1# Schema 2 3(This feature was released in v1.1.0) 4 5JSON 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. 6 7RapidJSON 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/). 8 9[TOC] 10 11## Basic Usage 12 13First of all, you need to parse a JSON Schema into `Document`, and then compile the `Document` into a `SchemaDocument`. 14 15Secondly, 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. 16 17~~~cpp 18#include "rapidjson/schema.h" 19 20// ... 21 22Document sd; 23if (!sd.Parse(schemaJson).HasParseError()) { 24 // the schema is not a valid JSON. 25 // ... 26} 27SchemaDocument schema(sd); // Compile a Document to SchemaDocument 28// sd is no longer needed here. 29 30Document d; 31if (!d.Parse(inputJson).HasParseError()) { 32 // the input is not a valid JSON. 33 // ... 34} 35 36SchemaValidator validator(schema); 37if (!d.Accept(validator)) { 38 // Input JSON is invalid according to the schema 39 // Output diagnostic information 40 StringBuffer sb; 41 validator.GetInvalidSchemaPointer().StringifyUriFragment(sb); 42 printf("Invalid schema: %s\n", sb.GetString()); 43 printf("Invalid keyword: %s\n", validator.GetInvalidSchemaKeyword()); 44 sb.Clear(); 45 validator.GetInvalidDocumentPointer().StringifyUriFragment(sb); 46 printf("Invalid document: %s\n", sb.GetString()); 47} 48~~~ 49 50Some notes: 51 52* One `SchemaDocment` can be referenced by multiple `SchemaValidator`s. It will not be modified by `SchemaValidator`s. 53* A `SchemaValidator` may be reused to validate multiple documents. To run it for other documents, call `validator.Reset()` first. 54 55## Validation during parsing/serialization 56 57Unlike 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. 58 59### DOM parsing 60 61For 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. 62 63~~~cpp 64#include "rapidjson/filereadstream.h" 65 66// ... 67SchemaDocument schema(sd); // Compile a Document to SchemaDocument 68 69// Use reader to parse the JSON 70FILE* fp = fopen("big.json", "r"); 71FileReadStream is(fp, buffer, sizeof(buffer)); 72 73// Parse JSON from reader, validate the SAX events, and store in d. 74Document d; 75SchemaValidatingReader<kParseDefaultFlags, FileReadStream, UTF8<> > reader(is, schema); 76d.Populate(reader); 77 78if (!reader.GetParseResult()) { 79 // Not a valid JSON 80 // When reader.GetParseResult().Code() == kParseErrorTermination, 81 // it may be terminated by: 82 // (1) the validator found that the JSON is invalid according to schema; or 83 // (2) the input stream has I/O error. 84 85 // Check the validation result 86 if (!reader.IsValid()) { 87 // Input JSON is invalid according to the schema 88 // Output diagnostic information 89 StringBuffer sb; 90 reader.GetInvalidSchemaPointer().StringifyUriFragment(sb); 91 printf("Invalid schema: %s\n", sb.GetString()); 92 printf("Invalid keyword: %s\n", reader.GetInvalidSchemaKeyword()); 93 sb.Clear(); 94 reader.GetInvalidDocumentPointer().StringifyUriFragment(sb); 95 printf("Invalid document: %s\n", sb.GetString()); 96 } 97} 98~~~ 99 100### SAX parsing 101 102For using SAX in parsing, it is much simpler. If it only need to validate the JSON without further processing, it is simply: 103 104~~~ 105SchemaValidator validator(schema); 106Reader reader; 107if (!reader.Parse(stream, validator)) { 108 if (!validator.IsValid()) { 109 // ... 110 } 111} 112~~~ 113 114This 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). 115 116If 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: 117 118~~~ 119MyHandler handler; 120GenericSchemaValidator<SchemaDocument, MyHandler> validator(schema, handler); 121Reader reader; 122if (!reader.Parse(ss, validator)) { 123 if (!validator.IsValid()) { 124 // ... 125 } 126} 127~~~ 128 129### Serialization 130 131It is also possible to do validation during serializing. This can ensure the result JSON is valid according to the JSON schema. 132 133~~~ 134StringBuffer sb; 135Writer<StringBuffer> writer(sb); 136GenericSchemaValidator<SchemaDocument, Writer<StringBuffer> > validator(s, writer); 137if (!d.Accept(validator)) { 138 // Some problem during Accept(), it may be validation or encoding issues. 139 if (!validator.IsValid()) { 140 // ... 141 } 142} 143~~~ 144 145Of course, if your application only needs SAX-style serialization, it can simply send SAX events to `SchemaValidator` instead of `Writer`. 146 147## Remote Schema 148 149JSON Schema supports [`$ref` keyword](http://spacetelescope.github.io/understanding-json-schema/structuring.html), which is a [JSON pointer](doc/pointer.md) referencing to a local or remote schema. Local pointer is prefixed with `#`, while remote pointer is an relative or absolute URI. For example: 150 151~~~js 152{ "$ref": "definitions.json#/address" } 153~~~ 154 155As `SchemaDocument` does not know how to resolve such URI, it needs a user-provided `IRemoteSchemaDocumentProvider` instance to do so. 156 157~~~ 158class MyRemoteSchemaDocumentProvider : public IRemoteSchemaDocumentProvider { 159public: 160 virtual const SchemaDocument* GetRemoteDocument(const char* uri, SizeTyp length) { 161 // Resolve the uri and returns a pointer to that schema. 162 } 163}; 164 165// ... 166 167MyRemoteSchemaDocumentProvider provider; 168SchemaDocument schema(sd, &provider); 169~~~ 170 171## Conformance 172 173RapidJSON passed 262 out of 263 tests in [JSON Schema Test Suite](https://github.com/json-schema/JSON-Schema-Test-Suite) (Json Schema draft 4). 174 175The 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. 176 177Besides, the `format` schema keyword for string values is ignored, since it is not required by the specification. 178 179### Regular Expression 180 181The schema keyword `pattern` and `patternProperties` uses regular expression to match the required pattern. 182 183RapidJSON implemented a simple NFA regular expression engine, which is used by default. It supports the following syntax. 184 185|Syntax|Description| 186|------|-----------| 187|`ab` | Concatenation | 188|`a|b` | Alternation | 189|`a?` | Zero or one | 190|`a*` | Zero or more | 191|`a+` | One or more | 192|`a{3}` | Exactly 3 times | 193|`a{3,}` | At least 3 times | 194|`a{3,5}`| 3 to 5 times | 195|`(ab)` | Grouping | 196|`^a` | At the beginning | 197|`a$` | At the end | 198|`.` | Any character | 199|`[abc]` | Character classes | 200|`[a-c]` | Character class range | 201|`[a-z0-9_]` | Character class combination | 202|`[^abc]` | Negated character classes | 203|`[^a-c]` | Negated character class range | 204|`[\b]` | Backspace (U+0008) | 205|`\|`, `\\`, ... | Escape characters | 206|`\f` | Form feed (U+000C) | 207|`\n` | Line feed (U+000A) | 208|`\r` | Carriage return (U+000D) | 209|`\t` | Tab (U+0009) | 210|`\v` | Vertical tab (U+000B) | 211 212For 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. 213 214## Performance 215 216Most 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. 217 218That 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). 219 220On a Mac Book Pro (2.8 GHz Intel Core i7), the following results are collected. 221 222|Validator|Relative speed|Number of test runs per second| 223|---------|:------------:|:----------------------------:| 224|RapidJSON|155%|30682| 225|[`ajv`](https://github.com/epoberezkin/ajv)|100%|19770 (± 1.31%)| 226|[`is-my-json-valid`](https://github.com/mafintosh/is-my-json-valid)|70%|13835 (± 2.84%)| 227|[`jsen`](https://github.com/bugventure/jsen)|57.7%|11411 (± 1.27%)| 228|[`schemasaurus`](https://github.com/AlexeyGrishin/schemasaurus)|26%|5145 (± 1.62%)| 229|[`themis`](https://github.com/playlyfe/themis)|19.9%|3935 (± 2.69%)| 230|[`z-schema`](https://github.com/zaggino/z-schema)|7%|1388 (± 0.84%)| 231|[`jsck`](https://github.com/pandastrike/jsck#readme)|3.1%|606 (± 2.84%)| 232|[`jsonschema`](https://github.com/tdegrunt/jsonschema#readme)|0.9%|185 (± 1.01%)| 233|[`skeemas`](https://github.com/Prestaul/skeemas#readme)|0.8%|154 (± 0.79%)| 234|tv4|0.5%|93 (± 0.94%)| 235|[`jayschema`](https://github.com/natesilva/jayschema)|0.1%|21 (± 1.14%)| 236 237That is, RapidJSON is about 1.5x faster than the fastest JavaScript library (ajv). And 1400x faster than the slowest one. 238