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jesse (JSON Schema Erlang) is an implementation of a JSON Schema validator for Erlang.

jesse implements the following specifications:

Erlang API Docs

Automatically generated docs are available .

Please keep in mind that the public API is the jesse.erl module alone.

Quick start - CLI

You can fire up jesse from the CLI, with

bin/jesse [path_to_json_schema] path_to_json_schema -- path_to_json_instance [path_to_json_instance]

You can also output the result in JSON format, with --json, and beautify it e.g. with python

bin/jesse [path_to_json_schema] path_to_json_schema --json -- path_to_json_instance [path_to_json_instance] | python -m json.tool

You can pass multiple JSON schemas which should be loaded into jesse in-memory storage, but JSON instances will be validated against the last JSON schema passed.

Quick start - Erlang

There are two ways of using jesse:

  • to use jesse internal in-memory storage to keep all your schema definitions In this case jesse will look up a schema definition in its own storage, and then validate given a JSON instance.
  • it is also possible to provide jesse with schema definitions when jesse is called.


NOTE: jesse doesn't have any parsing functionality. It currently works with four
      formats: mochijson2, jiffy, jsx and Erlang 17+ maps, so JSON needs to be
      parsed in advance, or you can specify a callback which jesse will use to
      parse JSON.

      In examples below and in jesse test suite jiffy parser is used.
  • Use jesse's internal in-memory storage:

(parse JSON in advance)

1> Schema = jiffy:decode(<<"{\"items\": {\"type\": \"integer\"}}">>).
2> jesse:add_schema("some_key", Schema).
3> Json1 = jiffy:decode(<<"[1, 2, 3]">>).
4> jesse:validate("some_key", Json1).
5> Json2 = jiffy:decode(<<"[1, \"x\"]">>).
6> jesse:validate("some_key", Json2).

The [1] in the error is the path in the original value to <<"x">> where the validation failed. See Validation errors below for the full error format.

(using a callback)

1> jesse:add_schema("some_key",
1>                  <<"{\"uniqueItems\": true}">>,
1>                  [{parser_fun, fun jiffy:decode/1}]).
2> jesse:validate("some_key",
2>                <<"[1, 2]">>,
2>                [{parser_fun, fun jiffy:decode/1}]).
{ok,[1, 2]}
3> jesse:validate("some_key",
3>                <<"[{\"foo\": \"bar\"}, {\"foo\": \"bar\"}] ">>,
3>                [{parser_fun, fun jiffy:decode/1}]).
  • Call jesse with schema definition in place (do not use internal storage)

(parse JSON in advance)

1> Schema = jiffy:decode(<<"{\"pattern\": \"^a*$\"}">>).
2> Json1 = jiffy:decode(<<"\"aaa\"">>).
3> jesse:validate_with_schema(Schema, Json1).
4> Json2 = jiffy:decode(<<"\"abc\"">>).
5> jesse:validate_with_schema(Schema, Json2).

(using a callback)

1> Schema = <<"{\"patternProperties\": {\"f.*o\": {\"type\": \"integer\"}}}">>.
<<"{\"patternProperties\": {\"f.*o\": {\"type\": \"integer\"}}}">>
2> jesse:validate_with_schema(Schema,
2>                            <<"{\"foo\": 1, \"foooooo\" : 2}">>,
2>                            [{parser_fun, fun jiffy:decode/1}]).
3> jesse:validate_with_schema(Schema,
3>                            <<"{\"foo\": \"bar\", \"fooooo\": 2}">>,
3>                            [{parser_fun, fun jiffy:decode/1}]).
  • Since 0.4.0 it's possible to instruct jesse to collect errors, and not stop immediately when it finds an error in the given JSON instance:
1> Schema = <<"{\"properties\": {\"a\": {\"type\": \"integer\"}, \"b\": {\"type\": \"string\"}, \"c\": {\"type\": \"boolean\"}}}">>.
<<"{\"properties\": {\"a\": {\"type\": \"integer\"}, \"b\": {\"type\": \"string\"}, \"c\": {\"type\": \"boolean\"}}}">>
2> jesse:validate_with_schema(Schema,
2>                            <<"{\"a\": 1, \"b\": \"b\", \"c\": true}">>,
2>                            [{parser_fun, fun jiffy:decode/1}]).

now let's change the value of the field "b" to an integer

3> jesse:validate_with_schema(Schema,
3>                            <<"{\"a\": 1, \"b\": 2, \"c\": true}">>,
3>                            [{parser_fun, fun jiffy:decode/1}]).

works as expected, but let's change the value of the field "c" as well

4> jesse:validate_with_schema(Schema,
4>                            <<"{\"a\": 1, \"b\": 2, \"c\": 3}">>,
4>                            [{parser_fun, fun jiffy:decode/1}]).

still works as expected, jesse stops validating as soon as finds an error.

Let's use the allowed_errors option, and set it to 1

5> jesse:validate_with_schema(Schema,
5>                            <<"{\"a\": 1, \"b\": 2, \"c\": 3}">>,
5>                            [{parser_fun, fun jiffy:decode/1},
5>                             {allowed_errors, 1}]).

now we got a list of two errors.

Let's now change the value of the field "a" to a boolean

6> jesse:validate_with_schema(Schema,
6>                            <<"{\"a\": true, \"b\": 2, \"c\": 3}">>,
6>                            [{parser_fun, fun jiffy:decode/1},
6>                             {allowed_errors, 1}]).

we stil got only two errors.

Let's try using 'infinity' as the argument for the allowed_errors option

7> jesse:validate_with_schema(Schema,
7>                            <<"{\"a\": true, \"b\": 2, \"c\": 3}">>,
7>                            [{parser_fun, fun jiffy:decode/1},
7>                             {allowed_errors, infinity}]).

Maps example

8> jesse:validate_with_schema(Schema,
8>                            <<"{\"a\": 1, \"b\": 2, \"c\": true}">>,
8>                            [{parser_fun, fun(Bin) -> jiffy:decode(Bin, [return_maps]) end}]).
{error,[{data_invalid,#{<<"type">> => <<"string">>},
9> jesse:validate_with_schema(Schema,
9>                            <<"{\"a\": 1, \"b\": \"val\", \"c\": true}">>,
9>                            [{parser_fun, fun(Bin) -> jiffy:decode(Bin, [return_maps]) end}]).
{ok, #{<<"a">> => 1, <<"b">> => <<"val">>, <<"c">> => true}}

JSON Schema versions

Currently there are two popular drafts of JSON Schema: draft3 and draft4. jesse supports both. To decide which validator to use jesse tries to read $schema property from the given schema, and checks if it's a supported one, otherwise it will return an error. If $schema property isn't provided in the given schema, jesse will use the default validator (currently the validator for draft3).

To specify which validator to use by default (if there's no $schema property in the given schema), one should use 'default_schema_ver' option when call jesse:validate/3 or jesse:validate_with_schema/3, the value should be a binary consisting a schema path, i.e. <<"">>.

Validation errors

The validation functions jesse:validate/2 and jesse:validate_with_schema/2,3 return {ok, Value} on success and {error, ListOfErrors} on failure. An error is either data_invalid or schema_invalid.

A data_invalid error is a tuple on the form {data_invalid, Schema, ErrorType, Value, Path} where

  • Schema is the part of the schema where validation failed
  • ErrorType is the type of error, usually an atom such as wrong_type, not_in_range or no_match
  • Value is The part of the value where failed validation agains Schema
  • Path is a path to where validation failed within the original value. The path is a list of property names and zero-based array indices referencing the properties and array items within a JSON document; e.g. in the JSON document {"foo": [42, 43, 44]}, the path [<<"foo">>, 0] refers to the value 42. An empty list refers to the whole JSON document.

A schema_invalid error is a tuple on the form {schema_invalid, Schema, ErrorType} where

  • Schema is the part of the schema which is invalid
  • ErrorType is an atom such as missing_id_field or a tuple such as {wrong_type_dependency, Dependency}.


  • pattern and patternProperty attributes:

    jesse uses standard erlang module re for regexp matching, therefore there could be some incompatible regular expressions in schemas you define.

    From erlang docs: "re's matching algorithms are currently based on the PCRE library, but not all of the PCRE library is interfaced"

    But most of common cases should work fine.

  • internal references (id attribute) are NOT supported


If you see something missing or incorrect, a pull request is most welcome!


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