Work that cannot be served properly is refused at the front, not missed at the back
A request the engine cannot stand behind is declined rather than accepted and quietly under-served — so the queue that would have formed does not.
The expensive failure in a loaded system is rarely a crash. It is accepting work you cannot finish to standard, discovering that at the far end, and paging somebody at three in the morning for a backlog that has been building since the afternoon.
Refusing is unfashionable and it is the correct behaviour. A declined request is a fact you can act on immediately; a request accepted and served badly is a fact you find out about later, from a customer.
We publish the count of attempts blocked and the number of guards, and we do not publish what any individual guard looks for. The count demonstrates the discipline. The list would be a specification for anyone who wanted to build around it.
The number that matters to an operator is not how clever the refusal is. It is that the pager does not go off.