Barrier request send mechanism in OutboundQueueManager is based on time and number of messages written.

If a barrier is scheduled - and there is another barrier written (because of exceeded number of messages written) between the time the first one was scheduled till it is sent - then the scheduled barrier is not sent. This happens due to the condition in scheduled barrier Runnable:

final long now = System.nanoTime();
final long sinceLast = now - lastBarrierNanos;
if (sinceLast >= maxBarrierNanos) {

• the exceeded-number-of-messages barrier resets the lastBarrierNanos time, so the (sinceLast >= maxBarrierNanos) condition is false -> resulting in not sending the barrier