How to handle external service failure in Open-Uri? - ruby-on-rails

Question

In my Rails app I am trying to fetch a number of currency exchange rates from an external service and store them in the cache:
require 'open-uri'
module ExchangeRate
def self.all
Rails.cache.fetch("exchange_rates", :expires_in => 24.hours) { load_all }
end
private
def self.load_all
hashes = {}
CURRENCIES.each do |currency|
begin
hash = JSON.parse(open(URI("http://api.fixer.io/latest?base=#{currency}")).read) #what if not available?
hashes[currency] = hash["rates"]
rescue Timeout::Error
puts "Timeout"
rescue OpenURI::Error => e
puts e.message
end
end
hashes
end
end
This works great in development but I am worried about the production environment. How can I prevent the whole thing from being cached if the external service is not available? How can I ensure ExchangeRate.all always contains data, even if it's old and can't be updated due to an external service failure?
I tried to add some basic error handling but I'm afraid it's not enough.

Answer 1

If you're worried about your external service not being reliable enough to keep up with caching every 24 hours, then you should disable the auto cache expiration, let users work with old data, and set up some kind of notification system to tell you if the load_all fails.
Here's what I'd do:
Assume ExchangeRate.all always returns a cached copy, with no expiration (this will return nil if no cache is found):
module ExchangeRate
def self.all
rates = Rails.cache.fetch("exchange_rates")
UpdateCurrenciesJob.perform_later if rates.nil?
rates
end
end
Create an ActiveJob that handles the updates on a regular basis:
class UpdateCurrenciesJob < ApplicationJob
queue_as :default
def perform(*_args)
hashes = {}
CURRENCIES.each do |currency|
begin
hash = JSON.parse(open(URI("http://api.fixer.io/latest?base=#{currency}")).read) # what if not available?
hashes[currency] = hash['rates'].merge('updated_at' => Time.current)
rescue Timeout::Error
puts 'Timeout'
rescue OpenURI::Error => e
puts e.message
end
if hashes[currency].blank? || hashes[currency]['updated_at'] < Time.current - 24.hours
# send a mail saying "this currency hasn't been updated"
end
end
Rails.cache.write('exchange_rates', hashes)
end
end
Set the job up to run every few hours (4, 8, 12, less than 24). This way, the currencies will load in the background, the clients will always have data, and you will always know if currencies aren't working.