feeds_cache.rb

# FeedsCache.set_backend(FeedsCache::FileBackend)
# FeedsCache.fetch('foo', 60) { 'bar' }
# FeedsCache.persist

module FeedsCache
  module_function

  def set_backend(backend)
    @backend = backend
  end

  def fetch(key, expiration = nil)
    set(key, yield, expiration) unless cache[key]
    get(key)
  end

  def get(key, default = nil)
    return default unless cache[key]
    cache[key]['l'] = Time.now.to_i
    cache[key]['v']
  end

  def set(key, value, expiration = nil)
    cache[key] = { 'v' => value, 'l' => Time.now.to_i }
    cache[key]['e'] = Time.now.to_i + expiration if expiration
  end

  def cache
    @cache ||= expire_items(@backend.load_cache)
  end

  def expire_items(cache)
    now = Time.now.to_i
    lru_expires = now - 30 * 24 * 3600
    cache.select do |_, v|
      (v['e'] && v['e'] > now) || v['l'] > lru_expires
    end
  end

  def persist
    @backend.persist(cache)
  end

  module S3Backend
    module_function

    def load_cache
      cache_string = S3.client.get_object(
        bucket: ENV['BUCKET_NAME'],
        key: 'cache/cache.json'
      ).body.string rescue '{}'

      JSON.parse(cache_string) rescue {}
    end

    def persist(cache)
      S3.client.put_object(
        acl: 'private',
        bucket: ENV['BUCKET_NAME'],
        key: 'cache/cache.json',
        body: JSON.dump(cache)
      )
    end
  end

  module FileBackend
    module_function

    def load_cache
      str = File.read('feeds_cache.json') rescue '{}'
      JSON.parse(str)
    end

    def persist(cache)
      File.open('feeds_cache.json', 'w') do |f|
        f.puts JSON.dump(cache)
      end
    end
  end
end