Simple game server with channels

Hello everyone,
I am working on a little 2-player game to help me learn elixir/phoenix. Before starting to code (and re-read some documentation), I would be glad to have some feedback about this scenario:

1- Players join their own “user:#user_id” private topic to receive/send private events (see why below). On join/2("user:" <> user_id,...):

  • check the user_id maps the one assigned on the socket during initial connection

2- Then they join a “match:lobby” channel where they can meet. On join/2("match:lobby",...):

  • enable Presence in a handle_info/2 callback

3- Players invite each other to start a match: client 1 sends an invitation event to client 2. On handle_in/3("invite", %{"dest_id" => dest_id}, socket):

  • check dest_id player is still available
  • then send invitation through the “user:#dest_id” topic

4- Let’s say player 2 accepts the challenge. On handle_in/3("accept", %{"match_id" => match_id}, socket):

  • push an event for both players so that they ask back to leave “match:lobby” (till the end of the game when they will join again)
  • create a Game (module agent) that implements the gameplay and is added to a GameSupervisor (through start_child/2?).

5- Depending on user input and game events, the Game agent dispatch events to users until the match ends.

  • here a specific channel with a topic “match:#match_id” could be used, or we could rely on the user channels (“user:#user_id” topics) previously created to send invitations
  • a bit off-topic: maybe I need to persist the Game state to the DB, and retrieve it if the agent crashes and is respawned

Regarding the communication between the Game agent and clients:

  • Game agent->players: the Game agent holds the relevant topics in its state (let it be “user:#user_id” or “match:#match_id”) and can send event to players thanks to the endpoint’s broadcast/3
  • Players->Game agent: on handle_in/3("game_event",...): we need to find the Game agent corresponding to the given user_id or match_id. I guess this is when Registry is useful, but for the moment I feel that implementing my own GameIndex agent, mapping user_id keys to process names would be simpler. Or I could use the socket assigns to store the Game agent process name. This is where I would really appreciate some feedback, I am a bit lost here.

6- When the game ends:

  • the result of the game is persisted to the DB
  • the Game agent is terminated and specific properties are reset (for instance if the Game agent process name was assigned to the socket)
  • players get back to “match:lobby”

In case of a client side reboot (a page refresh for instance) and if the user is currently in a game, we could jump directly from step 1 to 5, using the GameIndex.

Do you see any drawback/oversight in this scenario? Many thanks, Guillaume


Regarding the flow I think your approach is fine.

If you’re going to store things in the DB anyway then sure, use the DB to store intermediate “snapshots” of the game in case of failure - if you’re not going to store (things like results, etc) then a simple ETS table would suffice.

Regarding Game Agents and identification - erlang does provide a global namespace where you can name processes by term - it’s available through elixir at least on genservers and allows you to find processes by assigned ID. I’ve used that in GenServers to find the matching server - the genserver is registered with the ID, after joining the game channel, the ID is assigned to the socket, on msg in, it tries to find the genserver with that ID, if it finds it, calls/casts on the return pid to get the game state, if it doesn’t find an existing genserver with that ID it creates one loading the last saved state from the DB and in case there isn’t one yet, it creates one new game state.

You can use name: {:global, {:game, "string_or_whatever_id"}} to register the process. And then use GenServer.whereis({:global, {:game, "123"}}) which will give you back nil or a pid. MAke sure the terms are the same type, as 123 is diff. than “123” for registration purposes.


Thank you, I am going to try the {:global ...} option, and maybe I’ll look to Registry after that