One of the most important parameters of Application Server pooling is steady-pool-size. When steady-pool-size is set to greater than 0, the container not only pre-populates the bean pool with the specified number of beans, but also attempts to ensure that there is always this many beans in the free pool. This ensures that there are enough beans in the ready to serve state to process user requests.
This parameter does not necessarily guarantee that no more than steady-pool-size instances exist at a given time. It only governs the number of instances that are pooled over a long period of time. For example, suppose an idle stateless session container has a fully-populated pool with a steady-pool-size of 10. If 20 concurrent requests arrive for the EJB component, the container creates 10 additional instances to satisfy the burst of requests. The advantage of this is that it prevents the container from blocking any of the incoming requests. However, if the activity dies down to 10 or fewer concurrent requests, the additional 10 instances are discarded.
Another parameter, pool-idle-timeout-in-seconds, allows the administrator to specify, through the amount of time a bean instance can be idle in the pool. When pool-idle-timeout-in-seconds is set to greater than 0, the container removes or destroys any bean instance that is idle for this specified duration.