@Override
      public void run() {
        while (isRunning()) {
          processAssignQueue();
        }
        pendingAssignQueue.clear();
      }
Add the assign operation to the assignment queue. The pending assignment operation will be
processed, and each region will be assigned by a server using the balancer.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java