Backpressure and admission control

The JD frames this as making sure traffic spikes don't cascade into providers. Backpressure is the system saying no early and cleanly, so a surge doesn't melt the fleet or trip every provider's rate limiter at once.

The instinct under load is to queue everything. That's the trap. An unbounded queue trades a fast failure for a slow one: requests sit until they're stale, the client times out and retries and you've added load to a system that's already over capacity. Admission control decides what to accept before any of that happens.