Elixir JSON Schema Validator

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A JSON Schema validator with support for the draft 4, draft 6 and draft 7 specifications. Passes the official JSON Schema Test Suite.


Add the project to your Mix dependencies in mix.exs:

defp deps do
    {:ex_json_schema, "~> 0.10.2"}

Update your dependencies with:

$ mix deps.get

Loading remote schemata

If you have remote schemata that need to be fetched at runtime, you have to register a function that takes a URL and returns a Map of the parsed JSON. So in your Mix configuration in config/config.exs you should have something like this:

config :ex_json_schema,
  fn url -> HTTPoison.get!(url).body |> Jason.decode! end

Alternatively, you can specify a module and function name for situations where using anonymous functions is not possible (i.e. working with Erlang releases):

config :ex_json_schema,
  {MyModule, :my_resolver}

You do not have to do that for the official draft 4 meta-schema found at http://json-schema.org/draft-04/schema# though. That schema is bundled with the project and will work out of the box without any network calls.

Resolving a schema

In this step the schema is validated against its meta-schema (the draft 4 schema definition) and $refs are being resolved (making sure that the reference points to an existing fragment). You should only resolve a schema once to avoid the overhead of resolving it in every validation call.

schema = %{
  "type" => "object",
  "properties" => %{
    "foo" => %{
      "type" => "string"
} |> ExJsonSchema.Schema.resolve()

Note that Map keys are expected to be strings, since in practice that data will always come from some JSON parser.


If you're only interested in whether a piece of data is valid according to the schema:

iex> ExJsonSchema.Validator.valid?(schema, %{"foo" => "bar"})

iex> ExJsonSchema.Validator.valid?(schema, %{"foo" => 1})

Or in case you want to have detailed validation errors:

iex> ExJsonSchema.Validator.validate(schema, %{"foo" => "bar"})

iex> ExJsonSchema.Validator.validate(schema, %{"foo" => 1})
{:error, [{"Type mismatch. Expected String but got Integer.", "#/foo"}]}

Validation error formats

By default, errors are formatted using a string formatter that returns errors as tuples of error message and path. If you want to get raw validation error structs, you can pass the following option:

iex> ExJsonSchema.Validator.validate(schema, %{"foo" => 1}, error_formatter: false)
     error: %ExJsonSchema.Validator.Error.Type{
       actual: "integer",
       expected: ["string"]
     path: "#/foo"

Custom error formatter

You can also pass your own custom error formatter as a module that implements a format/1 function that takes a list of raw errors via the same option:

defmodule MyFormatter do
  def format(errors) do
    Enum.map(errors, fn %ExJsonSchema.Validator.Error{error: error, path: path} ->
      {error.__struct__, path}
iex> ExJsonSchema.Validator.validate(schema, %{"foo" => 1}, error_formatter: MyFormatter)
{:error, [{ExJsonSchema.Validator.Error.Type, "#/foo"}]}

Validating against a fragment

It is also possible to validate against a subset of the schema by providing either a fragment:

iex> fragment = ExJsonSchema.Schema.get_fragment!(schema, "#/properties/foo")
%{"type" => "string"}

iex> ExJsonSchema.Validator.valid_fragment?(schema, fragment, "bar")

iex> ExJsonSchema.Validator.validate_fragment(schema, fragment, "bar")

or a path:

iex> ExJsonSchema.Validator.valid_fragment?(schema, "#/foo", "bar")

Format support

The validator supports all the formats specified by draft 4 (date-time, email, hostname, ipv4, ipv6), with the exception of the uri format which has confusing/broken requirements in the official test suite (see https://github.com/json-schema/JSON-Schema-Test-Suite/issues/77).

Custom formats

The JSON schema spec states that the format property "allows values to be constrained beyond what the other tools in JSON Schema can do". To support this, you can configure a callback validator function which gets called when a format property is encountered that is not one of the builtin formats.

As a global configuration option:

config :ex_json_schema,
  {MyModule, :validate}

Or by passing an option when resolving the schema:

ExJsonSchema.Schema.resolve(%{"format" => "custom"}, custom_format_validator: {MyModule, :validate})

The configured function is called with the arguments (format, data) and is expected to return true, false or a {:error, %Error.Format{expected: "something"}} tuple, depending whether the data is valid for the given format. For compatibility with JSON schema, it is expected to return true when the format is unknown by your callback function.

The custom function can also be an anonymous function:

ExJsonSchema.Schema.resolve(%{"format" => "custom"}, custom_format_validator: fn format, data -> true end)

Note that the anonymous function version of the custom validator is only available in the option to Schema.resolve/2 and not as Application config, as using anonymous functions as configuration is not allowed.


Copyright (c) 2015 Jonas Schmidt

Released under the MIT license.


  • Add some source code documentation
  • Enable providing JSON for known schemata at resolve time