2023-12-11 20:30:44 -05:00

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Schema

(This feature was released in v1.1.0)

JSON Schema is a draft standard for describing the format of JSON data. The schema itself is also JSON data. By validating a JSON structure with JSON Schema, your code can safely access the DOM without manually checking types, or whether a key exists, etc. It can also ensure that the serialized JSON conform to a specified schema.

RapidJSON implemented a JSON Schema validator for JSON Schema Draft v4. If you are not familiar with JSON Schema, you may refer to Understanding JSON Schema.

[TOC]

Basic Usage

First of all, you need to parse a JSON Schema into Document, and then compile the Document into a SchemaDocument.

Secondly, construct a SchemaValidator with the SchemaDocument. It is similar to a Writer in the sense of handling SAX events. So, you can use document.Accept(validator) to validate a document, and then check the validity.

#include "rapidjson/schema.h"

// ...

Document sd;
if (sd.Parse(schemaJson).HasParseError()) {
    // the schema is not a valid JSON.
    // ...       
}

SchemaDocument schema(sd); // Compile a Document to SchemaDocument
if (!schema.GetError().ObjectEmpty()) {
    // there was a problem compiling the schema
    StringBuffer sb;
    Writer<StringBuffer> w(sb);
    schema.GetError().Accept(w);
    printf("Invalid schema: %s\n", sb.GetString());
}
// sd is no longer needed here.

Document d;
if (d.Parse(inputJson).HasParseError()) {
    // the input is not a valid JSON.
    // ...       
}

SchemaValidator validator(schema);
if (!d.Accept(validator)) {
    // Input JSON is invalid according to the schema
    // Output diagnostic information
    StringBuffer sb;
    validator.GetInvalidSchemaPointer().StringifyUriFragment(sb);
    printf("Invalid schema: %s\n", sb.GetString());
    printf("Invalid keyword: %s\n", validator.GetInvalidSchemaKeyword());
    sb.Clear();
    validator.GetInvalidDocumentPointer().StringifyUriFragment(sb);
    printf("Invalid document: %s\n", sb.GetString());
}

Some notes:

One SchemaDocument can be referenced by multiple SchemaValidators. It will not be modified by SchemaValidators.
A SchemaValidator may be reused to validate multiple documents. To run it for other documents, call validator.Reset() first.

Validation during parsing/serialization

Unlike most JSON Schema validator implementations, RapidJSON provides a SAX-based schema validator. Therefore, you can parse a JSON from a stream while validating it on the fly. If the validator encounters a JSON value that invalidates the supplied schema, the parsing will be terminated immediately. This design is especially useful for parsing large JSON files.

DOM parsing

For using DOM in parsing, Document needs some preparation and finalizing tasks, in addition to receiving SAX events, thus it needs some work to route the reader, validator and the document. SchemaValidatingReader is a helper class that doing such work.

#include "rapidjson/filereadstream.h"

// ...
SchemaDocument schema(sd); // Compile a Document to SchemaDocument

// Use reader to parse the JSON
FILE* fp = fopen("big.json", "r");
FileReadStream is(fp, buffer, sizeof(buffer));

// Parse JSON from reader, validate the SAX events, and store in d.
Document d;
SchemaValidatingReader<kParseDefaultFlags, FileReadStream, UTF8<> > reader(is, schema);
d.Populate(reader);

if (!reader.GetParseResult()) {
    // Not a valid JSON
    // When reader.GetParseResult().Code() == kParseErrorTermination,
    // it may be terminated by:
    // (1) the validator found that the JSON is invalid according to schema; or
    // (2) the input stream has I/O error.

    // Check the validation result
    if (!reader.IsValid()) {
        // Input JSON is invalid according to the schema
        // Output diagnostic information
        StringBuffer sb;
        reader.GetInvalidSchemaPointer().StringifyUriFragment(sb);
        printf("Invalid schema: %s\n", sb.GetString());
        printf("Invalid keyword: %s\n", reader.GetInvalidSchemaKeyword());
        sb.Clear();
        reader.GetInvalidDocumentPointer().StringifyUriFragment(sb);
        printf("Invalid document: %s\n", sb.GetString());
    }
}

SAX parsing

For using SAX in parsing, it is much simpler. If it only need to validate the JSON without further processing, it is simply:

SchemaValidator validator(schema);
Reader reader;
if (!reader.Parse(stream, validator)) {
    if (!validator.IsValid()) {
        // ...    
    }
}

This is exactly the method used in the schemavalidator example. The distinct advantage is low memory usage, no matter how big the JSON was (the memory usage depends on the complexity of the schema).

If you need to handle the SAX events further, then you need to use the template class GenericSchemaValidator to set the output handler of the validator:

MyHandler handler;
GenericSchemaValidator<SchemaDocument, MyHandler> validator(schema, handler);
Reader reader;
if (!reader.Parse(ss, validator)) {
    if (!validator.IsValid()) {
        // ...    
    }
}

Serialization

It is also possible to do validation during serializing. This can ensure the result JSON is valid according to the JSON schema.

StringBuffer sb;
Writer<StringBuffer> writer(sb);
GenericSchemaValidator<SchemaDocument, Writer<StringBuffer> > validator(s, writer);
if (!d.Accept(validator)) {
    // Some problem during Accept(), it may be validation or encoding issues.
    if (!validator.IsValid()) {
        // ...
    }
}

Of course, if your application only needs SAX-style serialization, it can simply send SAX events to SchemaValidator instead of Writer.

Remote Schema

JSON Schema supports $ref keyword, which is a JSON pointer referencing to a local or remote schema. Local pointer is prefixed with #, while remote pointer is an relative or absolute URI. For example:

{ "$ref": "definitions.json#/address" }

As SchemaDocument does not know how to resolve such URI, it needs a user-provided IRemoteSchemaDocumentProvider instance to do so.

class MyRemoteSchemaDocumentProvider : public IRemoteSchemaDocumentProvider {
public:
    virtual const SchemaDocument* GetRemoteDocument(const char* uri, SizeType length) {
        // Resolve the uri and returns a pointer to that schema.
    }
};

// ...

MyRemoteSchemaDocumentProvider provider;
SchemaDocument schema(sd, &provider);

Conformance

RapidJSON passed 262 out of 263 tests in JSON Schema Test Suite (Json Schema draft 4).

The failed test is "changed scope ref invalid" of "change resolution scope" in refRemote.json. It is due to that id schema keyword and URI combining function are not implemented.

Besides, the format schema keyword for string values is ignored, since it is not required by the specification.

Regular Expression

The schema keyword pattern and patternProperties uses regular expression to match the required pattern.

RapidJSON implemented a simple NFA regular expression engine, which is used by default. It supports the following syntax.

Syntax	Description
`ab`	Concatenation
`a\|b`	Alternation
`a?`	Zero or one
`a*`	Zero or more
`a+`	One or more
`a{3}`	Exactly 3 times
`a{3,}`	At least 3 times
`a{3,5}`	3 to 5 times
`(ab)`	Grouping
`^a`	At the beginning
`a$`	At the end
`.`	Any character
`[abc]`	Character classes
`[a-c]`	Character class range
`[a-z0-9_]`	Character class combination
`[^abc]`	Negated character classes
`[^a-c]`	Negated character class range
`[\b]`	Backspace (U+0008)
`\\|`, `\\`, ...	Escape characters
`\f`	Form feed (U+000C)
`\n`	Line feed (U+000A)
`\r`	Carriage return (U+000D)
`\t`	Tab (U+0009)
`\v`	Vertical tab (U+000B)

For C++11 compiler, it is also possible to use the std::regex by defining RAPIDJSON_SCHEMA_USE_INTERNALREGEX=0 and RAPIDJSON_SCHEMA_USE_STDREGEX=1. If your schemas do not need pattern and patternProperties, you can set both macros to zero to disable this feature, which will reduce some code size.

Performance

Most C++ JSON libraries do not yet support JSON Schema. So we tried to evaluate the performance of RapidJSON's JSON Schema validator according to json-schema-benchmark, which tests 11 JavaScript libraries running on Node.js.

That benchmark runs validations on JSON Schema Test Suite, in which some test suites and tests are excluded. We made the same benchmarking procedure in schematest.cpp.

On a Mac Book Pro (2.8 GHz Intel Core i7), the following results are collected.

Validator	Relative speed	Number of test runs per second
RapidJSON	155%	30682
`ajv`	100%	19770 (± 1.31%)
`is-my-json-valid`	70%	13835 (± 2.84%)
`jsen`	57.7%	11411 (± 1.27%)
`schemasaurus`	26%	5145 (± 1.62%)
`themis`	19.9%	3935 (± 2.69%)
`z-schema`	7%	1388 (± 0.84%)
`jsck`	3.1%	606 (± 2.84%)
`jsonschema`	0.9%	185 (± 1.01%)
`skeemas`	0.8%	154 (± 0.79%)
tv4	0.5%	93 (± 0.94%)
`jayschema`	0.1%	21 (± 1.14%)

That is, RapidJSON is about 1.5x faster than the fastest JavaScript library (ajv). And 1400x faster than the slowest one.

Schema violation reporting

(Unreleased as of 2017-09-20)

When validating an instance against a JSON Schema, it is often desirable to report not only whether the instance is valid, but also the ways in which it violates the schema.

The SchemaValidator class collects errors encountered during validation into a JSON Value. This error object can then be accessed as validator.GetError().

The structure of the error object is subject to change in future versions of RapidJSON, as there is no standard schema for violations. The details below this point are provisional only.

General provisions

Validation of an instance value against a schema produces an error value. The error value is always an object. An empty object {} indicates the instance is valid.

The name of each member corresponds to the JSON Schema keyword that is violated.
The value is either an object describing a single violation, or an array of such objects.

Each violation object contains two string-valued members named instanceRef and schemaRef. instanceRef contains the URI fragment serialization of a JSON Pointer to the instance subobject in which the violation was detected. schemaRef contains the URI of the schema and the fragment serialization of a JSON Pointer to the subschema that was violated.

Individual violation objects can contain other keyword-specific members. These are detailed further.

For example, validating this instance:

{"numbers": [1, 2, "3", 4, 5]}

against this schema:

{
  "type": "object",
  "properties": {
    "numbers": {"$ref": "numbers.schema.json"}
  }
}

where numbers.schema.json refers (via a suitable IRemoteSchemaDocumentProvider) to this schema:

{
  "type": "array",
  "items": {"type": "number"}
}

produces the following error object:

{
  "type": {
    "instanceRef": "#/numbers/2",
    "schemaRef": "numbers.schema.json#/items",
    "expected": ["number"],
    "actual": "string"
  }
}

Validation keywords for numbers

multipleOf

expected: required number strictly greater than 0. The value of the multipleOf keyword specified in the schema.
actual: required number. The instance value.

maximum

expected: required number. The value of the maximum keyword specified in the schema.
exclusiveMaximum: optional boolean. This will be true if the schema specified "exclusiveMaximum": true, and will be omitted otherwise.
actual: required number. The instance value.

minimum

expected: required number. The value of the minimum keyword specified in the schema.
exclusiveMinimum: optional boolean. This will be true if the schema specified "exclusiveMinimum": true, and will be omitted otherwise.
actual: required number. The instance value.

Validation keywords for strings

maxLength

expected: required number greater than or equal to 0. The value of the maxLength keyword specified in the schema.
actual: required string. The instance value.

minLength

expected: required number greater than or equal to 0. The value of the minLength keyword specified in the schema.
actual: required string. The instance value.

pattern

actual: required string. The instance value.

(The expected pattern is not reported because the internal representation in SchemaDocument does not store the pattern in original string form.)

Validation keywords for arrays

additionalItems

This keyword is reported when the value of items schema keyword is an array, the value of additionalItems is false, and the instance is an array with more items than specified in the items array.

disallowed: required integer greater than or equal to 0. The index of the first item that has no corresponding schema.

maxItems and minItems

expected: required integer greater than or equal to 0. The value of maxItems (respectively, minItems) specified in the schema.
actual: required integer greater than or equal to 0. Number of items in the instance array.

uniqueItems

duplicates: required array whose items are integers greater than or equal to 0. Indices of items of the instance that are equal.

(RapidJSON only reports the first two equal items, for performance reasons.)

Validation keywords for objects

maxProperties and minProperties

expected: required integer greater than or equal to 0. The value of maxProperties (respectively, minProperties) specified in the schema.
actual: required integer greater than or equal to 0. Number of properties in the instance object.

required

missing: required array of one or more unique strings. The names of properties that are listed in the value of the required schema keyword but not present in the instance object.

additionalProperties

This keyword is reported when the schema specifies additionalProperties: false and the name of a property of the instance is neither listed in the properties keyword nor matches any regular expression in the patternProperties keyword.

disallowed: required string. Name of the offending property of the instance.

(For performance reasons, RapidJSON only reports the first such property encountered.)

dependencies

errors: required object with one or more properties. Names and values of its properties are described below.

Recall that JSON Schema Draft 04 supports schema dependencies, where presence of a named controlling property requires the instance object to be valid against a subschema, and property dependencies, where presence of a controlling property requires other dependent properties to be also present.

For a violated schema dependency, errors will contain a property with the name of the controlling property and its value will be the error object produced by validating the instance object against the dependent schema.

For a violated property dependency, errors will contain a property with the name of the controlling property and its value will be an array of one or more unique strings listing the missing dependent properties.

Validation keywords for any instance type

enum

This keyword has no additional properties beyond instanceRef and schemaRef.

The allowed values are not listed because SchemaDocument does not store them in original form.
The violating value is not reported because it might be unwieldy.

If you need to report these details to your users, you can access the necessary information by following instanceRef and schemaRef.

type

expected: required array of one or more unique strings, each of which is one of the seven primitive types defined by the JSON Schema Draft 04 Core specification. Lists the types allowed by the type schema keyword.
actual: required string, also one of seven primitive types. The primitive type of the instance.

allOf, anyOf, and oneOf

errors: required array of at least one object. There will be as many items as there are subschemas in the allOf, anyOf or oneOf schema keyword, respectively. Each item will be the error value produced by validating the instance against the corresponding subschema.

For allOf, at least one error value will be non-empty. For anyOf, all error values will be non-empty. For oneOf, either all error values will be non-empty, or more than one will be empty.

not

This keyword has no additional properties apart from instanceRef and schemaRef.

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