当有一个队列失效时，其他队列会在临界值内以少量或无中断或变化接管负载。
When a queue fails, other queues pick up the load with little or no change interruption.
它是一种特定类型的BlockingQueue：如果一个项不可用，则从队列获取该项会受到阻止，直到延迟失效。
It is a specific type of BlockingQueue: getting an item from the queue will block until the delay has passed, if an item isn't available immediately.
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