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Frontmatter

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Author 1

Fetch.jl: a simple HTTP request function

inspired by fetch from JS

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253 μs

You can use Fetch.jl to request something from the web:

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177 μs
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fetch("https://api.github.com/users/JuliaLang").json()
364 ms
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66.2 μs

Response

The fetch function returns a special object, a FetchResponse, containing the response status and headers.

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230 μs
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response = fetch("https://api.github.com/users/JuliaLang")
7.1 ms
200
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response.status
14.4 μs
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response.headers
14.2 μs
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65.8 μs

Reading data

You can use .json(), .text() or .arrayBuffer() to read the response data. Let's read the response as JSON:

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253 μs
response.json()
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12.4 ms

Each response can only be read once. Reading it a second time will raise an error:

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175 μs
Error message

AssertionError: Body stream has already been read.

Stack trace

Here is what happened, the most recent locations are first:

  1. (::Main.workspace#5.var"#4#10"{Main.workspace#5.var"#5#11"{Main.workspace#5.FetchResponse}, Main.workspace#5.FetchResponse})()
    from Other cell: line 5
    		function()			ref = getfield(r, :bodyUsed_ref)			@assert !ref[] "Body stream has already been read."			ref[] = true			f()
  2. Show more...
response.json()
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---
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65.8 μs

There are three ways to read a response:

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170 μs
fetch("https://api.github.com/users/JuliaLang").json()
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6.8 ms
"{\n  \"login\": \"JuliaLang\",\n  \"id\": 743164,\n  \"node_id\": \"MDEyOk9yZ2FuaXphdGlvbjc0MzE2NA==\",\n  \"avatar_url\": \"https://avatars.githubusercontent.com/u/743164?v=4\",\n  \"gravatar_id\": \"\",\n  \"url\": \"https://api.github.com/users/JuliaLang\",\n  \"html_url\": \"https://github.com/JuliaLang\",\n  \"followers_url\": \"" ⋯ 820 bytes ⋯ "ocation\": null,\n  \"email\": null,\n  \"hireable\": null,\n  \"bio\": \"The Julia Programming Language\",\n  \"twitter_username\": \"JuliaLanguage\",\n  \"public_repos\": 59,\n  \"public_gists\": 0,\n  \"followers\": 1668,\n  \"following\": 0,\n  \"created_at\": \"2011-04-21T06:33:51Z\",\n  \"updated_at\": \"2025-06-09T21:01:31Z\"\n}\n"
fetch("https://api.github.com/users/JuliaLang").text()
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6.8 ms
fetch("https://api.github.com/users/JuliaLang").arrayBuffer()
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6.9 ms
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65.2 μs

Request

Besides getting data from a URL, you can also make requests with a body. Here is an example of a POST request:

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268 μs
fetch(
"https://httpbin.org/post";
method="POST",
body="hello!",
).json()
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766 ms
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84.7 μs

Implementation

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165 μs
using Downloads
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210 μs
fetch (generic function with 1 method)
function fetch(url;
method=nothing,
headers=Pair{String,String}[],
body::Union{Nothing,AbstractVector{UInt8},AbstractString}=nothing,
# credentials=nothing,
# cache=nothing,
# redirect=nothing,
# referrer,
# referrerPolicy,
# integrity,
# keepalive
# signal
)
has_input = body !== nothing
input = has_input ? IOBuffer(body_data(body); write=false, append=false, read=true) : nothing
output = IOBuffer()
result = request(url;
method,
headers,
input,
output,
)

FetchResponse(;
url,
body=take!(output),
status=result.status,
statusText=result.message,
headers=result.headers,
)
end
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2.6 ms
begin
Base.@kwdef struct FetchResponse
url::String
body::Vector{UInt8}
bodyUsed_ref::Ref{Bool}=Ref(false)
status::Int
statusText::String=""
headers :: Vector{Pair{String,String}}
ok::Bool=true
redirected::Bool=false
end
function Base.getproperty(r::FetchResponse, name::Symbol)
handle_getproperty(r, name)
end
end
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37.1 ms

Reading response data

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183 μs
using JSON
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97.2 ms
handle_getproperty (generic function with 1 method)
function handle_getproperty(r, name)
function with_use_body(f)
function()
ref = getfield(r, :bodyUsed_ref)
@assert !ref[] "Body stream has already been read."
ref[] = true
f()
end
end
if name === :json
with_use_body() do
JSON.parse(String(r.body))
end
elseif name === :text
with_use_body() do
String(r.body)
end
elseif name === :arrayBuffer
with_use_body() do
r.body
end
elseif name === :formData || name === :formdata
with_use_body() do
read_form_data(r)
end
elseif name === :bodyUsed
getfield(r, :bodyUsed_ref)[]
else
getfield(r, name)
end
end
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16.8 ms
body_data (generic function with 1 method)
body_data(x::AbstractVector{UInt8}) = x
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380 μs
body_data (generic function with 2 methods)
body_data(x::AbstractString) = codeunits(x)
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385 μs

Examples

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163 μs
"https://httpbin.org/uuid"
const url_uuid = "https://httpbin.org/uuid"
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109 μs
"https://registry.npmjs.org/react"
const url_npm_react = "https://registry.npmjs.org/react"
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110 μs
r = request(url_uuid)
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705 ms

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65.5 μs

Multipart form data

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169 μs
import HTTP
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118 ms
FormFile
Base.@kwdef struct FormFile
data
name
contenttype
filename
end
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3.0 ms
read_form_data (generic function with 1 method)
function read_form_data(r)
fs = HTTP.parse_multipart_form(
HTTP.Request(
"GET",
"http://asfd.com",
r.headers,
r.body
)
)

map(fs) do f
FormFile(;
data = take!(f.data),
name = f.name,
filename = f.filename,
contenttype = f.contenttype,
)
end
end
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1.7 ms
rm = HTTP.parse_multipart_form(HTTP.Request("GET", "http://asfd.com", ["Content-Type" => "multipart/form-data; boundary=WebKitFormBoundary"], mpt))
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353 ms
HTTP.Multipart(filename="invoice.pdf", data=::Base.GenericIOBuffer{SubArray{UInt8, 1, Vector{UInt8}, Tuple{UnitRange{Int64}}, true}}, contenttype="text/plain", contenttransferencoding=""))
rm1 = rm[3]
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18.2 μs
Error message

syntax: incomplete: premature end of input

computer bad, you GREAT!
rm1.
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---
take!(rm1.data)
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19.9 μs
"\r\n--WebKitFormBoundary\r\nContent-Disposition: form-data; name=\"text\"\r\n\r\ninvoice_text\r\n--WebKitFormBoundary\r\nContent-Disposition: form-data; name=\"title\"\r\n\r\ninvoice_title\r\n--WebKitFormBoundary\r\nContent-Disposition: form-data; name=\"invoice\"; filename=\"invoice.pdf\"\r\nContent-Type: application/pdf\r\n\r\n\x90\xd7?\x1e\x13\xb9&\x1c\xef\xd3\xefb\xb2>\r\xa8\xb5\x90\x92Hc\xa4{\xcazA\u05cee\xa2\x87\x94SH\xaa8\x90\xb6\x19\xc4\xc18\xc0\xab0)|\xab\xed\xf5\r\n--WebKitFormBoundary--\r\n"
const mpt = replace("""

--WebKitFormBoundary
Content-Disposition: form-data; name="text"

invoice_text
--WebKitFormBoundary
Content-Disposition: form-data; name="title"

invoice_title
--WebKitFormBoundary
Content-Disposition: form-data; name="invoice"; filename="invoice.pdf"
Content-Type: application/pdf

$(String(rand(UInt8, 50)))
--WebKitFormBoundary--
""", "\n" => "\r\n")
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108 ms
# 1 method for generic function parse_multipart_form:
methods(HTTP.parse_multipart_form)
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31.8 μs

Experiments

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187 μs
let
io = IOBuffer(; maxsize=100)

xs = zeros(Int, 10000)

r = request(url_npm_react;
output=io
)
for i in eachindex(xs)
xs[i] = io.ptr
end
xs
end
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469 ms
f1 (generic function with 1 method)
function f1(url)
pipe = Pipe()
Base.link_pipe!(pipe; reader_supports_async = true, writer_supports_async = true)
json_task = @async JSON.parse(read(pipe, String))
request(url;
output=pipe.in,
)
fetch(json_task)
end
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1.8 ms
f2 (generic function with 1 method)
function f2(url)
s = sprint() do io
request(url;
output=io,
)
end

JSON.parse(s)
end
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1.0 ms
@time f2(url_uuid)
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❔
  0.291953 seconds (346 allocations: 19.594 KiB, 1.33% compilation time)
335 ms
Error message

MethodError: no method matching request(::Task; method=nothing, headers=Pair{String, String}[], input=nothing, output=IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=Inf, ptr=1, mark=-1))

Closest candidates are:

request(::AbstractString; input, output, method, headers, timeout, progress, verbose, debug, throw, downloader) at /opt/hostedtoolcache/julia/1.7.3/x64/share/julia/stdlib/v1.7/Downloads/src/Downloads.jl:293

Stack trace

Here is what happened, the most recent locations are first:

  1. fetch(url::Task; method::Nothing, headers::Vector{Pair{String, String}}, body::Nothing)
    from Other cell: line 18
    	output = IOBuffer()		result = request(url;		method,		headers,
  2. Show more...
computer bad, you GREAT!
@time f1(url_uuid)
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---
@time f2(url_npm_react)
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❔
  0.323751 seconds (816.37 k allocations: 63.980 MiB, 9.85% gc time, 17.04% compilation time)
330 ms
Error message

MethodError: no method matching request(::Task; method=nothing, headers=Pair{String, String}[], input=nothing, output=IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=Inf, ptr=1, mark=-1))

Closest candidates are:

request(::AbstractString; input, output, method, headers, timeout, progress, verbose, debug, throw, downloader) at /opt/hostedtoolcache/julia/1.7.3/x64/share/julia/stdlib/v1.7/Downloads/src/Downloads.jl:293

Stack trace

Here is what happened, the most recent locations are first:

  1. fetch(url::Task; method::Nothing, headers::Vector{Pair{String, String}}, body::Nothing)
    from Other cell: line 18
    	output = IOBuffer()		result = request(url;		method,		headers,
  2. Show more...
@time f1(url_npm_react)
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---
"<html>\r\n<head><title>502 Bad Gateway</title></head>\r\n<body>\r\n<center><h1>502 Bad Gateway</h1></center>\r\n</body>\r\n</html>\r\n"
sprint() do io
request(url_uuid;
output=io
)
end
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2.1 s

JSON.parse(io::IO) benchmark

Some benchmarks with https://github.com/JuliaIO/JSON.jl/issues/339 as conclusion.

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j1 (generic function with 1 method)
j1(io) = JSON.parse(read(io, String))
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430 μs
j2 (generic function with 1 method)
j2(io) = JSON.parse(io)
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"{\"_id\":\"react\",\"_rev\":\"4556-38606e4086d9df83b101248af9d3d7b0\",\"name\":\"react\",\"dist-tags\":{\"beta\":\"19.0.0-beta-26f2496093-20240514\",\"rc\":\"19.0.0-rc.1\",\"latest\":\"19.1.0\",\"experimental\":\"0.0.0-experimental-12bc60f5-20250613\",\"next\":\"19.2.0-canary-12bc60f5-20250613\",\"canary\":\"19.2.0-canary-12bc60f5-2" ⋯ 5779370 bytes ⋯ "ue,\"javascriptismagic\":true,\"sebastian-schmidt\":true,\"arulsakthiprakasam\":true,\"ashtonsoftwarelabs\":true,\"avanthikameenakshi\":true,\"emesfun_blueraster\":true,\"vision_tecnologica\":true,\"azulejosmetrosubway\":true,\"robertocarvalho7458\":true,\"dylanthomasfernandez\":true,\"nguyenvanhoang26041994\":true}}"
nrs = sprint() do io
request(url_npm_react;
output=io,
)
end
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211 ms
using BenchmarkTools
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768 ms
BenchmarkTools.Trial: 87 samples with 1 evaluation.
 Range (min … max):  41.291 ms … 106.524 ms  ┊ GC (min … max):  0.00% … 48.97%
 Time  (median):     53.670 ms               ┊ GC (median):    20.11%
 Time  (mean ± σ):   57.682 ms ±  13.545 ms  ┊ GC (mean ± σ):  19.33% ± 14.43%

   █   ▂  █     ▄▆▂       ▂                                     
  ██▆▁▆██▄██▄▁▁▄███▄▄▁▁▄▄▆█▆▄▆▄▁▁▁▁█▄▆▄▁▄▁▁▁▁▆▄█▁▄▁▄▄▄▁▄▄▁▁▁▁▄ ▁
  41.3 ms         Histogram: frequency by time         88.7 ms <

 Memory estimate: 57.59 MiB, allocs estimate: 762476.
@benchmark let
io = IOBuffer()
write(io, $nrs)
seekstart(io)
r = j1(io)
close(io)
r
end
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12.4 s
BenchmarkTools.Trial: 46 samples with 1 evaluation.
 Range (min … max):   82.137 ms … 249.694 ms  ┊ GC (min … max):  0.00% … 66.38%
 Time  (median):     102.759 ms               ┊ GC (median):    17.69%
 Time  (mean ± σ):   111.442 ms ±  27.698 ms  ┊ GC (mean ± σ):  23.09% ± 13.85%

  ▂   ▂  ▄        █                                              
  █▁▃▇█▃▁█▁▁▁▁▅▁▃▁█▆▁▁▃▃▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▃ ▁
  82.1 ms          Histogram: frequency by time          250 ms <

 Memory estimate: 103.57 MiB, allocs estimate: 1282427.
@benchmark let
io = IOBuffer()
write(io, $nrs)
seekstart(io)
r = j2(io)
close(io)
r
end
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12.5 s
parse(io::IO; dicttype, inttype, allownan, null) in JSON.Parser at /home/runner/.julia/packages/JSON/NeJ9k/src/Parser.jl:474
@which JSON.parse(stdin)
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105 ms
parse(str::AbstractString; dicttype, inttype, allownan, null) in JSON.Parser at /home/runner/.julia/packages/JSON/NeJ9k/src/Parser.jl:443
@which JSON.parse("asdf")
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83.7 μs