private String buildCacheKey(Throwable throwable, String url) {
      if (null == throwable || null == url) {
        return "";
      }

      String throwableName = throwable.getClass().getSimpleName();
      return throwableName + "-" + url;
    }
Builds a unique cache key for the combination of {@link Throwable} and
{@link String} URL.

@param throwable
@param url
@return the key, such as <code>IOException-http://www.server.com/jmx</code>.
ambari-server/src/main/java/org/apache/ambari/server/state/services/MetricsRetrievalService.java