Erlang HTTP linked-in driver
C Erlang
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gen_http driver for erlang

This module is a high efficient special linked-in driver that handled HTTP on C level.

It uses code based on Nginx http parser, which is beleived to be one of the fastest on earth.

gen_http has non-gen_tcp API. It has other call for opening socket, accepting connection and receiving data.

gen_http is built around idea of handling HTTP protocol.

% listener.erl:
{ok, Listener} = gen_http:listen(8080, [{reuseaddr,true},{backlog,400}]),
microtcp:accept_once(Socket), % For accepting next connection
  {http_connection, Listen, Socket} ->
    Pid = spawn(fun() -> handle_client() end),
    gen_http:controlling_process(Socket, Pid),
    Pid ! {socket, Socket},
      {done, Pid}

handle_client() ->
  Socket = receive
    {socket, S} -> S

client_loop(Socket) ->
  {http, Socket, Method, URL, Keepalive, Version, Headers} = Req ->
    gen_http:send(Socket, "HTTP/1.1 200 OK\r\n\r\n"),
    {http_closed, Socket} ->
    {http_error, Socket, Error} ->


gen_http can be used in server and in client mode.

Server mode

Server mode is divided in two parts: listener and worker.

Listener socket is created with {ok, Listen} = gen_http:listen(Port, Options) call.

Then listener socket must be activated with gen_http:accept_once(Listen) call. This call may be done from many processes, multiaccept is supported.

Listener socket will send message {http_connection, Listen, Client} to some of accepting processes. Synchronous call accept(Listen, Timeout) is emulated via receive .. after ..

Standard call gen_http:controlling_process(Client, HandlerPid) is supported.

After receiving socket it is handler task to work with it.

Handler mode when being server is very similar to client mode. Slight difference in request/response message format.

Client mode

To establish connection to remote server {ok, Socket} = gen_http:connect(Host, Port, Options) should be used.

After this call make request: gen_http:send(Socket, ["GET / HTTP/1.1\r\nHost: localhost\r\n\r\n"])

When this call returns, use next chapter API, because socket enters in the same {active,once} state, as in handler mode.

Client/Handler mode

After receiving request or response headers handler process should call gen_http:active_once(Socket) to receive messages from socket.

Socket can send following list of messages:

  • {http, Socket, Method, URL, Keepalive, Version, Headers} received when remote peer sends request to us (in server mode);
  • {http, Socket, Status, Keepalive, Version, Headers} received when remote peer replies with some response to us (in client mode);
  • {http, Socket, Bin} when is_binary(Bin) part of body;
  • {http, Socket, eof} when body is over;
  • {http, Socket, connected} when remote server accepts our connection and we can send HTTP request there;
  • {http, Socket, empty} when socket has sent all scheduled data to remote peer. It can be used to emulate blocking gen_http:send(Socket, Data) call;
  • {http_error, Socket, Error} when some error has happened;
  • {http_closed, Socket} after socket was closed. It can be received for example when accepting

Mention Keepalive option. It can be keepalive or close. Use this value to determine whether you should close connection or send next message on this wire.

Receiving body issues

If you wish to receive body, than take a look at gen_http:setopts(Socket, [{chunk_size,ChunkSize}]).

gen_tcp driver has performance problems in active mode: messages are received of MTU size. If you download 1GB file, you may receive hundreds of thousands small messages, while you require only several tens of them, but big.

gen_http driver will try to accumulate binaries in rather efficient way inside in chunks not more than ChunkSize + 10KB (default buffer size). Don't rely that ChunkSize is maximum size. You can receive more data than you have asked to, but not too much.

Also received data may be less than ChunkSize if body is over. Mention, that there is no timeout for sending if remote peer doesn't send data. It may highly increase latency. Sorry, but you are worried.