Queues

Backpressure connects a consumer’s limited capacity to the behavior of its producers. A full channel may make a producer wait, reject new work, or apply a deliberately chosen shedding policy. The important property is that the limit changes what upstream components do. A growing queue or a dashboard showing lag is evidence of imbalance, but neither is automatically a mechanism that controls admission. ...

A queue is a promise about delivery between a producer and a consumer. That promise can be “at least once” (never lose) or “at most once” (never duplicate), but never both for free. Choosing the guarantee is choosing which failure you can tolerate. ...