@Override
    public void onCreate(TezAmInstance si, int ephSeqVersion) {
      String sessionId = si.getSessionId();
      SessionType session = bySessionId.get(sessionId);
      if (session != null) {
        LOG.info("AM for " + sessionId + ", v." + ephSeqVersion + " has registered; updating ["
            + session + "] with an endpoint at " + si.getPluginPort());
        session.updateFromRegistry(si, ephSeqVersion);
      } else {
        LOG.warn("AM for an unknown " + sessionId + " has registered; ignoring");
      }
    }
Puts session back into the pool.
@return true if the session has been put back; false if it's not needed and should be killed.
ql/src/java/org/apache/hadoop/hive/ql/exec/tez/TezSessionPool.java