Steady Pool Size

An integer indicating the minimum number of message–driven beans to maintain. When the value is set to a number greater than 0 (zero), the container pre-populates the MDB pool with the specified number and tries to ensure that there are always this number of beans in the free pool. This ensures that there are enough MDB beans in the ready-to-serve state to process user requests. 

This property 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, if an idle stateless session container has a fully-populated pool with a steady-pool-size of 10 and 20 concurrent requests arrive for the MDB 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 is reduced to 10 or fewer concurrent requests, the additional 10 instances are discarded. 

Max Pool Size

An integer indicating the maximum number of message–driven beans in the pool. A value of 0 (zero) indicates that the pool is unbounded.

Pool Idle Timeout in Seconds

The maximum amount of time (in seconds) that an MDB instance can remain idle in the pool. When an MDB has exceeded the configured timeout, a timer thread removes the unused MDB. This property defines the interval at which this thread runs. 

A value greater than 0 indicates that the container removes or destroys any MDB instance that is idle at this specified duration. A value of 0 (zero) specifies that idle MDB instances can remain in the pool indefinitely.