Building an event-sourced game with Phoenix Liveview: Game Server

Feb 15, 2021

In the previous articles, we’ve looked at the design of the game’s rules. Now is an excellent time to focus on the runtime perspective of the application.

We’ve seen that we can interact with a game via the GameState module by providing a list of events and a command. We still need some way to store the events, and for this, we’ll use a GenServer.

Skeleton of GameServer

GameServer module is a GenServer which allows keeping track of the events in a process.

defmodule DoctorP.GameServer do
  use GenServer

def start_link(args) do
  game_id = Keyword.get(args, :game_id)
  GenServer.start_link(__MODULE__, args, name: process_name(game_id))
end

def init(args) do
  game_id = Keyword.get(args, :game_id)
  {:ok, %{game_id: game_id, events: []}}
end

def process_name(game_id), do:
  {:via, Registry, {DoctorP.GamesRegistry, game_id}}

end

We can start a GameServer using the start_link function, passing a keyword list containing a game id. The server’s initial state is a map containing the game id and an empty list of events.

We also want to keep track of several games at the same time and to be able to interact with them using their game id. I’ve decided to use the via tuple method to link a game id and the server PID in a Registry. process_name returns a tuple that indicates which registry and key to use to find the server.

Playing

The next thing to do is to be able to act on the GameServer.

All actions follow the same patterns. For the sake of brevity, let’s focus solely on adding a player.

First, I’ve introduced a client side function. It allows for an easy to use interface for other modules:

def add_player(game_id, player_name), do:
  GenServer.call(process_name(game_id), {:add_player, player_name})

When called, add_player issues a call to the server, using the process_name based on the game_id and asking to add a player with the provided player name.

Its server-side counterpart is more interesting as it is responsible of dispatching a command to the game, storing the events, and replying to the caller.

def handle_call({:add_player, player_name}, _from, state), do:
  %AddPlayer{player_name: player_name}
  |> handle_command(state)
  |> reply()

First, the AddPlayer command is built and piped into to a private handle_command function, alongside the state . The result of this call is, in turn, piped into a private reply function.

Let’s dive into these!

Handling the command

defp handle_command(command, state), do:
  command
    |> dispatch_command(state)
    |> build_new_state(state)

defp dispatch_command(command, state), do:
  GameState.dispatch_message(command, state.events)

defp build_new_state(%ActionResult{events: events} = result, state), do:
  {result, %{state | events: state.events ++ events}}

handle_command is made of two parts, each one in its function.

First, we use the GameState module to dispatch the command. We can see that all previous events, currently stored in the state, are passed as a second parameter. It ensures the command handler knows the current state.

Secondly, the command dispatch result, an ActionResult is piped into build_new_state. Here the new state is created by appending the news events to the events currently stored. Note that build_new_state returns a tuple containing the result of the dispatch and the new state.

Replying to the caller

Once the command is dispatched, the game logic applied and, the new state created we need to reply to the caller.

This is the job of the reply function we’ve seen in the handle_call function.

build_new_state returns a tuple containing an ActionResult and the desired state after the command handling, which serves as a parameter for our new reply function.

defp reply({ %ActionResult{}, new_state}), do:
  {:reply, :ok, new_state)

Here we simply reply with the :ok tuple and use the second part of the tuple for the new server’s state.

The first part of the tuple seems useless by now, but it’s actually important.

We’ve seen in the article about error handling that part of ActionResult’s job is to keep track of errors. So far, reply only handles the positive cases.

I’ve decided to communicate errors to the caller by responding with a tuple starting with the :error atom and containing the error provided by the game.

To do this we need to add a clause for the reply function matching with ActionResult containing errors:

defp reply({ %ActionResult{error: error}, new_state}, state) when not is_nil(error), do:
  {:reply, {:error, error}, new_state)

The GameServer can now deal with both positive outputs, made of new events and errors, and communicates them to callers.

There are still exciting things to say about the GameServer, from how it communicates with the Liveviews, to stopping them and more. These are the things we’ll cover in the next articles!