protected void maybeSubscribeChangeTopic(boolean reportNull) {
    if (notifier != null) {
      notifier.subscribe(ACCOUNT_METADATA_CHANGE_TOPIC, changeTopicListener);
      logger.info("Subscribed to {} for change notifications.", ACCOUNT_METADATA_CHANGE_TOPIC);
    } else if (reportNull) {
      logger.warn("Notifier is null. Account updates cannot be notified to other entities. Local account cache may not "
          + "be in sync with remote account data.");
      accountServiceMetrics.nullNotifierCount.inc();
    }
  }
Return the {@link Notifier}.
@return The {@link Notifier}
ambry-account/src/main/java/com/github/ambry/account/AbstractAccountService.java