code: protected function getRateKey($currencyCode, $date, $rateType, $sourceName)
      {
          return str_replace(
              ['%currency_code%', '%date%', '%rate_type%', '%source_name%'],
              [$currencyCode, $date->format('Y-m-d'), $rateType, $sourceName],
              '%currency_code%_%date%_%rate_type%_%source_name%'
          );
      }
docs: Builds rate key to speed up search.
  
  @param string    $currencyCode
  @param \DateTime $date
  @param string    $rateType
  @param string    $sourceName
  
  @return string