One slave doesn't respond correctly when having concurrent requests.
When two running workflow are requesting concurrently to one slave, the executions will be twisted. Usually only one workflow can get correct results. I think it is because multiple client requests for the same slave only get the same object reference. And the attributes of the current SlaveController class created a stateful server. A discussion about it can be found at
A solution to is to refactor the current SlaveController so that its attributes are in a separate class and SlaveController is only a container.