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Fastest Json parser for Ruby

Ruby wrapper for simdjson (Parsing gigabytes of JSON per second)

The simdjson library uses commonly available SIMD instructions and microparallel algorithms to parse JSON 2.5 times faster than anything else out there.

Initial code imported from simdjson_ruby

Comparison with Oj gem & JSON.parse

  1. Parsing JSON from file
FastJsonparser.load(src) is 40% faster than Oj.load_file(src)
FastJsonparser.load(src) is 155% faster than JSON.parse(

Benchmark result

# virus-info.json 1.2M
#                 user     system      total        real
# FastJsonparser  4.772700   0.321748   5.094448 (  5.100354)
# OJ              6.944966   0.267082   7.212048 (  7.253477) ~ 42% more
# standard JSON   13.311654   0.167076  13.478730 ( 13.504489) ~  164% more
  1. Parsing local JSON
FastJsonparser.parse(json) is 19% faster than Oj.load(json)
FastJsonparser.parse(json) is 96% faster than JSON.parse(json)

Benchmark result

#                 user     system      total        real
# FastJsonparser  6.732325   0.005716   6.738041 (  6.750285)
# OJ              8.029406   0.006582   8.035988 (  8.055000) ~ 19% more
# standard JSON   13.025884   0.031287  13.057171 ( 13.264931) ~ 96% more
  1. Streaming json from file
FastJsonparser.load_many(src) is 153% faster than, 'r'))

Benchmark result

#                   user     system      total        real
# FastJsonparser  3.844446   0.141822   3.986268 (  3.884655)
# YAJL            9.699621   0.110060   9.809681 (  9.826104) ~ 150% more


Add this line to your application's Gemfile:

gem 'fast_jsonparser'

And then execute:

$ bundle

Or install it yourself as:

$ gem install fast_jsonparser


  1. Parsing JSON from file
FastJsonparser.load(src) # src is source path for json file



  1. Parsing local JSON


FastJsonparser.parse('{"one": 1, "two": 2}')

  1. Streaming JSON from file

File with multiple json can be stream with load_many method

Example: logs.json with following content

{"time": "17/May/2015:08:05:32 +0000", "remote_ip": "", "remote_user": "-"}
{"time": "17/May/2015:08:05:23 +0000", "remote_ip": "", "remote_user": "-"}
{"time": "17/May/2015:08:05:24 +0000", "remote_ip": "", "remote_user": "-"}

load_many accepts file_path & block as arguments

> FastJsonparser.load_many(file_path) { |obj| p obj[:time]}
"17/May/2015:08:05:32 +0000"
"17/May/2015:08:05:23 +0000"
"17/May/2015:08:05:24 +0000"

If size of json batch is greater than 1 MB then use batch_size option

FastJsonparser.load_many(f.path, batch_size: 2_000) {}
  1. Accept optional param :symbolize_keys (default symbolize_keys: true)

If string key is expected in parsed result then use

FastJsonparser.parse('{"one": 1, "two": 2}', symbolize_keys: false)

  1. Raise FastJsonparser::ParseError when invalid JSON provided for parsing
FastJsonparser.parse("123: 1") # FastJsonparser::ParseError (parse error)

Known Incompatibilities with stdlib JSON

FastJsonparser behaves mostly like stdlib's JSON, but there are a few corner cases:

  • FastJsonparser will use symbols for hash keys by default. You can pass symbolize_keys: false to have strings instead like JSON.
  • FastJsonparser will raise on integers outside of the 64bits range (-9223372036854775808..18446744073709551615), JSON will parse them fine.
  • FastJsonparser will raise on invalid string escapings ("\x15"), JSON will often handle some of them.
  • FastJsonparser will raise on /**/ comments. JSON will sometimes ignore them, sometimes raise.


> sample_json = '{"a":"Alpha","b":true,"c":12345,"d":[true,[false,[-123456789,null],3.9676,["Something else.",false],null]],"e":{"zero":null,"one":1,"two":2,"three":[3],"four":[0,1,2,3,4]},"f":null,"h":{"a":{"b":{"c":{"d":{"e":{"f":{"g":null}}}}}}},"i":[[[[[[[null]]]]]]]}'
> FastJsonparser.parse(sample_json)
=> {:a=>"Alpha", :b=>true, :c=>12345, :d=>[true, [false, [-123456789, nil], 3.9676, ["Something else.", false], nil]], :e=>{:zero=>nil, :one=>1, :two=>2, :three=>[3], :four=>[0, 1, 2, 3, 4]}, :f=>nil, :h=>{:a=>{:b=>{:c=>{:d=>{:e=>{:f=>{:g=>nil}}}}}}}, :i=>[[[[[[[nil]]]]]]]} 


After checking out the repo, run rake compile to install dependencies. Then, run rake test to run the tests.

For more option, refer


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the FastJsonparser project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.


Fastest Json parser for Ruby, wrapper for simdjson



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