HTTP Guide

This guide covers hackney's HTTP features in depth.

Request Anatomy

hackney:request(Method, URL, Headers, Body, Options) ->
    {ok, StatusCode, RespHeaders, Body} | {error, Reason}

Body is always returned directly in the response for consistent behavior across HTTP/1.1, HTTP/2, and HTTP/3.

Request Bodies

Binary Body

hackney:post(URL,
    [{<<"content-type">>, <<"application/json">>}],
    <<"{\"key\": \"value\"}">>
).

Form-Encoded Body

hackney:post(URL, [], {form, [{<<"key">>, <<"value">>}]}).

Multipart Body

Multipart requests are used to upload files and send form data together.

Basic File Upload

hackney:post(URL, [], {multipart, [
    {<<"field">>, <<"value">>},
    {file, <<"/path/to/file.txt">>}
]}).

File Upload with Custom Field Name

Use {file, Path, FieldName, ExtraHeaders} to specify the form field name:

%% Upload file to "attachment" field instead of default "file"
hackney:post(URL, [], {multipart, [
    {file, <<"/path/to/document.pdf">>, <<"attachment">>, []}
]}).

File Upload with Full Control

For complete control over the Content-Disposition header:

Path = <<"/path/to/photo.jpg">>,
FName = hackney_bstr:to_binary(filename:basename(Path)),
Disposition = {<<"form-data">>,
               [{<<"name">>, <<"photo">>},
                {<<"filename">>, FName}]},
hackney:post(URL, [], {multipart, [
    {file, Path, Disposition, []}
]}).

Mixed File and Text Fields

Combine file uploads with text fields:

hackney:post(URL, [], {multipart, [
    {file, <<"/path/to/image.jpg">>, <<"image">>, []},
    {<<"title">>, <<"My Photo">>},
    {<<"description">>, <<"A nice picture">>}
]}).

Text Fields with Explicit Content-Type

Some servers require explicit content-type for text fields:

hackney:post(URL, [], {multipart, [
    {file, <<"/path/to/doc.pdf">>, <<"document">>, []},
    {<<"name">>, <<"Report">>, [{<<"content-type">>, <<"text/plain">>}]}
]}).

Supported Part Formats

Streaming Body

{ok, Ref} = hackney:post(URL, Headers, stream),
ok = hackney:send_body(Ref, <<"chunk1">>),
ok = hackney:send_body(Ref, <<"chunk2">>),
ok = hackney:finish_send_body(Ref),
{ok, Status, RespHeaders, Ref} = hackney:start_response(Ref).

Response Handling

Read Full Body

%% Body is returned directly
{ok, 200, Headers, Body} = hackney:get(URL).

Automatic Decompression

Hackney can automatically decompress gzip and deflate encoded responses:

{ok, 200, Headers, Body} = hackney:get(URL, [], <<>>, [
    {auto_decompress, true}
]).

When auto_decompress is enabled:

• Adds Accept-Encoding: gzip, deflate header to requests
• Automatically decompresses the response body based on Content-Encoding
• Supports gzip, deflate, and x-gzip encodings
• Non-compressed responses are returned unchanged

Stream Response Body (Async Mode)

For incremental body streaming, use async mode:

{ok, Ref} = hackney:get(URL, [], <<>>, [async]),
stream_loop(Ref).

stream_loop(Ref) ->
    receive
        {hackney_response, Ref, {status, Status, _}} ->
            io:format("Status: ~p~n", [Status]),
            stream_loop(Ref);
        {hackney_response, Ref, {headers, Headers}} ->
            io:format("Headers: ~p~n", [Headers]),
            stream_loop(Ref);
        {hackney_response, Ref, done} ->
            ok;
        {hackney_response, Ref, Chunk} when is_binary(Chunk) ->
            process_chunk(Chunk),
            stream_loop(Ref)
    end.

HTTP/2 Support

Hackney automatically negotiates HTTP/2 for HTTPS connections via ALPN.

Response format is consistent across all protocols - body is always returned directly.

Automatic HTTP/2

%% HTTP/2 used automatically when server supports it
{ok, 200, Headers, Body} = hackney:get(<<"https://nghttp2.org/">>).

Force Protocol

%% HTTP/2 only
hackney:get(URL, [], <<>>, [{protocols, [http2]}]).

%% HTTP/1.1 only
hackney:get(URL, [], <<>>, [{protocols, [http1]}]).

Detect Protocol

HTTP/2 responses have lowercase header names:

case hd(Headers) of
    {<<"date">>, _} -> http2;
    {<<"Date">>, _} -> http1
end.

For details on multiplexing, server push, and architecture, see the HTTP/2 Guide.

Async Responses

{ok, Ref} = hackney:get(URL, [], <<>>, [async]),
receive
    {hackney_response, Ref, {status, Status, _}} -> ok
end,
receive
    {hackney_response, Ref, {headers, Headers}} -> ok
end,
receive
    {hackney_response, Ref, done} -> ok;
    {hackney_response, Ref, Bin} -> ok
end.

Async Once

{ok, Ref} = hackney:get(URL, [], <<>>, [{async, once}]),
receive {hackney_response, Ref, Msg} -> ok end,
hackney:stream_next(Ref).  %% Request next message

Stream to Another Process

Use stream_to to send async messages to a different process:

Receiver = spawn(fun() -> receive_loop() end),
{ok, Ref} = hackney:get(URL, [], <<>>, [
    async,
    {stream_to, Receiver}
]).

When stream_to is specified:

• The connection is owned by the stream_to process, not the caller
• If stream_to dies, the connection terminates
• If the original caller dies, the connection continues as long as stream_to is alive
• This ensures proper cleanup when the message recipient terminates

Connection Pooling

Default Pool

hackney:get(URL).  %% Uses default pool

Named Pools

hackney_pool:start_pool(my_api, [
    {max_connections, 100},
    {timeout, 150000}
]),
hackney:get(URL, [], <<>>, [{pool, my_api}]).

Manual Connection Management

For fine-grained control, you can create a connection and reuse it for multiple requests. This works for both HTTP/1.1 and HTTP/2.

Get a Connection

%% Connect to a host (returns a connection PID)
{ok, ConnPid} = hackney:connect(hackney_ssl, "example.com", 443, []).

%% Or from a URL
{ok, ConnPid} = hackney:connect(<<"https://example.com">>).

Check the Protocol

%% See which protocol was negotiated
Protocol = hackney_conn:get_protocol(ConnPid).  %% http1 | http2 | http3

Send Requests on the Connection

%% Send multiple requests on the same connection
{ok, 200, Headers1, Body1} = hackney:send_request(ConnPid, {get, <<"/api/users">>, [], <<>>}).
{ok, 201, Headers2, Body2} = hackney:send_request(ConnPid, {post, <<"/api/users">>,
    [{<<"content-type">>, <<"application/json">>}],
    <<"{\"name\": \"Alice\"}">>}).
{ok, 200, Headers3, Body3} = hackney:send_request(ConnPid, {get, <<"/api/users/1">>, [], <<>>}).

Close the Connection

hackney:close(ConnPid).

Complete Example

%% Reuse a connection for multiple API calls
{ok, Conn} = hackney:connect(hackney_ssl, "api.example.com", 443, []),

%% Check protocol (optional)
case hackney_conn:get_protocol(Conn) of
    http2 -> io:format("Using HTTP/2 multiplexing~n");
    http1 -> io:format("Using HTTP/1.1 keep-alive~n")
end,

%% Make requests
{ok, 200, _, Token} = hackney:send_request(Conn, {post, <<"/auth">>, [], Credentials}),
{ok, 200, _, Users} = hackney:send_request(Conn, {get, <<"/users">>, AuthHeaders, <<>>}),
{ok, 200, _, Data} = hackney:send_request(Conn, {get, <<"/data">>, AuthHeaders, <<>>}),

%% Clean up
hackney:close(Conn).

HTTP/1.1 vs HTTP/2 Behavior

For HTTP/2 multiplexing (parallel requests on one connection), see the HTTP/2 Guide.

Redirects

{ok, 200, Headers, Body} = hackney:get(URL, [], <<>>, [
    {follow_redirect, true},
    {max_redirect, 5}
]).

Proxies

HTTP Proxy

hackney:get(URL, [], <<>>, [
    {proxy, <<"http://proxy:8080">>}
]).

Environment Variables

hackney reads HTTP_PROXY, HTTPS_PROXY, NO_PROXY automatically.

SSL/TLS

Custom CA Certificate

hackney:get(URL, [], <<>>, [
    {ssl_options, [{cacertfile, "/path/to/ca.crt"}]}
]).

Skip Verification

hackney:get(URL, [], <<>>, [insecure]).

Timeouts

hackney:get(URL, [], <<>>, [
    {connect_timeout, 5000},
    {recv_timeout, 30000}
]).

Cookies

hackney:get(URL, [], <<>>, [{cookie, <<"session=abc">>}]).

%% Parse response cookies
{ok, 200, Headers, _} = hackney:get(URL),
Cookies = hackney:cookies(Headers).

Basic Authentication

hackney:get(URL, [], <<>>, [
    {basic_auth, {<<"user">>, <<"pass">>}}
]).

Next Steps

• HTTP/2 Guide - Multiplexing, server push, architecture
• WebSocket Guide
• Migration Guide