Defines the cache behaviour for exchange rates.

Most callbacks receive a cache/0 term - the value returned by init/1 - identifying which retriever's storage to operate on. This allows a single cache module (such as the bundled Money.ExchangeRates.Cache.Ets and Money.ExchangeRates.Cache.Dets) to be shared safely by multiple named retrievers, each backed by its own separated storage.

Migrating from the deprecated singleton callbacks

Older cache modules implemented init/0, terminate/0, latest_rates/0, historic_rates/1, last_updated/0, store_latest_rates/2 and store_historic_rates/2, storing rates as fixed, module-wide state. These still work, but named retrievers sharing such a module overwrite each other's rates.

To migrate, add the cache/0 value returned by init/1 as the leading argument to every other callback, and key storage off the name given to init/1 instead of a fixed identifier:

def init(name), do: :ets.new(name, [:named_table, :public])
def latest_rates(cache), do: :ets.lookup(cache, :latest_rates)
def store_latest_rates(cache, rates, retrieved_at) do
  :ets.insert(cache, {:latest_rates, rates})
  :ets.insert(cache, {:last_updated, retrieved_at})
end

terminate/1, historic_rates/2 and store_historic_rates/3 follow the same pattern.