Websocket

This chapter describes how to use the Gun client for communicating with a Websocket server.

HTTP upgrade

Websocket is a protocol built on top of HTTP. To use Websocket, you must first request for the connection to be upgraded. Only HTTP/1.1 connections can be upgraded to Websocket, so you might need to restrict the protocol to HTTP/1.1 if you are planning to use Websocket over TLS.

You must use the gun:ws_upgrade/2,3,4 function to upgrade to Websocket. This function can be called anytime after connection, so you can send HTTP requests before upgrading to Websocket.

gun:ws_upgrade(ConnPid, "/websocket").

Gun will set all the necessary headers for performing the Websocket upgrade, but you can specify additional headers if needed. For example you can request a custom sub-protocol.

gun:ws_upgrade(ConnPid, "/websocket", [
    {<<"sec-websocket-protocol">>, "mychat"}
]).

You can pass the Websocket options as part of the gun:open/2,3 call when opening the connection, or using the gun:ws_upgrade/4. The fourth argument is those same options.

When the upgrade succeeds, a gun_upgrade message is sent. If the server does not understand Websocket or refused the upgrade, a gun_response message is sent. If Gun couldn't perform the upgrade due to an error (for example attempting to upgrade to Websocket on an HTTP/1.0 connection) then a gun_error message is sent.

When the server does not understand Websocket, it may send a meaningful response which should be processed. In the following example we however ignore it:

receive
    {gun_upgrade, ConnPid, StreamRef, [<<"websocket">>], Headers} ->
        upgrade_success(ConnPid, StreamRef);
    {gun_response, ConnPid, _, _, Status, Headers} ->
        exit({ws_upgrade_failed, Status, Headers});
    {gun_error, ConnPid, StreamRef, Reason} ->
        exit({ws_upgrade_failed, Reason})
    %% More clauses here as needed.
after 1000 ->
    exit(timeout)
end.

Sending data

Once the Websocket upgrade has completed successfully, you no longer have access to functions for performing requests. You can only send and receive Websocket messages.

Use gun:ws_send/2 to send messages to the server.

gun:ws_send(ConnPid, {text, "Hello!"}).

Note that if you send a close frame, Gun will close the connection cleanly but will attempt to reconnect afterwards.

Receiving data

Gun sends an Erlang message to the owner process for every Websocket message it receives.

receive
    {gun_ws, ConnPid, StreamRef, Frame} ->
        handle_frame(ConnPid, StreamRef, Frame)
end.