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# Schema

## Status: experimental, shall be included in v1.1

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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.
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RapidJSON implemented a JSON Schema validator for [JSON Schema Draft v4](http://json-schema.org/documentation.html). If you are not familiar with JSON Schema, you may refer to [Understanding JSON Schema](http://spacetelescope.github.io/understanding-json-schema/).
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[TOC]

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## Basic Usage

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First of all, you need to parse a JSON Schema into `Document`, and then compile the `Document` into a `SchemaDocument`.
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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.
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~~~cpp
#include "rapidjson/schema.h"

// ...

Document sd;
if (!sd.Parse(schemaJson)) {
    // the schema is not a valid JSON.
    // ...       
}
SchemaDocument schema(sd); // Compile a Document to SchemaDocument
// sd is no longer needed here.

Document d;
if (!d.Parse(inputJson)) {
    // 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 `SchemaDocment` can be referenced by multiple `SchemaValidator`s. It will not be modified by `SchemaValidator`s.
* A `SchemaValidator` may be reused to validate multiple documents. To run it for other documents, call `validator.Reset()` first.

## Validation during parsing/serialization

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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.
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### DOM parsing

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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.
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~~~cpp
#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()) {
        // ...    
    }
}
~~~

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This is exactly the method used in the [schemavalidator](example/schemavalidator/schemavalidator.cpp) 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).
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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()) {
        // ...
    }
}
~~~

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

JSON Schema supports [`$ref` keyword](http://spacetelescope.github.io/understanding-json-schema/structuring.html), which is a [JSON pointer](pointer.md) referencing to a local or remote schema. Local pointer is prefixed with `#`, while remote pointer is an relative or absolute URI. For example:

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

As `SchemaValidator` 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, SizeTyp length) {
        // Resolve the uri and returns a pointer to that schema.
    }
};

// ...

MyRemoteSchemaDocumentProvider provider;
SchemaValidator validator(schema, &provider);
~~~

## Conformance

RapidJSON passed 262 out of 263 tests in [JSON Schema Test Suite](https://github.com/json-schema/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.

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RapidJSON implemented a simple NFA regular expression engine, which is used by default. It supports the following syntax.
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|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)
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|`\|`, `\\`, ...  | Escape characters
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|`\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

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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](https://github.com/ebdrup/json-schema-benchmark), which tests 11 JavaScript libraries running on Node.js.
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That benchmark runs validations on [JSON Schema Test Suite](https://github.com/json-schema/JSON-Schema-Test-Suite), in which some test suites and tests are excluded. We made the same benchmarking procedure in [`schematest.cpp`](test/perftest/schematest.cpp).
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On a Mac Book Pro (2.8 GHz Intel Core i7), the following results are collected.
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|Validator|Relative speed|Number of test runs per second|
|---------|:------------:|:----------------------------:|
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|RapidJSON|155%|30682|
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|[`ajv`](https://github.com/epoberezkin/ajv)|100%|19770 (± 1.31%)|
|[`is-my-json-valid`](https://github.com/mafintosh/is-my-json-valid)|70%|13835 (± 2.84%)|
|[`jsen`](https://github.com/bugventure/jsen)|57.7%|11411 (± 1.27%)|
|[`schemasaurus`](https://github.com/AlexeyGrishin/schemasaurus)|26%|5145 (± 1.62%)|
|[`themis`](https://github.com/playlyfe/themis)|19.9%|3935 (± 2.69%)|
|[`z-schema`](https://github.com/zaggino/z-schema)|7%|1388 (± 0.84%)|
|[`jsck`](https://github.com/pandastrike/jsck#readme)|3.1%|606 (± 2.84%)|
|[`jsonschema`](https://github.com/tdegrunt/jsonschema#readme)|0.9%|185 (± 1.01%)|
|[`skeemas`](https://github.com/Prestaul/skeemas#readme)|0.8%|154 (± 0.79%)|
|tv4|0.5%|93 (± 0.94%)|
|[`jayschema`](https://github.com/natesilva/jayschema)|0.1%|21 (± 1.14%)|

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