rjson

rjson is a json parser that relies on Ragel-generated state machines for most parsing. rjson's api is minimal and focussed on efficient parsing.

Ragel state machines

This whole thing is built around a few Ragel-generated state machines. They are defined in .rl files, and the generated code is in .rl.go files. If you peek at the generated code, beware that it doesn't look like anything intended to be read by a human.

Read functions

rjson provides Read functions for simple json values (strings, numbers, booleans and null). They each take a data []byte argument and read the first json value in data. They return a p int value that is the offset of the first byte after the value they read. When used in a handler, this p should be returned as the handler's p.

Read functions return an error for any json type other than the type they are meant to read. This includes null. So if there is a value that could be either a string or null, be sure to use NextTokenType to check which one it is.

ReadString and ReadStringBytes take a buf []byte argument. ReadStringBytes returns the result appended to buf and ReadString uses it as a byte buffer when decoding strings to avoid memory allocations. buf may be nil in both cases.

Handlers

rjson uses handlers to parse complex json values (objects and arrays). The handler will implement either HandleArrayValue(data []byte) (p int, err error) or HandleObjectValue(fieldname, data []byte) (p int, err error). data is the document being parsed from the current position to the end of the document.

The handler has to return p as either 0, or the offset after the last byte of the value it handled. This is important as it is how rjson knows where to resume parsing the document. When the handler returns 0, rjson uses SkipValue to find the next offset, so it is best to write handlers that return the next offset and avoid an extra call to SkipValue.

When your handler returns an error, HandleArrayValues and HandleObjectValues will immediately return the same error. You can use this to stop parsing a document once you have found all values you are after.

See HandleObjectValues's example

Standard Library Compatibility

rjson endeavors to decode json to the same values as the encoding/json functions. In the source code, most of rjson's exported functions are followed by an unexported version that produces the same output using encoding/json instead of rjson. These are used to ensure compatibility.

The RuneError compatibility exception

When decoding strings, encoding/json will replace bytes with values over 127 that aren't part of a valid utf8 rune with utf8.RuneError. I don't think this is correct behavior This is probably correct behavior for the standard library, but rjson keeps those bytes as-is when decoding strings.

If you need standard library compatibility here, you can use StdLibCompatibleString, StdLibCompatibleSlice, and StdLibCompatibleMap.

Differential Fuzz Testing

rjson uses fuzz testing extensively to test compatibility with encoding/json. See fuzzers.go for the fuzz functions and testdata/fuzz/corpus for the increasingly unwieldy corpus.

What rjson doesn't do

• drop in replacement for "encoding/json" - rjson doesn't aim to be a replacement for "encoding/json". It probably won't ever unmarshal json to a struct, and it definitely won't ever marshal json. It does one thing well, and that one thing is parsing json fast and with minimal memory allocations.

• import "unsafe" - Not that there is necessarily anything wrong with using unsafe where it is called for. It's just that you are better off avoiding it where possible, and it is possible here.

• return unchecked strings - Some json parsers use techniques like strings.IndexByte(data, '"') to skip to the end of a string and return the contents uninspected. This can be dangerous when dealing with json from untrusted sources. Those strings can contain control characters and other invalid json.

• parse from an io.Reader - I would like to implement this in the future, but it's off the table for now.

• keep a global resource pool - Some json packages have handy Borrow and Return functions that let you borrow resources and avoid new allocations. rjson leaves it up to the user to decide how to do resource pooling.

Why you should still use the standard library most of the time

While rjson is much faster than "encoding/json", it is faster at the cost of being more difficult to use. If you use rjson, your code will be more verbose and less readable than if you used the standard library. In most cases, this will cost more in developer time than you will make up for in time saved parsing json.

To make rjson worthwhile, one of these should apply:

• You have a very high volume of json to parse.
• You need very low latency but are stuck using json serialization.

rjson is originally written for a project where we have to process thousands of json documents per second. This is the sort of project that should be using rjson.

Performance principles and tricks

Handle each byte once

This is the main guiding principle of rjson performance. Do everything you can to avoid handling the same byte twice.

fp.ParseJSONFloatPrefix

fp.ParseJSONFloatPrefix is a rewrite of strconv.ParseFloat that deals in bytes instead of strings and eliminates everything not needed for parsing json numbers. This reduces the time to read numbers by about 60%. It's an internal package for now, but I encourage anybody who is parsing json numbers to consider copying this code or writing something similar.

Predict object and array size based on previous values

ReadObject and ReadArray can create a lot of nested maps and slices. It would be nice if we knew how big they would be ahead of time so we can make([]interface{}, 0, size), but we can't know that until we reach the end. In place of that, rjson keeps track of how big the values end up being and allocates values that are as big as the previous sibling value of the same type.

Always Be Benchmarking

Running benchmarks for every change keeps me from going too far down bad paths. Also, consistently benchmarking against other parsers shows me where rjson can be improved.

While I'm coding I frequently run benchdiff with script/benchdiff --benchtime 10ms --bench WhateverBenchmark to get a quick comparison with the previous commit. It takes just a couple of seconds to run, and it saves a lot of time down the road.

Benchmarks

Benchmarks are in ./benchmarks and run with script/compbench. Each package that is benchmarked has its own file that implements the various benchmarks.

JSON data

With the exception of github_repo.json, this is all taken from nativejson-benchmark

All tested JSON data are in UTF-8.

JSON file	Size	Description
canada.json source	2199KB	Contour of Canada border in GeoJSON format. Contains a lot of real numbers.
citm_catalog.json source	1737KB	A big benchmark file with indentation used in several Java JSON parser benchmarks.
twitter.json	632KB	Search "一" (character of "one" in Japanese and Chinese) in Twitter public time line for gathering some tweets with CJK characters.
github_repo.json	6KB	The golang/go repository from GitHub's API. curl https://api.github.com/repos/golang/go

Compared packages

The go community is fortunate to have multiple high-performance options for parsing JSON. The benchmarks include some better known packages. They don't all have the same features, so not all the compared packages are represented in every benchmark. There are also some packages with incorrect Valid() functions. Those are excluded until they are fixed.

Every package should be able to put their best foot forward in the benchmarks. I have tried to write the best code for each package in the benchmark. If anybody sees a way to improve the benchmarks for any package, please create an issue to let me know.

This import statement has all the players seen in the benchmarks below:

import (
encoding_json "encoding/json"

jsonparser "github.com/buger/jsonparser"
jsoniter "github.com/json-iterator/go"
gjson "github.com/tidwall/gjson"
fastjson "github.com/valyala/fastjson"
rjson "github.com/willabides/rjson"
goccyjson "github.com/goccy/go-json"
)

Benchmarks

GetRepoValues

GetRepoValues gets three values of different types from github_repo.json and writes them to a struct. The same as running json.Unmarshal with this struct:

type repoData struct {
Archived bool   `json:"archived"`
Forks    int64  `json:"forks"`
FullName string `json:"full_name"`
}

Valid

Valid runs checks whether json data is valid. It's the equivalent of json.Valid(data)

DistinctUserIDs

DistinctUserIDs returns a slice of user ids from twitter.json.

ReadObject

ReadObject decodes json data to a map[string]interface{}.

ReadFloat64

ReadFloat64 parses a json number and returns its value as a float64.

ReadInt64

ReadInt64 parses a json number and returns its value as an int64.

ReadString

ReadString decodes a json string and returns its value as a string.

ReadBool

ReadBool decodes a json true or false and returns the value.

Results

These are the results from running the benchmarks 10x and using benchstat to compare each package to rjson. You can run them yourself by running script/compbench.

encoding_json

name	encoding_json (ns/op)	rjson (ns/op)	delta
GetRepoValues	39772.3	2156.9	-94.58%
Valid_canada	6981750	2413060	-65.44%
Valid_citm_catalog	5380290	1645390	-69.42%
Valid_github_repo	19912	5485.7	-72.45%
Valid_twitter	2141580	611203	-71.46%
DistinctUserIDs	4227210	743022	-82.42%
ReadObject_canada	52882000	32002000	-39.48%
ReadObject_citm_catalog	22354300	11808100	-47.18%
ReadObject_github_repo	92965.4	30023.4	-67.70%
ReadObject_twitter	9228560	4593510	-50.23%
DecodeInt64_zero	328.378	13.328	-95.94%
DecodeInt64_small	360.122	14.794	-95.89%
DecodeInt64_big_negative	563.4	29.224	-94.81%

name	encoding_json (B/op)	rjson (B/op)	delta
GetRepoValues	250	16	-93.60%
Valid_canada	145800	47619	-67.34%
Valid_citm_catalog	86129.9	24020	-72.11%
Valid_github_repo	1	0	-100.00%
Valid_twitter	12613.7	3345.5	-73.48%
DistinctUserIDs	26794	4072.67	-84.80%
ReadObject_canada	12070600	8434380	-30.12%
ReadObject_citm_catalog	5468880	5229700	-4.37%
ReadObject_github_repo	25704.1	17036.9	-33.72%
ReadObject_twitter	2190850	2717220	+24.03%
DecodeInt64_zero	184	0	-100.00%
DecodeInt64_small	186	0	-100.00%
DecodeInt64_big_negative	208	0	-100.00%

fastjson

name	fastjson (ns/op)	rjson (ns/op)	delta
GetRepoValues	4919.22	2156.9	-56.15%
ReadFloat64_zero	28.9222	14.504	-49.85%
ReadFloat64_smallInt	30.047	17.713	-41.05%
ReadFloat64_negExp	43.04	27.6944	-35.65%
ReadInt64_zero	28.564	12.406	-56.57%
ReadInt64_small	29.473	13.144	-55.40%
ReadInt64_big_negative	89.229	27.828	-68.81%
ReadString_short_ascii	62.68	39.98	-36.22%
ReadString_medium_ascii	574.967	1872.8	+225.72%
ReadString_medium	301.4	817.07	+171.09%
ReadBool	19.39	9.0604	-53.27%

name	fastjson (B/op)	rjson (B/op)	delta
GetRepoValues	18	16	-11.11%
ReadFloat64_zero	0	0	~
ReadFloat64_smallInt	0	0	~
ReadFloat64_negExp	0	0	~
ReadInt64_zero	0	0	~
ReadInt64_small	0	0	~
ReadInt64_big_negative	0	0	~
ReadString_short_ascii	5	5	~
ReadString_medium_ascii	896	896	~
ReadString_medium	384	384	~
ReadBool	0	0	~

gjson

name	gjson (ns/op)	rjson (ns/op)	delta
GetRepoValues	2408.33	2156.9	-10.44%
Valid_canada	2531630	2413060	-4.68%
Valid_citm_catalog	1665450	1645390	~
Valid_github_repo	7892.7	5485.7	-30.50%
Valid_twitter	806403	611203	-24.21%
DistinctUserIDs	1471150	743022	-49.49%
ReadObject_canada	64774800	32002000	-50.59%
ReadObject_citm_catalog	16946300	11808100	-30.32%
ReadObject_github_repo	31775.6	30023.4	-5.51%
ReadObject_twitter	7668040	4593510	-40.10%
ReadFloat64_zero	45.7111	14.504	-68.27%
ReadFloat64_smallInt	55.236	17.713	-67.93%
ReadFloat64_negExp	89.102	27.6944	-68.92%
ReadInt64_zero	51.251	12.406	-75.79%
ReadInt64_small	62.192	13.144	-78.87%
ReadInt64_big_negative	156.53	27.828	-82.22%
DecodeInt64_zero	52.371	13.328	-74.55%
DecodeInt64_small	60.197	14.794	-75.42%
DecodeInt64_big_negative	155.967	29.224	-81.26%
ReadString_short_ascii	106.93	39.98	-62.61%
ReadString_medium_ascii	3896.33	1872.8	-51.93%
ReadString_medium	1669.1	817.07	-51.05%
ReadBool	44.706	9.0604	-79.73%

name	gjson (B/op)	rjson (B/op)	delta
GetRepoValues	200	16	-92.00%
Valid_canada	49880.1	47619	-4.53%
Valid_citm_catalog	24880.3	24020	-3.46%
Valid_github_repo	0	0	~
Valid_twitter	4417.56	3345.5	-24.27%
DistinctUserIDs	32438.1	4072.67	-87.44%
ReadObject_canada	11677900	8434380	-27.78%
ReadObject_citm_catalog	6697300	5229700	-21.91%
ReadObject_github_repo	22679.1	17036.9	-24.88%
ReadObject_twitter	2431030	2717220	+11.77%
ReadFloat64_zero	0	0	~
ReadFloat64_smallInt	2	0	-100.00%
ReadFloat64_negExp	16	0	-100.00%
ReadInt64_zero	0	0	~
ReadInt64_small	2	0	-100.00%
ReadInt64_big_negative	24	0	-100.00%
DecodeInt64_zero	0	0	~
DecodeInt64_small	2	0	-100.00%
DecodeInt64_big_negative	24	0	-100.00%
ReadString_short_ascii	24	5	-79.17%
ReadString_medium_ascii	2960	896	-69.73%
ReadString_medium	1168	384	-67.12%
ReadBool	4	0	-100.00%

jsoniter

name	jsoniter (ns/op)	rjson (ns/op)	delta
GetRepoValues	2565.33	2156.9	-15.92%
DistinctUserIDs	1661190	743022	-55.27%
ReadObject_canada	73041000	32002000	-56.19%
ReadObject_citm_catalog	13473400	11808100	-12.36%
ReadObject_github_repo	45984.9	30023.4	-34.71%
ReadObject_twitter	6713870	4593510	-31.58%
ReadFloat64_zero	107.57	14.504	-86.52%
ReadFloat64_smallInt	112.99	17.713	-84.32%
ReadFloat64_negExp	149.69	27.6944	-81.50%
ReadInt64_zero	14.027	12.406	-11.56%
ReadInt64_small	21.35	13.144	-38.44%
ReadInt64_big_negative	46.1238	27.828	-39.67%
DecodeInt64_zero	99.286	13.328	-86.58%
DecodeInt64_small	105.21	14.794	-85.94%
DecodeInt64_big_negative	134.32	29.224	-78.24%
ReadString_short_ascii	50.339	39.98	-20.58%
ReadString_medium_ascii	4839.3	1872.8	-61.30%
ReadString_medium	2226.4	817.07	-63.30%
ReadBool	24.8511	9.0604	-63.54%

name	jsoniter (B/op)	rjson (B/op)	delta
GetRepoValues	360	16	-95.56%
DistinctUserIDs	385571	4072.67	-98.94%
ReadObject_canada	17315200	8434380	-51.29%
ReadObject_citm_catalog	5787990	5229700	-9.65%
ReadObject_github_repo	25469.3	17036.9	-33.11%
ReadObject_twitter	2452640	2717220	+10.79%
ReadFloat64_zero	16	0	-100.00%
ReadFloat64_smallInt	16	0	-100.00%
ReadFloat64_negExp	16	0	-100.00%
ReadInt64_zero	0	0	~
ReadInt64_small	0	0	~
ReadInt64_big_negative	0	0	~
DecodeInt64_zero	0	0	~
DecodeInt64_small	0	0	~
DecodeInt64_big_negative	0	0	~
ReadString_short_ascii	5	5	~
ReadString_medium_ascii	2936	896	-69.48%
ReadString_medium	1400	384	-72.57%
ReadBool	0	0	~

jsonparser

name	jsonparser (ns/op)	rjson (ns/op)	delta
GetRepoValues	2447.22	2156.9	-11.86%
DistinctUserIDs	1022040	743022	-27.30%
ReadFloat64_zero	43.557	14.504	-66.70%
ReadFloat64_smallInt	46.0313	17.713	-61.52%
ReadFloat64_negExp	67.8511	27.6944	-59.18%
ReadInt64_zero	25.7944	12.406	-51.90%
ReadInt64_small	27.1367	13.144	-51.56%
ReadInt64_big_negative	59.38	27.828	-53.14%
ReadString_short_ascii	49.6714	39.98	-19.51%
ReadString_medium_ascii	1806.89	1872.8	+3.65%
ReadString_medium	811.09	817.07	~
ReadBool	30.078	9.0604	-69.88%

name	jsonparser (B/op)	rjson (B/op)	delta
GetRepoValues	16	16	~
DistinctUserIDs	6170.9	4072.67	-34.00%
ReadFloat64_zero	0	0	~
ReadFloat64_smallInt	0	0	~
ReadFloat64_negExp	0	0	~
ReadInt64_zero	0	0	~
ReadInt64_small	0	0	~
ReadInt64_big_negative	0	0	~
ReadString_short_ascii	5	5	~
ReadString_medium_ascii	1920	896	-53.33%
ReadString_medium	768	384	-50.00%
ReadBool	0	0	~

goccyjson

name	goccyjson (ns/op)	rjson (ns/op)	delta
GetRepoValues	7488.44	2156.9	-71.20%
ReadObject_canada	47965900	32002000	-33.28%
ReadObject_citm_catalog	15102800	11808100	-21.82%
ReadObject_github_repo	43846.8	30023.4	-31.53%
ReadObject_twitter	14948500	4593510	-69.27%

name	goccyjson (B/op)	rjson (B/op)	delta
GetRepoValues	6152	16	-99.74%
ReadObject_canada	10980000	8434380	-23.18%
ReadObject_citm_catalog	7411250	5229700	-29.44%
ReadObject_github_repo	26592.9	17036.9	-35.93%
ReadObject_twitter	2811040	2717220	-3.34%

simdjson

name	simdjson (ns/op)	rjson (ns/op)	delta
GetRepoValues	11510.1	2156.9	-81.26%
DistinctUserIDs	992518	743022	-25.14%
ReadObject_canada	30483200	32002000	+4.98%
ReadObject_citm_catalog	10949300	11808100	+7.84%
ReadObject_github_repo	36584.2	30023.4	-17.93%
ReadObject_twitter	4399670	4593510	+4.41%

name	simdjson (B/op)	rjson (B/op)	delta
GetRepoValues	712	16	-97.75%
DistinctUserIDs	21623.2	4072.67	-81.17%
ReadObject_canada	12702400	8434380	-33.60%
ReadObject_citm_catalog	8301120	5229700	-37.00%
ReadObject_github_repo	23330.7	17036.9	-26.98%
ReadObject_twitter	2652500	2717220	+2.44%