "=lt=" yields True if the point in time specified in the argument
follows that indicated by the string-value of the selected node;
otherwise False.
o "=le=" yields True if the point in time specified in the argument
follows that indicated by the string-value of the selected node,
or is equal to it; otherwise False.
o "=gt=" yields True if the point in time specified in the argument
precedes that indicated by the string-value of the selected node;
otherwise False.
o "=ge=" yields True if the point in time specified in the argument
precedes that indicated by the string-value of the selected node,
or is equal to it; otherwise False.
A point in time can be specified in two ways;
o Absolutely, expressed as an XML Schema dateTime
[W3C.REC-xmlschema-2-20010502].
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o Relatively, expressed as an XML Schema duration
[W3C.REC-xmlschema-2-20010502]. By default, such arguments are
relative to the time that the query is processed.
White space in the string value of the selected node MUST be ignored.
For example, given an entry
<entry>
<title>Hello World</title>
<updated>2003-12-13T18:30:02Z</updated>
<!-- other stuff -->
</entry>
"updated==2003-12-13T18:30:02Z" yields True
"updated=gt=2003-12-13T00:00:00Z" yields True
"updated=lt=2005-01-01T00:00:00Z yields True
"updated=gt=-P1D12H" yields False
"updated=gt=-P5Y" yields True
(assuming processing on July 1st, 2006)
This specification does not define the appropriate behaviour if a
selector with a date type returns multiple nodes.
This comparison type can be identied with the URI
"http://purl.org/syndication/query/date".
3.2.2.3. Numeric Comparisons
Numeric comparison allows filtering of numeric values.
number-arg = [ "+" / "-" ] 1*DIGIT [ "." 1*DIGIT ]
Four operators are relevant to numeric comparisons;
o "==" yields True if the string-value of the selected node is
numerically equal to the argument; otherwise False.
o "!=" yields True if the string-value of the selected node is not
numerically equal to the argument; otherwise False.
o "=lt=" yields True if the string-value of the selected node
evaluates as numerically less than the argument; otherwise, False.
o "=le=" yields True if the string-value of the selected node
evaluates as numerically less than the argument, or as equal to
it; otherwise, False.
o "=gt=" yields True if the string-value of the selected node
evaluates as numerically greater than the argument; otherwise,
False.
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o "=ge=" yields True if the string-value of the selected node
evaluates as numerically greater than the argument, or as equal to
it; otherwise, False.
White space in the string-value of selected node MUST be ignored.
For example, given an entry
<entry>
<title>Hello World</title>
<x:foo>123</x:foo>
<x:bar> 456</x:foo>
"x:foo==123" yields True
"x:foo==123.00" yields True
"x:foo!=123.1" yields True
"x:foo=lt=200" yields True
"x:bar==456" yields True
"x:foo=gt=500" yields False
This specification does not define the appropriate behaviour if a
selector with a numeric type returns multiple nodes.
This comparison type can be identied with the URI
"http://purl.org/syndication/query/numeric".
4. Feed Queries and HTTP
Although FIQL can be used in many contexts, it is optimised and
intended for use in the query component [RFC3986] of an HTTP
[RFC2616] URI. E.g.,
http://example.com/feed.atom?query=title==*new*,author==bob*
http://example.org/feed.rss?title==*great*;ex:rating=gt=4
Note that a FIQL query can be used as the entire query component, or
as a sub-component. In the latter case, it is important to account
for any encoding conventions to be used with the sub-component; for
example, if queries are generated from HTML forms, the sub-component
will not be a bare FIQL query, but instead an encoded one.
The output of a HTTP query SHOULD be returned in the HTTP response
body, with an appropriate media type. HTTP resources SHOULD return a
400 Bad Request status code if an FIQL expression includes an
unsupported or unknown selector.
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5. Feed Extensions for Queries
Although FIQL is suitable for use with unmodified Web feeds, it is
often desirable to override default comparison types, and to
advertise what item elements are available for queries.
This can be accomplished using a top-level feed metadata extension,
fq:interface. For example,
<feed xmlns="http://www.w3.org/2005/Atom"
xmlns:ex="http://other.example.net/"
xmlns:fq="http://purl.org/syndication/query">
<title>example</title>
<link href="http://www.example.org/"/>
<id>http://example.org/</id>
<updated>2006-09-12T12:28:02Z</updated>
<fq:interface
xmlns:fiql="http://purl.org/syndication/query"
template="/feed-search?{fiql-exp}">
<fq:index name="title"/>
<fq:index name="ex:foo"
type="http://purl.org/syndication/query/date"/>
<fq:index name="ex:bar"
type="http://purl.org/syndication/query/numeric"/>
</fq:interface>
<entry>
<title>test entry</title>
<description>something...</description>
<id>http://example.org/1124</id>
<ex:foo>2003-12-13T18:30:02Z</ex:foo>
<ex:bar>15.4</ex:bar>
</entry>
</feed>
Here, the fq:interface element indicates that queries can be made at
the URI template "http://www.example.org/feed-search?{fiql-exp}",
where {fiql-exp} is replaced with the desired FIQL expression.
Expressions will support the selectors "title", "ex:foo" and "ex:
bar", and the latter two will be compared as a date and number,
respectively.
5.1. The fq:interface element
The fq:interface element describes an interface that processes
queries for the feed it occurs in. One or more fq:interface elements
MAY occur in a feed's head section.
It MUST have a "template" attribute, whose content is a URI template,
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as defined in [I-D.gregorio-uritemplate]. This template SHOULD have
exactly one variable, "fiql-exp", that indicates where a FIQL
expression should occur. If it does not have a "fiql-exp" variable,
it MUST have some other variable that indicates how to submit queries
(e.g., using another query language).
For example, given a feed with the fq:interface declaration:
<fq:interface template="http://www.example.com/search?{fiql-exp}"/>
someone desiring to search for all entries with a title would use
this URI:
http://www.example.com/search?title
fq:interface MAY have any number of fq:index child elements.
Additional extension elements and attributes MAY also occur.
5.2. The fq:index element
The fq:index element indicates a selector that is available for use
in queries. Optionally, it can override its default type, and refine
it with an XPath expression.
It MUST have a "name" attribute, containing a selector (see
Section 3.2) to be used to reference the indexed element.
It MAY have a "path" attribute, containing an XPath
[W3C.REC-xpath-19991116] expression that yields the selected node(s)
when evaluated with an entry's element as the context node.
It MAY have a "type" attribute, containing a URI indicating the
comparison type that will be applied to that selector.
For example,
<fq:interface template="http://www.example.com/search?{fiql-exp}">
<fq:index name="ex:foo"
type="http://purl.org/syndication/query/text"/>
<fq:index name="foo-case-insensitive" path="ex:foo"
type="http://example.org/comparison-types/case-insensitive"/>
<fq:interface>
Here, the ex:foo element can compared using the simple textual
algorithm described in Section 3.2.2.1 with the "ex:foo" selector, or
using a (here, fictional) case-insensitive extension selector "foo-
case-insensitive".
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Or,
<fq:interface template="http://www.example.com/search;{fiql-exp}">
<fq:index name="foo-num" path="ex:foo/ex:bar/@num"
type="http://purl.org/syndication/query/numeric"/>
<fq:interface>
Here, queries using the selector "foo-num" will be evaluated against
the content(s) of the "num" attribute of the "ex:bar" child element
of "ex:foo".
By default, fq:index is an empty element; however, comparison types
MAY define extension elements and attributes.
6. Security Considerations
Servers processing queries should be aware of the potential resource
and security issues of allowing arbitrarily long and complex queries.
Nothing in this specification requires queries to be successfully
processed; e.g., a HTTP response may respond with a 403 Forbidden
status code if they believe a request to be a security risk.
7. IANA Considerations
This memo has no registration requirements.
8. Normative References
[I-D.gregorio-uritemplate] Gregorio, J., "URI Template",
draft-gregorio-uritemplate-00 (work
in progress), October 2006.
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J.,
Frystyk, H., Masinter, L., Leach, P.,
and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1",
RFC 2616, June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L.
Masinter, "Uniform Resource
Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.
[RFC4234] Crocker, D. and P. Overell,
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"Augmented BNF for Syntax
Specifications: ABNF", RFC 4234,
October 2005.
[RFC4287] Nottingham, M. and R. Sayre, "The
Atom Syndication Format", RFC 4287,
December 2005.
[W3C.REC-xml-infoset-20040204] Cowan, J. and R. Tobin, "XML
Information Set (Second Edition)",
W3C REC REC-xml-infoset-20040204,
February 2004.
[W3C.REC-xml-names-19990114] Bray, T., Hollander, D., and A.
Layman, "Namespaces in XML", W3C
REC REC-xml-names-19990114,
January 1999.
[W3C.REC-xmlschema-2-20010502] Biron, P. and A. Malhotra, "XML
Schema Part 2: Datatypes", W3C
REC REC-xmlschema-2-20010502,
May 2001.
[W3C.REC-xpath-19991116] Clark, J. and S. DeRose, "XML Path
Language (XPath) Version 1.0", W3C
REC REC-xpath-19991116,
November 1999.
[unicode] The Unicode Consortium, "The Unicode
Standard, Version 4.1.0", 2003, <http
://www.unicode.org/versions/
Unicode4.1.0>.
[unicode-norm] Davis, M. and M. Duerst, "Unicode
Normalization Forms", 10 2006, <http:
//www.unicode.org/reports/tr15/>.
Appendix A. Acknowledgements
Thanks to Wendell Craig Baker, Dave Beckett, Jason Douglas, Tom
Gordon, Hugo Haas, John Nienart, Addison Phillips, Pasha Sadri,
Jayavel Shanmugasundaram, Tex Texin, Evan Torrie, and Chris Westin
for their suggestions. The author takes all responsibility for
errors and omissions.
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Appendix B. Default Element Comparison Types
Below are a selection of default comparison types for existing
elements.
B.1. Atom 1.0
o atom:author - simple text (Section 3.2.2.1)
o atom:category - simple text (Section 3.2.2.1)
o atom:content - simple text (Section 3.2.2.1)
o atom:contributor - simple text (Section 3.2.2.1)
o atom:id - simple text (Section 3.2.2.1)
o atom:link - simple text (Section 3.2.2.1)
o atom:published - date (Section 3.2.2.2)
o atom:rights - simple text (Section 3.2.2.1)
o atom:source - simple text (Section 3.2.2.1)
o atom:summary - simple text (Section 3.2.2.1)
o atom:title - simple text (Section 3.2.2.1)
o atom:updated - date (Section 3.2.2.2)
B.2. RSS 2.0
o title - simple text (Section 3.2.2.1)
o link - simple text (Section 3.2.2.1)
o description - simple text (Section 3.2.2.1)
o author - simple text (Section 3.2.2.1)
o category - simple text (Section 3.2.2.1)
o comments - simple text (Section 3.2.2.1)
o enclosure - simple text (Section 3.2.2.1)
o guid - simple text (Section 3.2.2.1)
o pubDate - date (Section 3.2.2.2)
o source - simple text (Section 3.2.2.1)
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Appendix C. Collected ABNF
expression = [ "(" ]
( constraint / expression )
[ operator ( constraint / expression ) ]
[ ")" ]
operator = ";" / ","
constraint = selector [ comparison argument ]
selector = 1*( unreserved / pct-encoded )
comparison = ( ( "=" 1*ALPHA ) / fiql-delim ) "="
argument = 1*arg-char
arg-char = unreserved / pct-encoded / fiql-delim / "="
fiql-delim = "!" / "$" / "'" / "*" / "+"
text-arg = [ "*" ] 1*arg-char [ "*" ]
date-arg = dateTime / duration ; as defined in XML Schema Datatypes
number-arg = [ "+" / "-" ] 1*DIGIT [ "." 1*DIGIT ]
Author's Address
Mark Nottingham
EMail: mnot@mnot.net
URI: http://www.mnot.net/
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