FAFX fetches data from the 90 day European Central Bank (ECB) feed and offers a simple CLI and API to interect with.
$ gem install fafx
$ fafx
Usage: fafx [options]
--recent Lists most recent rates
--currencies Lists available currencies
--dates Lists available dates
--update Fetches new data from the web
require 'Date'
require 'fafx'
# Gets the most recent rates
Fafx::ExchangeRate.get('GBP', 'USD')
# => 1.2951645399597045
Fafx::ExchangeRate.at(Date.today, 'GBP', 'USD')
# => 1.2951645399597045
Fafx::ExchangeRate.currencies_available
# => ["USD", "JPY", "BGN", "CZK", ...]
Fafx::ExchangeRate.dates_available
# => ["2018-09-10", "2018-09-07", "2018-09-06", "2018-09-05", ...]
Fafx::ExchangeRate.most_recent
# => {"USD"=>1.1571, "JPY"=>128.54, "BGN"=>1.9558, "CZK"=>25.648, ...}The at function may raise a DateError or CurrencyError exception, should the date or currency be unavailable. Passing an object other than Date to at raises DateError as well.
You can easily build an API on top of fafx, an example is given here with sinatra.
You can update the exchange rates values either via the CLI, Rake task or programmatically:
$ fafx --update
$ rake update_data
require 'fafx'
Fafx::ExchangeRate.update_data