Does use-strict-min permit closure of idle connections dropping the pool below min-pool-size?

Environment

• Red Hat JBoss Enterprise Application Platform (EAP)
  • 5
  • 6
  • 7
  • 8

Issue

• When using <use-strict-min>true</use-strict-min>, can idle connection closure drop the pool size below <min-pool-size>?

Resolution

When use-strict-min is set to true, the idle connection scan will not mark for closure any further connections once the min-pool-size has been reached.

In JBoss EAP 8, the two parameters <use-strict-min>true</use-strict-min> and <prefill>true</prefill> have almost the same effect. When <use-strict-min>true</use-strict-min> is used in combination with <prefill>true</prefill>, there is a behavior that can only be achieved with use-strict-min. Specifically, when idle connections are closed based on idle-timeout-minutes, connections will not be closed below min-pool-size, even if some connections in the pool have already exceeded the idle-timeout-minutes threshold. For example:

• When min-pool-size > 0, prefill=false, and use-strict-min=false, min-pool-size is effectively ignored, and the pool eventually shrinks to zero if there is no access to the datasource.

• When min-pool-size > 0, prefill=true, and use-strict-min=false, if all connections in the pool are detected as long idle, the pool size is first reduced to zero and then prefilled up to min-pool-size. In other words, min-pool-size acts as the target number of connections to prefill.

• When min-pool-size > 0, prefill=true, and use-strict-min=true, during the idle connection cleanup process triggered by idle-timeout-minutes, the pool does not allow the number of connections to temporarily drop below min-pool-size. Even if the connections are old, they are not closed, and the number of connections specified by min-pool-size is always kept in the pool. In the JBoss EAP 8 datasource implementation, connections are handed out in Last-In-First-Out (LIFO) order, meaning the most recently returned connection is issued first. Due to this connection allocation order and the behavior of use-strict-min=true, a side effect may occur where older connections corresponding to min-pool-size remain in the pool and are never closed by idle-timeout-minutes.

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