A queue with a ceiling

An unbounded queue never makes anybody wait, which sounds like a feature until the producer is faster than the consumer and the queue is the thing that runs the machine out of memory. A bounded queue moves the waiting to the producer, where it belongs.

This is the same idea as the concurrency limit in the async track, seen from the other side. There, a semaphore stopped four jobs from starting at once. Here, a full buffer stops a producer from running ahead. Both are backpressure, and both work by making somebody wait somewhere visible.