@Override
  public String toString() {
    return "QueryIdentifier{" +
        "appIdentifier='" + appIdentifier + '\'' +
        ", dagIdentifier=" + dagIdentifier +
        '}';
  }
An identifier for a query, which is unique.

This is based on the AppId and dagId.

At the moment, It's possible for Hive to use the same "hive.query.id" if a single query
is split into stages - which prevents this identifier being used as the unique id.

Not exposing getters to allow this to evolve - i.e. retain the uniqueness constraint.
llap-server/src/java/org/apache/hadoop/hive/llap/daemon/impl/QueryIdentifier.java