Websockex and a payload

Questions & Help Questions
1

Heyo. Trying to figure out the best way to use websockex to stream Kraken trades. Their API requires a payload to define the subscription, possibly including an auth token for private data. I’m able to connect and get a basic status event back but am not sure how to send the payload from the start_link function.

Any ideas? TIA.

2 
defmodule Streamer.Kraken do
  use WebSockex

  require Logger

  @stream_endpoint "wss://beta-ws.kraken.com"

  def start_link(symbol) do

    ## Get Kraken Websocket Token only needed for private endpoints
    ## {:ok, %{"expires" =>  expires, "token" => token}} = Krakex.websockets_token()}

    lowercased_symbol = String.downcase(symbol)

    WebSockex.start_link(
      "#{@stream_endpoint}",
      __MODULE__,
      nil,
      name: :"#{__MODULE__}-#{lowercased_symbol}"
    )

  end

  def subscribe(client, symbol) do
    json = """
    {"event": "subscribe", "pair": ["#{symbol}"], "subscription": {"name": "trade"}}
    """

    Logger.info("Sending message: #{json}")
    WebSockex.send_frame(client, {:text, json})
  end

  def handle_connect(_conn, state) do
    Logger.info("Kraken WS Connected!")
    subscribe(self(), "ADA/USDT")
    {:ok, state}
  end

  def handle_frame({_type, msg}, state) do
    IO.inspect msg
    case Jason.decode(msg) do
      {:ok, event} -> process_event(event)
      {:error, _} -> Logger.error("Unable to parse msg: #{msg}")
    end

    {:ok, state}
  end

And when I run this I get:

** (MatchError) no match of right hand side value: {:error, %WebSockex.CallingSelfError{function: :send_frame}}

I’m using a DynamicSupervisor to run the worker - so was looking for a way to get all the way through the subscribe from the start_link flow.

If I don’t use the supervisor and run it manually in two steps it works.

3

WebSockex.send_frame has specific “nope” code for that calling pattern:

https://github.com/Azolo/websockex/blob/ad2010048b8126b4229744957ad2d4f5dfed414d/lib/websockex.ex#L421-L423

The message for this case suggests a fix:

https://github.com/Azolo/websockex/blob/ce7064e1d6e8dfb1bba1bb8d3e75b71b88aabc1e/lib/websockex/errors.ex#L144-L150

4

Hi,
did you get how to subscribe to a subject after the connection has been established ?

5

for future ref :
call WebSockex.cast\2 in the connect\ callback as such :

def handle_connect(_conn, state) do
    WebSockex.cast(self(), :subscribe)
    {:ok, state}
end

def handle_cast(:subscribe, state) do
    {:reply, {:text, subscription_frame()}, state}
end