public void start() throws Exception {
      ListenableFuture<WmTezSession> getFuture = tezAmPool.getSessionAsync();
      Futures.addCallback(getFuture, this, MoreExecutors.directExecutor());
    }
The class that serves as a synchronization point, and future callback,
for async session initialization, as well as parallel cancellation.
ql/src/java/org/apache/hadoop/hive/ql/exec/tez/WorkloadManager.java