Elixir stream to all subscribers

Question

I'm trying to implement a radio server in Elixir

One process is always working and reading a file (mp3) and publish to topic ":radio", currently for test purpose when it finishes it starts over

Each connection subscribes to topic ":radio"

I don't understand how to send the chunks to all subscribed connections, the connection closed after 2 or 3 chunks

defmodule Plugtest do
  import Plug.Conn

  def init(opts), do: opts

  def start() do
    Plug.Adapters.Cowboy.http(Plugtest, [])
    {:ok, _pid} = PubSub.start_link()
    spawn(fn -> stream_from_file("./song.mp3", 128) end)
  end

  def call(conn, _opts) do
    conn = conn
    |> send_chunked(200)
    |> put_resp_content_type("audio/mpeg")

    :ok = PubSub.subscribe(spawn(fn -> send_chunk_to_connection(conn) end), :radio)
#    File.stream!("./song.mp3", [], 128) |> Enum.into(conn) # test purpose only
  end

  defp send_chunk_to_connection(conn) do
    receive do
      {:radio_data, data} ->
        IO.inspect "* #{inspect self()} * [ #{inspect conn.owner} ] [ #{inspect data} ]"
#        Enum.into(data, conn) # not working TODO send chunk to connection
        {:ok, conn} = chunk(conn, data)
        send_chunk_to_connection(conn)
    end
  end

  defp stream_from_file(fpath, bytes) do
    File.stream!(fpath, [], bytes)
    |> Enum.each(fn chunk ->
      PubSub.publish(:radio, {:radio_data, chunk})
    end)
    stream_from_file(fpath, bytes)
  end

end

Stacktrace :

[error] Process #PID<0.274.0> raised an exception
        ** (MatchError) no match of right hand side value: {:error, :closed}    
            (plugtest) lib/plugtest.ex:26: Plugtest.send_chunk_to_connection/1

dependencies :

  defp deps do
    [{:plug, "~> 1.0"}, {:cowboy, "~> 1.0"}, {:pubsub, "~> 0.0.2"}]
  end

edit after @maxdec comment

defmodule Plugtest do
  import Plug.Conn

  @file_path "./song.mp3"
  @port 4000
  @chunk_size 128

  def init(opts), do: opts

  def start() do
    Plug.Adapters.Cowboy.http Plugtest, [], port: @port
    {:ok, _pid} = PubSub.start_link()
    spawn fn ->
        stream_from_file(@file_path, @chunk_size)
    end
  end

  def call(conn, _opts) do
    conn = conn
    |> send_chunked(200)
    |> put_resp_content_type("audio/mpeg")

    :ok = PubSub.subscribe(spawn(fn -> send_chunk_to_connection(conn) end), :radio)
#    File.stream!("./song.mp3", [], 128) |> Enum.into(conn) # test purpose only
    conn
  end
  defp send_chunk_to_connection(conn) do
    receive do
      {:radio_data, data} ->
        case chunk(conn, data) do
          {:ok, conn} -> send_chunk_to_connection(conn)
          {:error, err} -> IO.puts err # do nothing, as something went wrong (client disconnection or something else...)
        end
    end
  end

  defp stream_from_file(fpath, bytes) do
    File.stream!(fpath, [], bytes)
    |> Enum.each(fn chunk ->
      PubSub.publish(:radio, {:radio_data, chunk})
    end)
    stream_from_file(fpath, bytes)
  end

end

Answer 1

After a quick look I think there are 2 things you should fix:

1. PlugTest is a Plug so call/2 should return conn (that's not your issue though). It should also blocks while waiting for events (receive):

   def call(conn, _opts) do
     conn = conn
     |> send_chunked(200)
     |> put_resp_content_type("audio/mpeg")
   
     :ok = PubSub.subscribe(self(), :radio)
     send_chunk_to_connection(conn)
   end
   

2. In send_chunk_to_connection you should do:

   defp send_chunk_to_connection(conn) do
     receive do
       {:radio_data, data} ->
         case chunk(conn, data) do
           {:ok, conn} -> send_chunk_to_connection(conn)
           {:error, err} -> IO.puts err; conn # do nothing, as something went wrong (client disconnection or something else...)
         end
     end
   end