solution.ex

defmodule Router do
    def get do
        
    end

    def owner_id_currency_to_name(owner_id, currency_id) do
        try do
            String.to_existing_atom("wallet-#{owner_id}-#{currency_id}")
          rescue
            ArgumentError -> String.to_atom("wallet-#{owner_id}-#{currency_id}")
          end
    end
end


defmodule Wallet do
    use GenServer

    def start_link(name) do
        GenServer.start_link(__MODULE__, 0, name: name)
    end

    def balance(name) do
        GenServer.call(name, :balance)
    end

    def credit(name, value) do
        GenServer.call(name, {:credit, value})
    end

    def handle_call(:balance, _from, state) do
        {:reply, state, state}
    end
    
    def handle_call({:credit, value}, _from, state) do
        new_state = state + value
        {:reply, new_state, new_state}
    end
end

defmodule MySupervisor do
    use DynamicSupervisor

    def start_link(arg) do
        DynamicSupervisor.start_link(__MODULE__, arg, name: __MODULE__)
    end

    def init(_arg), do: DynamicSupervisor.init(strategy: :one_for_one)

    def ensure_running(name) do
        spec = {Wallet, name}
        DynamicSupervisor.start_child(__MODULE__, spec)
    end
end

MySupervisor.start_link([])

# in router
owner_id_1 = 1
currency_id_1 = 2
name_1 = Router.owner_id_currency_to_name(owner_id_1, currency_id_1)
MySupervisor.ensure_running(name_1)
IO.inspect(Wallet.balance(name_1))
IO.inspect(Wallet.credit(name_1, 10))
IO.inspect(Wallet.balance(name_1))