Stream large JSON files in Elixir
Branch: master
Clone or download
boudra Merge pull request #12 from boudra/fix/empty-string-key-path
Handle empty strings as keys properly
Latest commit fb638f7 Feb 16, 2019

Jaxon ⚡️ Build Status Inline docs Coverage Status

Jaxon is the fastest JSON parser that can stream any JSON document without holding it all in memory.

Jaxon fully conforms to the RFC 8259 and ECMA 404 standards and is tested against JSONTestSuite.


  • Make an alternative parser in Elixir, for those who don't want to use NIFs.
  • JSON events to string Encoder.



def deps do
    {:jaxon, "~> 1.0"}

Simple decoding

Decode a binary:

iex> Jaxon.decode(~s({"jaxon":"rocks","array":[1,2]}))
{:ok, %{"array" => [1, 2], "jaxon" => "rocks"}}

iex> Jaxon.decode!(~s({"jaxon":"rocks","array":[1,2]}))
%{"array" => [1, 2], "jaxon" => "rocks"}


Query a binary JSON stream:

iex> stream = [~s({"jaxon":"rocks","array":[1,2]})]
iex> stream |> Jaxon.Stream.query([:root, "array", :all]) |> Enum.to_list()
[1, 2]

Query a binary JSON stream using JSON path expressions:

iex> stream = [~s({"jaxon":"rocks","array":[1,2]})]
iex> stream |> Jaxon.Stream.query(Jaxon.Path.decode!("$.array[*]")) |> Enum.to_list()
[1, 2]

Query a large file without holding the whole file in memory:

|> Jaxon.Stream.query([:root, "users", :all, "id"])
|> Enum.to_list()

How does Jaxon work?

Jaxon first parses the JSON string into a list of events/tokens:

iex(1)> Jaxon.Parsers.NifParser.parse(~s({"key":true}))
[:start_object, {:string, "key"}, :colon, {:boolean, true}, :end_object]

These are all the available events:

{:string, binary}
{:integer, integer}
{:decimal, float}
{:boolean, boolean}
{:incomplete, binary}
{:error, binary}

Which means that it can also parse a list of JSON tokens, even if the string is not a valid JSON representation:

iex> Jaxon.Parser.parse(~s("this is a string" "another string"))
[{:string, "this is a string"}, {:string, "another string"}]

This makes it very flexible when decoding files and lets us use different implementations for parsers, at the moment the default parser is written in C as a NIF. It can be changed in the config like this:

config :jaxon, :parser, Jaxon.Parsers.NifParser # only NifParser is supported at the moment

Then, the decoder's job is to take a list of events and aggregate it into a Elixir term:

iex(4)> Jaxon.Decoder.events_to_term([:start_object, {:string, "key"}, :colon, {:boolean, true}
, :end_object])
{:ok, %{"key" => true}}

About the NIF parser

All the parser does is take a binary and return a list of JSON events, the NIF respects the Erlang scheduler and tries to run for a maximum of one millisecond, yielding to the VM for another call if it runs over the limit.


Jaxon (using the NIF parser) performance is similar and often faster than jiffy and jason.

To run the benchmarks, execute:

mix bench.decode

See the benchmarks here: benchmarks


Copyright © 2018 Mohamed Boudra <>

This project is under the Apache 2.0 license. See the LICENSE file for more details.

Developed at for big data JSON parsing.