code: public Collection<ExecutorDetails> getUnassignedExecutors(TopologyDetails topology) {
          if (topology == null) {
              return new ArrayList<>(0);
          }
  
          Collection<ExecutorDetails> ret = new HashSet<>(topology.getExecutors());
  
          SchedulerAssignment assignment = this.getAssignmentById(topology.getId());
          if (assignment != null) {
              Set<ExecutorDetails> assignedExecutors = assignment.getExecutors();
              ret.removeAll(assignedExecutors);
          }
  
          return ret;
      }
docs: get the unassigned executors of the topology.