| Network Working Group | J.M. Snell |
| Internet-Draft | October 2012 |
| Intended status: Informational | |
| Expires: April 02, 2013 |
JSON Predicate
draft-snell-json-test-00
JSON Predicates defines a syntax for serializing various predicate expressions as JSON Objects.
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This specification defines JSON Predicates, a JSON-based syntax for the description and serialization of logical boolean predicate operations intended to be used in conjunction with other JSON-based mechanisms, such as JSON Patch, as a means of incorporating conditional handling during the processing of a JSON document.
For example, JSON Predicates can be used to extend a JSON Patch [I-D.ietf-appsawg-json-patch] document to provide a broader range of conditional processing options not currently supported by JSON Patch.
Example: Given a source JSON document
{
"a": {
"b": {
"c": "ABC!XYZ"
}
}
}
The following JSON Patch with JSON Predicates document will first test that the value of the "c" property is a string containing the character sequence "ABC" prior to applying the specified "replace" operation.
[
{
"op": "and",
"path": "/a/b",
"apply": [
{
"op": "type-of",
"path": "/a/b/c",
"value": "string"
},
{
"op": "contains",
"path": "/a/b/c",
"value": "ABC"
}
]
},
{
"op": "replace",
"path": "/a/b/c",
"value": 123
}
]
It is important to note this specification does not define a distinct JSON Predicates Document format. Rather, it is the intent for JSON Predicates to be used within other JSON-based document formats, like JSON Patch.
TODO: Lots more needs to be said here.
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119].
A JSON Predicate is a JSON Object whose name value pairs describe a testable conditions that evaluate as either true or false.
The essential components of a JSON Predicate include:
Predicate objects MUST have exactly one "op" member whose value indicates the type of predicate operation to perform. It's value MUST be one of: "and", "contains", "defined", "ends", "less", "matches", "more", "not", "or", "starts", "type", or "undefined". The semantics for each are defined in the sections that follow.
Note that the value of the "op" member is case-sensitive and that each of the operations listed are in lower-case. The value "Starts", for example, is not equivalent to "starts".
If the "op" member specifies any value other than one of those listed above, the evaluation of the predicate operation MUST cease and treated as if a boolean value of "false" was returned. The application processing the predicate operation MAY signal that an error condition has occurred depending on the specific requirements of the domain within which JSON Predicates are being used.
The remaining structure of each predicate operation depends on it's specific type. There are two basic types of predicates.
In addition to the required "op" member, First Order Predicates MUST have exactly one "path" member whose value MUST be a string containing a JSON-Pointer value referencing the location of the name value pair that is to be tested.
Second Order Predicates MUST have exactly one "apply" member whose value is a JSON Array containing one or more First or Second Order Predicate Objects.
Additional members can be required depending on the specific predicate operation. All other members not explicitly defined by this specification MUST be ignored.
Note that the ordering of members in JSON objects is not significant; therefore the following operations are equivalent:
{"op": "contains", "path": "/a/b/c", "value": "ABC"}
{"path": "/a/b/c", "op": "contains", "value": "ABC"}
{"value": "ABC", "path": "/a/b/c", "op": "contains"}
All JSON Predicates are evaluated against a given base context. The nature of this context is dependent entirely on the application within which JSON Predicates is being used. For instance, when used together with JSON Patch, the JSON Predicate operations are evaluated relative to the JSON document that is the target of the JSON Patch operation.
Although Predicate Objects use JSON Pointer references to identify values against which a predicate operation is evaluated, the base context is not required to be a JSON object or array. In such cases, however, it is the responsibility of the application implementation to determine how to interpret the JSON Pointer reference relative to the base context.
The "contains" predicate evaluates as true if the referenced element is defined and has a value whose string representation contains the exact sequence of characters given by the predicate object's "value" member.
For example, given the JSON document:
{
"a": {
"b": "This is a test"
}
}
The following predicate would evaluate as "true":
{
"op": "contains",
"path": "/a/b",
"value": " is a "
}
By default, character matching MUST be performed in a case-sensitive manner. To override this default behavior, the predicate object MAY contain an "ignore_case" member whose value is either "true" (to perform case-insensitive matching) or false.
For instance, the following will evaluate as "true":
{
"op": "contains",
"path": "/a/b/",
"value": " Is A ",
"ignore_case": true
}
The "defined" predicate evaluates as true if the referenced element exists within the target context.
For example, given the JSON document:
{
"a": {
"b": null
}
}
The following predicate would evaluate as "true" because the path "/a/b" exist within the document despite being explicitly set to null:
{
"op": "defined",
"path": "/a/b"
}
The following predicate would evaluate as "false" because the path "/a/c" does exist within the document.
{
"op": "defined",
"path": "/a/c"
}
The "ends" predicate evaluates as true if the referenced element is defined and has a value whose string representation ends with the exact sequence of characters given by the predicate object's "value" member.
For example, given the JSON document:
{
"a": {
"b": "This is a test"
}
}
The following predicate would evaluate as "true":
{
"op": "ends",
"path": "/a/b",
"value": " test"
}
By default, character matching MUST be performed in a case-sensitive manner. To override this default behavior, the predicate object MAY contain an "ignore_case" member whose value is either "true" (to perform case-insensitive matching) or false.
For instance, the following will evaluate as "true":
{
"op": "ends",
"path": "/a/b/",
"value": " TEST",
"ignore_case": true
}
The "less" predicate evaluates as true if the referenced element is defined and specifies a number whose value is less than that specified by the predicate object's "value" member.
For example, given the JSON document:
{
"a": {
"b": 10
}
}
The following will evaluate as "true":
{
"op": "less",
"path": "/a/b",
"value": 15
}
The "matches" predicate evaluates as true if the referenced element is defined and has a value whose string representation matches the regular expression provided by the predicate object's "value" member.
For example, given the JSON document:
{
"a": {
"b": "this is a test"
}
}
The following evalutes as "true":
{
"op": "matches",
"path": "/a/b",
"value": "[\\w\\s]*"
}
The predicate's matching pattern is expressed as a string value conforming to the JavaScript Regular Expression syntax.
By default, character matching MUST be performed in a case-sensitive manner. To override this default behavior, the predicate object MAY contain an "ignore_case" member whose value is either "true" (to perform case-insensitive matching) or false. Setting the value of "ignore_case" to true is equivalent to using the "i" modifier flag within the JavaScript Regular Expression syntax (e.g. "/\w\s/*/i").
For instance:
{
"op": "ends",
"path": "/a/b/",
"value": "[\\w\\s]*",
"ignore_case": true
}
The "more" predicate evaluates as true if the referenced element is defined and specifies a number whose value is greater than that specified by the predicate object's "value" member.
For example, given the JSON document:
{
"a": {
"b": 10
}
}
The following will evaluate as "true":
{
"op": "more",
"path": "/a/b",
"value": 5
}
The "starts" predicate evaluates as true if the referenced element is defined and has a value whose string representation begins with the exact sequence of characters given by the predicate object's "value" member.
For example, given the JSON document:
{
"a": {
"b": "This is a test"
}
}
The following predicate would evaluate as "true":
{
"op": "starts",
"path": "/a/b",
"value": "This "
}
By default, character matching MUST be performed in a case-sensitive manner. To override this default behavior, the predicate object MAY contain an "ignore_case" member whose value is either "true" (to perform case-insensitive matching) or false.
For instance, the following will evaluate as "true":
{
"op": "starts",
"path": "/a/b/",
"value": "this ",
"ignore_case": true
}
The "type" predicate evaluates as true if the referenced element exists and specifies a value whose JSON type is equal to that specified by the predicate's "value" member.
The "value" member MUST specify one of: "number", "string", "boolean", "object", "array", "null" or "undefined".
For example, given the JSON document:
{
"a": {
"b": "this is a test",
"c": [1,2,3]
}
}
The following predicate would evaluate as "true"
{
"op": "type",
"path": "/a/b",
"value": "string"
}
Note that the "type" predicate is generally identical to JavaScript's built in "typeof" operator with the exception that JavaScript's operator does not distinguish between Array and Object types. That is, when executing the JavaScript code "typeof doc.a.c" against the example JSON document given above, JavaScript will report the type as "object" rather than "array". By contrast, the "type" predicate distinguishes the two types of values.
The "undefined" predicate evaluates as true if the referenced element does not exist within the target context.
For example, given the JSON document:
{
"a": {
"b": null
}
}
The following predicate would evaluate as "true" because the path "/a/c" does not exist within the document:
{
"op": "undefined",
"path": "/a/c"
}
However, the following predicate would evaluate as "false" because the path "/a/b" does exist within the document, despite specifying an explicit null value.
{
"op": "undefined",
"path": "/a/b"
}
Second Order Predicates are defined as sets of one or more subordinate First and Second Order Predicates.
All Second Order Predicates MAY contain a "path" member whose value specifies a root path prefix for all contained predicates. For example, given the JSON document:
{
"a": {
"b": {
"c": "ABC!"
}
}
}
The following would evaluate as true because the path "/a/b/c" is defined.
{
"op": "and",
"path": "/a/b",
"apply": [
{
"op": "defined",
"path": "/c"
}
]
}
The above example is equivalent to:
{
"op": "and",
"apply": [
{
"op": "defined",
"path": "/a/b/c"
}
]
}
The "and" predicate evaluates as "true" if all of it's contained set of predicate operations evaluate as "true".
For example, given the JSON document:
{
"a" : {
"b" : "foo",
"c" : {
"d": 10
}
}
}
The following would evaluate as "true" because the element "/a/b" is defined and the value of element "/a/c/d" is less than 15.
{
"op": "and",
"apply" [
{
"op": "defined",
"path": "/a/b"
},
{
"op": "less",
"path": "/a/c/d",
"value": 15
}
]
}
However, the following would evaluate as "false" because the while element "/a/c" exists, the value of that element is not a string.
{
"op": "and",
"apply": [
{
"op": "test",
"path": "/a/c"
},
{
"op": "type",
"path": "/a/c",
"value": "string"
}
]
}
The "not" predicate evaluates as "true" if all of it's contained set of predicate operations evaluate as "false".
For example, given the JSON document:
{
"a" : {
"b" : "foo",
"c" : {
"d": 10
}
}
}
The following would evaluate as "true" because the element "/a/b/e" is undefined and the value of element "/a/c/d" is not less than 5.
{
"op": "not",
"apply": [
{
"op": "defined",
"path": "/a/b/e"
},
{
"op": "less",
"path": "/a/c/d",
"value": 5
}
]
}
However, the following would evaluate as "false" because the element "/a/c" exists and the value for element "/a/b" begins with the letter "f"
{
"op": "not",
"apply": [
{
"op": "undefined",
"path": "/a/c"
},
{
"op": "starts",
"path": "/a/b",
"value": "f"
}
]
}
The "or" predicate evaluates as "true" if at least one of it's contained set of predicate operations evaluate as "true".
For example, given the JSON document:
{
"a" : {
"b" : "foo",
"c" : {
"d": 10
}
}
}
The following would evaluate as "true" because the element "/a/b" is defined.
{
"op": "or",
"apply": [
{
"op": "defined",
"path": "/a/b"
},
{
"op": "less",
"path": "/a/c/d",
"value": 5
}
]
}
However, the following would evaluate as "false" because neither elements "/a/e" or "/a/f" exist.
{
"op": "or",
"apply": [
{
"op": "test",
"path": "/a/e"
},
{
"op": "test",
"path": "/a/f"
}
]
}
Second Order Predicates can be combined in a variety of ways to define more complex test operations. For example:
{
"op": "or",
"apply": [
{
"op": "not",
"apply": [
{
"op": "defined",
"path": "/a/b/c"
},
{
"op": "starts",
"path": "/a/b/c",
"value": "f"
}
]
},
{
"op": "not",
"apply": [
{
"op": "defined",
"path": "/a/b/d"
},
{
"op": "type",
"path": "/a/b/d",
"value": "number"
}
]
}
]
}
When an error condition is encounted during the processing of a JSON Predicate, the predicate MUST evaluate as false.
Error conditions can arise in each of the following conditions:
While JSON Predicate objects can be used in a variety of applications, the syntax has been specifically designed for compatibility with the JSON Patch Document format. JSON Predicate objects MAY be used directly within a JSON Patch Document as tests to evaluate whether or not the application of a set of patch operations should succeed or fail.
For example, given the following JSON document:
{
"a": {
"b": {
"c": "123"
}
}
}
The following JSON Patch + JSON Predicates document will first test that the path "/a/b/c" references a value matching the given regular expression prior to replacing that value:
[
{
"op": "matches",
"path": "/a/b/c",
"value": "\\d{3}"
},
{
"op": "replace",
"path": "/a/b/c",
"value": "ABC"
}
]
When a JSON Predicate object within a JSON Patch document evaluates as false, processing of the JSON Patch Document MUST be handled exactly the same as an unsuccessful JSON Patch operation would be handled as defined in JSON-PATCH [I-D.ietf-appsawg-json-patch], Section 5. Specifically, processing of the JSON Patch document SHOULD terminate and application of the entire patch document SHALL NOT be deemed successful.
Note that JSON Patch implementations that do not implement or recognize JSON Predicate objects will treat such objects as unknown patch operations that will cause evaluation of the Patch document to fail.
JSON Predicate objects do not, by themselves, introduce any particular security concerns. Note that JSON documents that consist of an arbitrary number of nested Second Order Predicate objects can have a detrimental impact on overall performance and could be leveraged by a malicious entity as part of a denial of service attack.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [I-D.ietf-appsawg-json-patch] | Bryan, P and M Nottingham, "JSON Patch", Internet-Draft draft-ietf-appsawg-json-patch-05, September 2012. |