public ApplicationId createExtClientAppId() {
    // Note that we cannot allow users to provide app ID, since providing somebody else's appId
    // would give one LLAP token (and splits) for that app ID. If we could verify it somehow
    // (YARN token? nothing we can do in an UDF), we could get it from client already running on
    // YARN. As such, the clients running on YARN will have two app IDs to be aware of.
    return ApplicationId.newInstance(startTime, appIdCounter.incrementAndGet());
  }
We'll keep signers per cluster around for some time, for reuse.
llap-client/src/java/org/apache/hadoop/hive/llap/coordinator/LlapCoordinator.java