Session.ex

defmodule Session do
    def get key, req do
        case :cowboy_req.cookie "session_id", req do
            { :undefined, req } -> { nil, req }

            { session_id, req } ->
                case :gen_server.call SessionStore, { :get, session_id, key } do
                    { :ok, value } -> { value, req }
                    nil            -> { nil, req }
                end
        end
    end

    def set key, value, req do
        { session_id, req } =
            case :cowboy_req.cookie "session_id", req do
                { :undefined, req } ->
                    session_id = :uuid.to_string :uuid.v4
                    req = :cowboy_req.set_resp_cookie "session_id", session_id, [], req
                    { session_id, req }

                { session_id, req } -> { session_id, req }
            end

        :gen_server.cast SessionStore, { :set, session_id, key, value }

        req
    end
end

defmodule SessionStore do
    use GenServer.Behaviour

    def start_link do
        :gen_server.start_link { :local, __MODULE__ }, __MODULE__, [], []
    end

    def init _args do
        tid = :ets.new :sessions, [:set]
        { :ok, tid }
    end

    def handle_call { :get, session, key }, _from, tid do
        result =
            case :ets.lookup tid, { session, key } do
                []               -> nil
                [ { _, value } ] -> { :ok, value }
            end

        { :reply, result, tid }
    end

    def handle_cast { :set, session, key, value }, tid do
        :ets.insert tid, { { session, key }, value }

        { :noreply, tid }
    end
end