Backpressure (Slow Down Producers)

Picture pouring water into a funnel that drains slowly into a bottle. Pour faster than the funnel can drain, and water overflows everywhere. Backpressure is the funnel pushing back before that happens. A slow consumer tells a fast producer, 'I'm getting full — pause for a bit,' instead of silently overflowing or crashing. Once the consumer catches up and there's room again, it tells the producer it's safe to resume.

1. 1Put a bounded buffer — a queue with a maximum size — between the producer and the consumer.
2. 2Before the producer pushes a new item, check the buffer against a high-water mark — an upper threshold. If the buffer has reached that mark, tell the producer to pause instead of accepting more.
3. 3As the consumer drains items from the buffer, the buffer shrinks. Once it drops back to a low-water mark — a lower threshold — tell the producer it's safe to resume pushing.

Use backpressure once your app is popular and a fast producer — like an API taking in incoming events — can generate work faster than a slower consumer, like a database writer, can keep up. Without backpressure, the buffer between them grows without limit, and the whole process eventually runs out of memory and crashes.

• ⚠Letting the buffer grow with no maximum size at all. That only delays the crash instead of preventing it, since memory use is still unlimited.
• ⚠Using the same threshold to pause and to resume. The producer would flip between paused and resumed on every single item right at that boundary. Using a lower resume mark than the pause mark gives it breathing room instead.