An Elixir JSON Schema validator
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Elixir JSON Schema validator

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A JSON Schema validator with full support for the draft 4 specification and zero dependencies. Passes the official JSON Schema Test Suite.


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

defp deps do
  [{:ex_json_schema, "~> 0.5.4"}]

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 |> Poison.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 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.

iex> 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"}]}

Errors are tuples of a message and the path to the element not matching the schema. The path is following the same conventions used in JSON Schema for referencing JSON elements.

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


Released under the MIT license.


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