Implements a GenServer to retrieve exchange rates from a configured retrieveal module on a periodic basis. By default exchange rates are retrieved from Open Exchange Rates.

Retrieved data is stored in an :ets table.

By default the period of execution is 5 minutes (360_000 microseconds). The period of retrieval is configured in config.exs or the appropriate environment configuration. For example:

config :ex_money,
  open_exchange_rates_app_id: "app_id_string",
  open_exchange_rates_retrieve_every: 360_000