Impala's admission controller relies on the YARN fair-scheduler.xml for configuration. That configuration is loaded using YARN directly (ie. as a library by the frontend). In Hadoop 3, a number of changes were made to the YARN resourcemanager which break Impala. While we eventually want to rethink the admission control configuration (IMPALA-4159), in the meantime we at least should avoid using unsupported YARN APIs.