hackney - HTTP client library in Erlang

Copyright (c) 2012-2019 Benoît Chesneau.

Version: 1.15.1


hackney is an HTTP client library for Erlang.

Build Status Hex pm

Main features:

  • no message passing (except for asynchronous responses): response is directly streamed to the current process and state is kept in a #client{} record.
  • binary streams
  • SSL support
  • Keepalive handling
  • basic authentication
  • stream the response and the requests
  • fetch a response asynchronously
  • multipart support (streamed or not)
  • chunked encoding support
  • Can send files using the sendfile API
  • Optional socket pool
  • REST syntax: hackney:Method(URL) (where a method can be get, post, put, delete, ...)

Supported versions of Erlang are R16B03-1, 17.3.4 and above. It is reported to work with R14B04 and R15B03-1.

WARNING: Erlang 17.3 and 17.3.1 have a broken SSL module which prevents the usage of SSL connection with some servers. You must upgrade in that case to Erlang 17.3.4 or superior.

Note: This is a work in progress, see the TODO for more information on what still needs to be done.

Useful modules are:

  • hackney: main module. It contains all HTTP client functions.

  • hackney_http: HTTP parser in pure Erlang. This parser is able to parse HTTP responses and requests in a streaming fashion. If not set it will be autodetected if it's a request or a response that's needed.

  • hackney_headers Module to manipulate HTTP headers.

  • hackney_cookie: Module to manipulate cookies.

  • hackney_multipart: Module to encode/decode multipart.

  • hackney_url: Module to parse and create URIs.

  • hackney_date: Module to parse HTTP dates.

Read the NEWS file to get the last changelog.


Download the sources from our Github repository

To build the application simply run 'rebar3 compile'.

To run tests run 'rebar3 eunit'. To generate doc, run 'rebar3 edoc'.

Or add it to your rebar config

{deps, [
    {hackney, ".*", {git, "git://", {branch, "master"}}}

Basic usage

The basic usage of hackney is:

Start hackney

hackney is an OTP application. You have to start it first before using any of the functions. The hackney application will start the default socket pool for you.

To start in the console run:

$ ./rebar3 shell

It is suggested that you install rebar3 user-wide as described here. This fixes zsh (and maybe other shells) escript-related bugs. Also this should speed things up.

> application:ensure_all_started(hackney).

It will start hackney and all of the application it depends on:


Or add hackney to the applications property of your .app in a release

Simple request

Do a simple request that will return a client state:

Method = get,
URL = <<"">>,
Headers = [],
Payload = <<>>,
Options = [],
{ok, StatusCode, RespHeaders, ClientRef} = hackney:request(Method, URL,
                                                        Headers, Payload,

The request method returns the tuple {ok, StatusCode, Headers, ClientRef} or {error, Reason}. A ClientRef is simply a reference to the current request that you can reuse.

If you prefer the REST syntax, you can also do:

hackney:Method(URL, Headers, Payload, Options)

where Method, can be any HTTP method in lowercase.

Read the body

{ok, Body} = hackney:body(ClientRef).

hackney:body/1 fetch the body. To fetch it by chunk you can use the hackney:stream_body/1 function:

read_body(MaxLength, Ref, Acc) when MaxLength > byte_size(Acc) ->
	case hackney:stream_body(Ref) of
		{ok, Data} ->
			read_body(MaxLength, Ref, << Acc/binary, Data/binary >>);
		done ->
			{ok, Acc};
		{error, Reason} ->
			{error, Reason}

Note: you can also fetch a multipart response using the functions hackney:stream_multipart/1 and hackney:skip_multipart/1.

Note 2: using the with_body option will return the body directy instead of a reference.

Reuse a connection

By default all connections are created and closed dynamically by hackney but sometimes you may want to reuse the same reference for your connections. It's especially useful if you just want to handle serially a couple of requests.

A closed connection will automatically be reconnected.

To create a connection:

Transport = hackney_ssl,
Host = << "" >>,
Port = 443,
Options = [],
{ok, ConnRef} = hackney:connect(Transport, Host, Port, Options).

To create a connection that will use an HTTP proxy use hackney_http_proxy:connect_proxy/5 instead.

To get local and remote ip and port information of a connection:

> hackney:peername(ConnRef).
> hackney:sockname(ConnRef).

Make a request

Once you created a connection use the hackney:send_request/2 function to make a request:

ReqBody = << "{	\"snippet\": \"some snippet\" }" >>,
ReqHeaders = [{<<"Content-Type">>, <<"application/json">>}],
NextPath = <<"/">>,
NextMethod = post,
NextReq = {NextMethod, NextPath, ReqHeaders, ReqBody},
{ok, _, _, ConnRef} = hackney:send_request(ConnRef, NextReq),
{ok, Body1} = hackney:body(ConnRef).

Here we are posting a JSON payload to '/' on the friendpaste service to create a paste. Then we close the client connection.

If your connection supports keepalive the connection will be kept open until you close it exclusively.

Send a body

hackney helps you send different payloads by passing different terms as the request body:

  • {form, PropList} : To send a form
  • {multipart, Parts} : to send your body using the multipart API. Parts follow this format:
    • eof: end the multipart request
    • {file, Path}: to stream a file
    • {file, Path, ExtraHeaders}: to stream a file
    • {file, Path, Name, ExtraHeaders} : to send a file with DOM element name and extra headers
    • {Name, Content}: to send a full part
    • {Name, Content, ExtraHeaders}: to send a full part
    • {mp_mixed, Name, MixedBoundary}: To notify we start a part with a a mixed multipart content
    • {mp_mixed_eof, MixedBoundary}: To notify we end a part with a a mixed multipart content
  • {file, File} : To send a file
  • Bin: To send a binary or an iolist

Note: to send a chunked request, just add the Transfer-Encoding: chunked header to your headers. Binary and Iolist bodies will be then sent using the chunked encoding.

Send the body by yourself

While the default is to directly send the request and fetch the status and headers, if the body is set as the atom stream the request and send_request function will return {ok, Client}. Then you can use the function hackney:send_body/2 to stream the request body and hackney:start_response/1 to initialize the response.

Note: The function hackney:start_response/1 will only accept a Client that is waiting for a response (with a response state equal to the atom waiting).


ReqBody = << "{
      \"id\": \"some_paste_id2\",
      \"rev\": \"some_revision_id\",
      \"changeset\": \"changeset in unidiff format\"
}" >>,
ReqHeaders = [{<<"Content-Type">>, <<"application/json">>}],
Path = <<"">>,
Method = post,
{ok, ClientRef} = hackney:request(Method, Path, ReqHeaders, stream, []),
ok  = hackney:send_body(ClientRef, ReqBody),
{ok, _Status, _Headers, ClientRef} = hackney:start_response(ClientRef),
{ok, Body} = hackney:body(ClientRef),

Note: to send a multipart body in a streaming fashion use the hackney:send_multipart_body/2 function.

Get a response asynchronously

Since the 0.6 version, hackney is able to fetch the response asynchronously using the async option:

Url = <<"">>,
Opts = [async],
LoopFun = fun(Loop, Ref) ->
            {hackney_response, Ref, {status, StatusInt, Reason}} ->
                io:format("got status: ~p with reason ~p~n", [StatusInt,
                Loop(Loop, Ref);
            {hackney_response, Ref, {headers, Headers}} ->
                io:format("got headers: ~p~n", [Headers]),
                Loop(Loop, Ref);
            {hackney_response, Ref, done} ->
            {hackney_response, Ref, Bin} ->
                io:format("got chunk: ~p~n", [Bin]),
                Loop(Loop, Ref);

            Else ->
                io:format("else ~p~n", [Else]),

{ok, ClientRef} = hackney:get(Url, [], <<>>, Opts),
LoopFun(LoopFun, ClientRef).

Note 1: When {async, once} is used the socket will receive only once. To receive the other messages use the function hackney:stream_next/1.

Note 2: Asynchronous responses automatically checkout the socket at the end.

Note 3: At any time you can go back and receive your response synchronously using the function hackney:stop_async/1 See the example test_async_once2 for the usage.

Note 4: When the option {follow_redirect, true} is passed to the request, you will receive the folllowing messages on valid redirection:

  • {redirect, To, Headers}
  • {see_other, To, Headers} for status 303 and POST requests.

Note 5: You can send the messages to another process by using the option {stream_to, Pid} .

Use the default pool

To reuse a connection globally in your application you can also use a socket pool. On startup, hackney launches a pool named default. To use it do the following:

Method = get,
URL = <<"">>,
Headers = [],
Payload = <<>>,
Options = [{pool, default}],
{ok, StatusCode, RespHeaders, ClientRef} = hackney:request(Method, URL, Headers,
                                                        Payload, Options).

By adding the tuple {pool, default} to the options, hackney will use the connections stored in that pool.

You can also use different pools in your application which allows you to maintain a group of connections.

PoolName = mypool,
Options = [{timeout, 150000}, {max_connections, 100}],
ok = hackney_pool:start_pool(PoolName, Options),

timeout is the time we keep the connection alive in the pool, max_connections is the number of connections maintained in the pool. Each connection in a pool is monitored and closed connections are removed automatically.

To close a pool do:


Note: Sometimes you want to disable the default pool in your app without having to set the client option each time. You can now do this by setting the hackney application environment key use_default_pool to false.

Use a custom pool handler.

Since the version 0.8 it is now possible to use your own Pool to maintain the connections in hackney.

A pool handler is a module that handles the hackney_pool_handler behaviour.

See for example the hackney_disp a load-balanced Pool dispatcher based on dispcount].

Note: for now you can`t force the pool handler / client.

Automatically follow a redirection

If the option {follow_redirect, true} is given to the request, the client will be able to automatically follow the redirection and retrieve the body. The maximum number of connections can be set using the {max_redirect, Max} option. Default is 5.

The client will follow redirects on 301, 302 & 307 if the method is get or head. If another method is used the tuple {ok, maybe_redirect, Status, Headers, Client} will be returned. It will only follow 303 redirects (see other) if the method is a POST.

Last Location is stored in the location property of the client state.


Method = get,
URL = "",
ReqHeaders = [{<<"accept-encoding">>, <<"identity">>}],
ReqBody = <<>>,
Options = [{follow_redirect, true}, {max_redirect, 5}],
{ok, S, H, Ref} = hackney:request(Method, URL, ReqHeaders,
                                     ReqBody, Options),
{ok, Body1} = hackney:body(Ref).

Proxy a connection

HTTP Proxy

To use an HTTP tunnel add the option {proxy, ProxyUrl} where ProxyUrl can be a simple url or an {Host, Port} tuple. If you need to authenticate set the option {proxy_auth, {User, Password}}.

SOCKS5 proxy

Hackney supports the connection via a socks5 proxy. To set a socks5 proxy, use the following settings:

  • {proxy, {socks5, ProxyHost, ProxyPort}}: to set the host and port of the proxy to connect.
  • {socks5_user, Username}: to set the user used to connect to the proxy
  • {socks5_pass, Password}: to set the password used to connect to the proxy

SSL and TCP connections can be forwarded via a socks5 proxy. hackney is automatically upgrading to an SSL connection if needed.


Hackney offers the following metrics

You can enable metrics collection by adding a mod_metrics entry to hackney's app config. Metrics are disabled by default. The module specified must have an API matching that of the hackney metrics module.

To use folsom, specify {mod_metrics, folsom}, or if you want to use exometer, specify{mod_metrics, exometer} and ensure that folsom or exometer is in your code path and has been started.

Generic Hackney metrics

Name Type Description
hackney.nb_requests counter Number of running requests
hackney.total_requests counter Total number of requests
hackney.finished_requests counter Total number of requests finished

Metrics per Hosts

Name Type Description
hackney.HOST.nb_requests counter Number of running requests
hackney.HOST.request_time histogram Request time
hackney.HOST.connect_time histogram Connect time
hackney.HOST.response_time histogram Response time
hackney.HOST.connect_timeout counter Number of connect timeout
hackney.HOST.connect_error counter Number of timeout errors

Metrics per Pool

Name Type Description
hackney.POOLNAME.take_rate meter meter recording rate at which a connection is retrieved from the pool
hackney.POOLNAME.no_socket counter Count of new connections
hackney.POOLNAME.in_use_count histogram How many connections from the pool are used
hackney.POOLNAME.free_count counter Number of free sockets in the pool
hackney.POOLNAME.queue_counter histogram queued clients


For issues, comments or feedback please create an issue.

Notes for developers

If you want to contribute patches or improve the docs, you will need to build hackney using the rebar_dev.config file. It can also be built using the Makefile:

$ rebar3 update
$ rebar3 compile

For successfully running the hackney test suite locally it is necessary to install httpbin.

An example installation using virtualenv::

$ mkvirtualenv hackney
$ pip install gunicorn httpbin

Running the tests:

$ gunicorn --daemon --pid httpbin:app
$ make test
$ kill `cat`