public static QueryHistoryService getInstance() {
    if (instance == null) {
      throw new RuntimeException("QueryHistoryService is not present when asked for the singleton instance");
    }
    return instance;
  }
Start method actually starts the service in a sense that it initializes the repository (which might involve
database operations) and starts the executor that's responsible for flushing records.
ql/src/java/org/apache/hadoop/hive/ql/queryhistory/QueryHistoryService.java