Websocket server in Elixir or phoenix

I’m trying to make a websocket server in Phoenix or raw Elixir. I heard about gun, I think I could use cowboy, but since I’m not that smart I thought I’d go as high level as possible.

I found this thread:

and this one:

and forked his code to:

I as I was bringing it up to date I ran into this issue:

Phoenix.Socket.Transport.connect/6 is undefined or private. Did you mean:

 * connect_info/3Elixir

Call to missing or unexported function ‘Elixir.Phoenix.Socket.Transport’:connect/6ElixirLS Dialyzer

on this line:
case Transport.connect(endpoint, handler, transport, __MODULE__, nil, conn.params) do

so after consulting the docs, which I don’t understand I came up with this, which I;m sure is wildly incorrect as well:

case Transport.connect(
          %{:endpoint=>endpoint, :transport=>transport, :params=>conn.params, :options=>[
            handler: handler, __MODULE__: __MODULE__, serializer: nil]}) do

But it told me I shouldn’t use this function anyway:

Phoenix.Socket.Transport.connect/1 is undefined or private. Did you mean:

 * connect_info/3Elixir

Call to missing or unexported function ‘Elixir.Phoenix.Socket.Transport’:connect/1ElixirLS Dialyzer

I think the code is almost updated, but what do I have to do to get it to a working state?



Using cowboy might be easier than you think. Here’s a small example of how it could be done. The important bits are the plug child spec and the module implementing cowboy_websocket behaviour. You can ignore the Web.UserSocket and “pubsub” specifics.

More info on cowboy_websocket behaviour.


There is a simpler API in Phoenix for custom websockets, you can follow the example here:



Is there a way to make this thing listen to the root path, such as ws://localhost?

From what I understand, it listens to the /websocket path if the endpoint’s socket is configured like so socket "/", RelayWeb.Sockets.EchoSocket.

I’ve seen comments saying it’s better for whatever reason. Thing is, I have to fill a spec that requires that the websockets endpoint answers to the root path and I have no idea how to do that with Phoenix.Socket.Transport as we stand.

Also would like to know how to implement heartbeat such as sending a PING and expecting a PONG after a while instead of simply closing the connection after 60s of inactivity.

Figured out this part

socket("/", RelayWeb.Sockets.EchoSocket, websocket: [path: ""])

Also figured out…

In init/1,

Process.send_after(self(), :ping, ping_timeout)

Then, add a handle_info

  @impl true
  def handle_info(:ping, %{options: options} = state) do
    ping_timeout = Keyword.get(options, :ping)

    Process.send_after(self(), :ping, ping_timeout)

    {:push, {:ping, ""}, state}

Will suggest the peer has to send a PONG, and if it does, that will count as a communication and thus will reset the timeout.

Thank you @RooSoft! I was trying to solve these 2 specific problems (path, ping) myself and your solutions to both work perfectly.

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Careful that sending pongs in response to client pings is explicitly handled for you by the underlying server in the WebSock world: WebSock — WebSock v0.4.3