I just design a system which use the entity bean to do data logic and some stateless session beans to do business logic,it seems that everything will be ok,and it real run OK.
  but yesterday we've got a software to test the capability of the system,we are suprised for that when 10 users log in and use the system to do some transations side by side in some short time,the system will halt!and also print that message "out of memory or connection available" etc.
  in fact ,in anothere system which there are few EJBs will run fine!and this system create more than 50 entity beans and more than 15 stateless session beans.
 thanks for any reply