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diff --git a/doc/schema.md b/doc/schema.md new file mode 100644 index 0000000..238d7a5 --- /dev/null +++ b/doc/schema.md @@ -0,0 +1,505 @@ +# 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](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/). + +[TOC] + +# Basic Usage {#Basic} + +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. + +~~~cpp +#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 +// 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 `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 {#Fused} + +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 {#DOM} + +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. + +~~~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 {#SAX} + +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/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). + +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 {#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 {#Remote} + +JSON Schema supports [`$ref` keyword](http://spacetelescope.github.io/understanding-json-schema/structuring.html), which is a [JSON pointer](doc/pointer.md) referencing to a local or remote schema. Local pointer is prefixed with `#`, while remote pointer is an relative or absolute URI. For example: + +~~~js +{ "$ref": "definitions.json#/address" } +~~~ + +As `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 {#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 {#Regex} + +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 | +|<code>a|b</code> | 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) | +|<code>\\|</code>, `\\`, ... | 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 {#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](https://github.com/ebdrup/json-schema-benchmark), which tests 11 JavaScript libraries running on Node.js. + +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). + +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`](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%)| + +That is, RapidJSON is about 1.5x faster than the fastest JavaScript library (ajv). And 1400x faster than the slowest one. + +# Schema violation reporting {#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 {#ReportingGeneral} + +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: + +~~~json +{"numbers": [1, 2, "3", 4, 5]} +~~~ + +against this schema: + +~~~json +{ + "type": "object", + "properties": { + "numbers": {"$ref": "numbers.schema.json"} + } +} +~~~ + +where `numbers.schema.json` refers +(via a suitable `IRemoteSchemaDocumentProvider`) +to this schema: + +~~~json +{ + "type": "array", + "items": {"type": "number"} +} +~~~ + +produces the following error object: + +~~~json +{ + "type": { + "instanceRef": "#/numbers/2", + "schemaRef": "numbers.schema.json#/items", + "expected": ["number"], + "actual": "string" + } +} +~~~ + +## Validation keywords for numbers {#Numbers} + +### multipleOf {#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 {#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 {#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 {#Strings} + +### maxLength {#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 {#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 {#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 {#Arrays} + +### additionalItems {#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 {#maxItems-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 {#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 {#maxProperties-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 {#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 {#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 {#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 {#AnyTypes} + +### enum {#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 {#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 {#allOf-anyOf-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 {#not} + +This keyword has no additional properties +apart from `instanceRef` and `schemaRef`. |