BACKPRESSURE

What the handler called is up but overloaded and asked it to come back later: a node refusing a command as busy (wow.command.busy), a wait that ran out (wow.wait.timeout), a query store's bulkhead (wow.query.busy), any 429 with a Retry-After. Never systemic: the subscription retries the entry with backoff and jitter (the Retry-After at least), throttles the partition's concurrency until handlings succeed again, and parks the aggregate once the backpressure budget is spent; it never pauses the partition.