Internet DRAFT - draft-goessner-dispatch-jsonpath
draft-goessner-dispatch-jsonpath
Network Working Group S. Gössner
Internet-Draft Fachhochschule Dortmund
Intended status: Standards Track C. Bormann, Ed.
Expires: 13 January 2021 Universität Bremen TZI
12 July 2020
JSONPath -- XPath for JSON
draft-goessner-dispatch-jsonpath-00
Abstract
insert abstract here
Contributing
This document picks up the popular JSONPath specification dated
2007-02-21 and provides a more normative definition for it. It is
intended as a submission to the IETF DISPATCH WG, in order to find
the right way to complete standardization of this specification. In
its current state, it is a strawman document showing what needs to be
covered.
Comments and issues can be directed at the github repository _insert
repo here_ as well as (for the time when the more permanent home is
being decided) at the dispatch@ietf.org mailing list.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 13 January 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Inspired by XPath . . . . . . . . . . . . . . . . . . . . 3
1.3. Overview of JSONPath Expressions . . . . . . . . . . . . 4
2. JSONPath Examples . . . . . . . . . . . . . . . . . . . . . . 7
3. Detailed definition . . . . . . . . . . . . . . . . . . . . . 9
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. IANA considerations . . . . . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Early JSONPath implementations . . . . . . . . . . . 11
A.1. Implementation . . . . . . . . . . . . . . . . . . . . . 11
A.2. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 12
A.3. Parameters . . . . . . . . . . . . . . . . . . . . . . . 12
A.4. Return value . . . . . . . . . . . . . . . . . . . . . . 12
A.5. JavaScript Example . . . . . . . . . . . . . . . . . . . 12
A.6. PHP example . . . . . . . . . . . . . . . . . . . . . . . 12
A.7. Results . . . . . . . . . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This document picks up the popular JSONPath specification dated
2007-02-21 [JSONPath-orig] and provides a more normative definition
for it. It is intended as a submission to the IETF DISPATCH WG, in
order to find the right way to complete standardization of this
specification. In its current state, it is a strawman document
showing what needs to be covered.
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1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The grammatical rules in this document are to be interpreted as
described in [RFC5234].
The terminology of [RFC8259] applies.
Data Item: A structure complying to the generic data model of JSON,
i.e., composed of containers such as arrays and maps (JSON
objects), and of atomic data such as null, true, false, numbers,
and text strings.
Object: Used in its generic sense, e.g., for programming language
objects. When a JSON Object as defined in [RFC8259] is meant, we
specifically say JSON Object.
Query: Short name for JSONPath expression.
Argument: Short name for the JSON data item a JSONPath expression is
applied to.
Output Path: A simple form of JSONPath expression that identifies a
Position by providing a query that results in exactly that
position. Similar to, but syntactically different from, a JSON
Pointer [RFC6901].
Position: A JSON data item identical to or nested within the JSON
data item to which the query is applied to, expressed either by
the value of that data item or by providing a JSONPath Output
Path.
1.2. Inspired by XPath
A frequently emphasized advantage of XML is the availability of
plenty tools to analyse, transform and selectively extract data out
of XML documents. [XPath] is one of these powerful tools.
In 2007, the need for something solving the same class of problems
for the emerging JSON community became apparent, specifically for:
* Finding data interactively and extracting them out of [RFC8259]
data items without special scripting.
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* Specifying the relevant parts of the JSON data in a request by a
client, so the server can reduce the data in the server response,
minimizing bandwidth usage.
So how does such a tool look like when done for JSON? When defining
a JSONPath, how should expressions look like?
The XPath expression
/store/book[1]/title
looks like
x.store.book[0].title
or
x['store']['book'][0]['title']
in popular programming languages such as JavaScript, Python and PHP,
with a variable x holding the JSON data item. Here we observe that
such languages already have a fundamentally XPath-like feature built
in.
The JSONPath tool in question should:
* be naturally based on those language characteristics.
* cover only essential parts of XPath 1.0.
* be lightweight in code size and memory consumption.
* be runtime efficient.
1.3. Overview of JSONPath Expressions
JSONPath expressions always apply to a JSON data item in the same way
as XPath expressions are used in combination with an XML document.
Since a JSON data item is usually anonymous and doesn't necessarily
have a "root member object", JSONPath used the abstract name "$" to
refer to the top level object of the data item.
JSONPath expressions can use the _dot-notation_
$.store.book[0].title
or the _bracket-notation_
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$['store']['book'][0]['title']
for paths input to a JSONPath processor. Where a JSONPath processor
uses JSONPath expressions for internal purposes or as output paths,
these will always be converted to the more general _bracket-
notation_.
JSONPath allows the wildcard symbol "*" for member names and array
indices. It borrows the descendant operator ".." from [E4X] and the
array slice syntax proposal "[start:end:step]" [SLICE] from
ECMASCRIPT 4.
JSONPath can employ an _underlying scripting language_, expressions
of which, written in parentheses: "(<expr>)", can be used as an
alternative to explicit names or indices as in:
$.store.book[(@.length-1)].title
The symbol "@" is used for the current object. Filter expressions
are supported via the syntax "?(<boolean expr>)" as in
$.store.book[?(@.price < 10)].title
Here is a complete overview and a side by side comparison of the
JSONPath syntax elements with its XPath counterparts.
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+=======+==================+=======================================+
| XPath | JSONPath | Description |
+=======+==================+=======================================+
| / | $ | the root object/element |
+-------+------------------+---------------------------------------+
| . | @ | the current object/element |
+-------+------------------+---------------------------------------+
| / | "." or "[]" | child operator |
+-------+------------------+---------------------------------------+
| .. | n/a | parent operator |
+-------+------------------+---------------------------------------+
| // | .. | nested descendants. JSONPath borrows |
| | | this syntax from E4X. |
+-------+------------------+---------------------------------------+
| * | * | wildcard. All objects/elements |
| | | regardless of their names. |
+-------+------------------+---------------------------------------+
| @ | n/a | attribute access. JSON data items |
| | | don't have attributes. |
+-------+------------------+---------------------------------------+
| [] | [] | subscript operator. XPath uses it to |
| | | iterate over element collections and |
| | | for predicates. In JavaScript and |
| | | JSON it is the native array operator. |
+-------+------------------+---------------------------------------+
| | | [,] | Union operator in XPath results in a |
| | | combination of node sets. JSONPath |
| | | allows alternate names or array |
| | | indices as a set. |
+-------+------------------+---------------------------------------+
| n/a | [start:end:step] | array slice operator borrowed from |
| | | ES4. |
+-------+------------------+---------------------------------------+
| [] | ?() | applies a filter (script) expression. |
+-------+------------------+---------------------------------------+
| n/a | () | script expression, using the |
| | | underlying script engine. |
+-------+------------------+---------------------------------------+
| () | n/a | grouping in Xpath |
+-------+------------------+---------------------------------------+
Table 1: Overview over JSONPath, comparing to XPath
XPath has a lot more to offer (location paths in unabbreviated
syntax, operators and functions) than listed here. Moreover there is
a significant difference how the subscript operator works in Xpath
and JSONPath:
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* Square brackets in XPath expressions always operate on the _node
set_ resulting from the previous path fragment. Indices always
start at 1.
* With JSONPath, square brackets operate on the _object_ or _array_
addressed by the previous path fragment. Indices always start at
0.
2. JSONPath Examples
This section provides some more examples for JSONPath expressions.
The examples are based on a simple JSON data item patterned after a
typical XML example representing a bookstore (that also has
bicycles):
{ "store": {
"book": [
{ "category": "reference",
"author": "Nigel Rees",
"title": "Sayings of the Century",
"price": 8.95
},
{ "category": "fiction",
"author": "Evelyn Waugh",
"title": "Sword of Honour",
"price": 12.99
},
{ "category": "fiction",
"author": "Herman Melville",
"title": "Moby Dick",
"isbn": "0-553-21311-3",
"price": 8.99
},
{ "category": "fiction",
"author": "J. R. R. Tolkien",
"title": "The Lord of the Rings",
"isbn": "0-395-19395-8",
"price": 22.99
}
],
"bicycle": {
"color": "red",
"price": 19.95
}
}
}
Figure 1: Example JSON data item
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The examples in Table 2 presume an underlying script language that
allows obtaining the number of items in an array, testing for the
presence of a map member, and performing numeric comparisons of map
member values with a constant.
+======================+=========================+==================+
| XPath | JSONPath | Result |
+======================+=========================+==================+
| /store/book/author | $.store.book[*].author | the authors of |
| | | all books in |
| | | the store |
+----------------------+-------------------------+------------------+
| //author | $..author | all authors |
+----------------------+-------------------------+------------------+
| /store/* | $.store.* | all things in |
| | | store, which |
| | | are some books |
| | | and a red |
| | | bicycle. |
+----------------------+-------------------------+------------------+
| /store//price | $.store..price | the price of |
| | | everything in |
| | | the store. |
+----------------------+-------------------------+------------------+
| //book[3] | $..book[2] | the third book |
+----------------------+-------------------------+------------------+
| //book[last()] | "$..book[(@.length-1)]" | the last book |
| | "$..book[-1:]" | in order. |
+----------------------+-------------------------+------------------+
| //book[position()<3] | "$..book[0,1]" | the first two |
| | "$..book[:2]" | books |
+----------------------+-------------------------+------------------+
| //book[isbn] | $..book[?(@.isbn)] | filter all |
| | | books with isbn |
| | | number |
+----------------------+-------------------------+------------------+
| //book[price<10] | $..book[?(@.price<10)] | filter all |
| | | books cheapier |
| | | than 10 |
+----------------------+-------------------------+------------------+
| //* | $..* | all Elements in |
| | | XML document. |
| | | All members of |
| | | JSON data item. |
+----------------------+-------------------------+------------------+
Table 2: Example JSONPath expressions applied to the example JSON
data item
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3. Detailed definition
[TBD: This section needs to be fleshed out in detail. The text given
here is intended to give the flavor of that detail, not to be the
actual definition that is to be defined.]
JSONPath expressions, "queries" for short in this specification, are
character strings, represented in UTF-8 unless otherwise required by
the context in which they are used.
When applied to a JSON data item, a query returns a (possibly empty)
list of "positions" in the data item that match the JSONPath
expression.
JSONPath = root *(step)
root = "$"
step = ".." name ; nested descendants
/ "." name ; child (dot notation)
/ "[" value-expression *("," value-expression) "]"
; child[ren] (bracket notation)
/ "[" value-expression *2(":" value-expression) "]" ; (slice)
value-expression = *DIGIT / name
/ script-expression / filter-expression
name = "'" text "'"
/ "*" ; wildcard
script-expression = "(" script ")"
filter-expression = "?(" script ")"
script = <To be defined in the course of standardization>
text = <any text, restrictions to be defined>
DIGIT = %x30-39
Figure 2: ABNF definition for JSONPath
Within a script, @ stands for the position under consideration.
[TBD: define underlying scripting language, if there is to be a
standard one]
[TBD: define handling of spaces]
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A JSONPath starts at the root of the argument; the "current list" of
positions is initialized to that root. Each step applies the
semantics of that step to each of the positions in the "current
list", returning another list; the "current list" is replaced by the
concatenation of all these returned lists, and the next step begins.
When all steps have been performed, the "current list" is returned,
depending on the choices of the context either as a list of data
items or as a list of output paths. [TBD: define the order of that
list]
[TBD: Define all the steps]
[TBD: Define details of Output Path]
4. Discussion
* Currently only single quotes allowed inside of JSONPath
expressions.
* Script expressions inside of JSONPath locations are currently not
recursively evaluated by jsonPath. Only the global "$" and local
"@" symbols are expanded by a simple regular expression.
* An alternative for jsonPath to return false in case of no match
may be to return an empty array in future. [This is already done
in the above.]
5. IANA considerations
TBD: Define a media type for JSON Path expressions.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
6.2. Informative References
[E4X] ISO, "Information technology -- ECMAScript for XML (E4X)
specification", ISO/IEC 22537:2006 , 2006.
[JSON-PHP] "JSON-PHP", January 2005,
<http://mike.teczno.com/json.html>.
[JSONPath-impl]
"jsonpath (Downloads)", n.d.,
<https://code.google.com/archive/p/jsonpath/downloads>.
[JSONPath-orig]
Gössner, S., "JSONPath - XPath for JSON", 21 February
2007, <https://goessner.net/articles/JsonPath/>.
[RFC6901] Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
"JavaScript Object Notation (JSON) Pointer", RFC 6901,
DOI 10.17487/RFC6901, April 2013,
<https://www.rfc-editor.org/info/rfc6901>.
[SLICE] "Slice notation", n.d.,
<https://github.com/tc39/proposal-slice-notation>.
[XPath] Berglund, A., Boag, S., Chamberlin, D., Fernandez, M.,
Kay, M., Robie, J., and J. Simeon, "XML Path Language
(XPath) 2.0 (Second Edition)", World Wide Web Consortium
Recommendation REC-xpath20-20101214, 14 December 2010,
<http://www.w3.org/TR/2010/REC-xpath20-20101214>.
Appendix A. Early JSONPath implementations
This appendix has been copied from the similar section in
[JSONPath-orig], with few changes. It is informative, intended to
supply more examples and give an impression for what could be a
typical JSONPath API.
A.1. Implementation
JSONPath is implemented in JavaScript for client-side usage and
ported over to PHP for use on the server.
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A.2. Usage
All you need to do is downloading either of the files
* "jsonpath.js" [JSONPath-impl]
* "jsonpath.php" [JSONPath-impl]
include it in your program and use the simple API consisting of one
single function.
jsonPath(obj, expr [, args])
A.3. Parameters
obj (object|array): Object representing the JSON data item.
expr (string): JSONPath expression string.
args (object|undefined): Object controlling path evaluation and
output. Currently only one member is supported.
args.resultType ("VALUE"|"PATH"): causes the result to be either
matching values (default) or normalized path expressions.
A.4. Return value
(array|false): Array holding either values or normalized path
expressions matching the input path expression, which can be used
for lazy evaluation. "false" in case of no match.
A.5. JavaScript Example
var o = { /*...*/ }, // the 'store' JSON object from above
res1 = jsonPath(o, "$..author").toJSONString(),
res2 = jsonPath(o, "$..author",
{resultType:"PATH"}).toJSONString();
A.6. PHP example
We need here to convert the JSON string to a PHP array first. I am
using Michal Migurski's JSON parser [JSON-PHP] for that.
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require_once('json.php'); // JSON parser
require_once('jsonpath.php'); // JSONPath evaluator
$json = '{ ... }'; // JSON data item from above
$parser = new Services_JSON(SERVICES_JSON_LOOSE_TYPE);
$o = $parser->decode($json);
$match1 = jsonPath($o, "$..author");
$match2 = jsonPath($o, "$..author",
array("resultType" => "PATH"));
$res1 = $parser->encode($match1);
$res2 = $parser->encode($match2);
A.7. Results
Both JavaScript and PHP example result in the following JSON arrays
(as strings):
res1:
[ "Nigel Rees",
"Evelyn Waugh",
"Herman Melville",
"J. R. R. Tolkien"
]
res2:
[ "$['store']['book'][0]['author']",
"$['store']['book'][1]['author']",
"$['store']['book'][2]['author']",
"$['store']['book'][3]['author']"
]
Please note that the return value of jsonPath is an array, which is
also a valid JSON data item. So you might want to apply jsonPath to
the resulting data item again or use one of your favorite array
methods as sort with it.
Acknowledgements
This specification is based on Stefan Gössner's original online
article defining JSONPath [JSONPath-orig].
The books example was taken from http://coli.lili.uni-
bielefeld.de/~andreas/Seminare/sommer02/books.xml -- a dead link now.
Authors' Addresses
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Stefan Gössner
Fachhochschule Dortmund
Sonnenstraße 96
D-44139 Dortmund
Germany
Email: stefan.goessner@fh-dortmund.de
Carsten Bormann (editor)
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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