Using Elixir Dynamic Supervisors

I have been working on my side project Grapevine, a community site for text based multiplayer games (MUDs.) The existing games primarily have a telnet interface for playing. This generally involves using a local client or hoping the game provides a web client for you. So I've been working on a general purpose client that any game can use. You can see it in action for my game MidMUD.

This involves a new web client that uses Phoenix Channels to spin up a backing :gen_tcp process. This process is started inside of a DynamicSupervisor, a new feature introduced in Elixir 1.6. We place the process inside of the supervisor to keep the process alive across page reloads. Lets see how it works!

Basic Flow

Supervision Tree

When a new web client starts they join a channel named play:client. The channel ensures that the game they are trying to connect to exists and starts the backing process.

{:ok, pid} = WebClient.connect(socket.assigns.user,
  game_id: socket.assigns.game.id,
  host: connection.host,
  port: connection.port,
  channel_pid: socket.channel_pid
)

This Client process starts itself inside of a DynamicSupervisor as a transient process and saves the calling channel PID in order to forward any received data to the front end. This uses the DynamicSupervisor.start_child/2 function.

def start_client(callback_module, opts) do  
  spec = {Client, [module: callback_module] ++ opts}
  DynamicSupervisor.start_child(__MODULE__, spec)
end  

Once the Client process is alive it parses the telnet stream and forwards any text directly to the web front end.

Handling Page Reloads

The Client process is globally registered with the user's ID and the game's ID as part of the name, {:webclient, {1, 1}}. With this, we can check :global to see if the process already exists before starting a new process. If the process exists we overtake the Client process with our new channel.

def connect(user, opts) do  
  case :global.whereis_name(pid(user, opts)) do
    :undefined ->
      ClientSupervisor.start_client(__MODULE__, opts ++ [name: {:global, pid(user, opts)}])

    pid ->
      set_channel(pid, opts[:channel_pid])
      {:ok, pid}
  end
end  

set_channel/2 sends a message to the process which sets the internal channel_pid state for forwarding text to the web client.

Conclusion

With this setup I'm able to have a place to supervise Client processes and allow for session "stickiness" on page reloads. The DynamicSupervisor will delete the child_spec of the Client process after it terminates with a :normal reason. Otherwise the process will be restarted allowing for minor hiccups in the network.

All of this code is open source on GitHub, check out the Grapevine repo. I also worked on this during my Elixir live coding stream. I'll be back doing more live coding on that channel every Monday from 12PM-1PM Eastern.

https://youtu.be/FPTF_ourFNM

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