Backpressure

Backpressure is a system's way of signaling upstream that it can't keep up, so producers slow down instead of overwhelming a consumer. When data arrives faster than it can be processed, something has to give: without backpressure, queues grow unbounded, memory fills, and the service eventually crashes. Well-designed streaming and messaging systems make this explicit. A consumer that's saturated stops pulling new messages; a bounded queue that's full blocks or rejects new writes; a reactive stream propagates a 'slow down' signal all the way back to the source. The alternative — dropping data silently or falling over — is almost always worse. For SaaS builders, backpressure is what keeps a traffic spike from turning into an outage. Practical note: prefer bounded queues and buffers over unbounded ones, decide up front what happens when a limit is hit (block, shed load, or route to a dead-letter queue), and load-test the failure path, not just the happy path.