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原文摘要

Workflow engines orchestrate pipelines of activities, which often call external APIs, on a fixed pool of worker slots, the threads that run them. When an API enforces a hard rate limit shared by all tenants and priorities, a throttled call that retries after a backoff keeps its worker slot and starves other tenants. Large Language Model provider APIs are particularly complex: their per-organization caps are low and bind independently on requests and tokens per minute. The operator can neither reconfigure the cap nor observe saturation before a request fails, so classical fair-queuing assumptions do not hold. We present Backpressure, a worker-side admission gate. It checks each call's cost atomically against shared admission state and enforces fairness over requests and tokens. A denied call releases its worker slot and waits on a durable timer, so waiting no longer blocks execution capacity. We compare against a naive retry, the AI gateway LiteLLM, and the built-in throttle of the workflow engine Temporal. On a workload with three priority classes, Backpressure raises fairness (Jain's index over each queue's fraction of its entitled share) from 0.60 to 0.89, where 1.0 is ideal. It also cuts high-priority tail latency roughly 8x. When tokens are the binding limit, all request-based baselines serve every priority equally, whereas Backpressure keeps the priority order and reaches a dominant-resource fairness of ~0.99. Admission costs one round trip to shared state, adds no measurable latency when the cap does not bind, and fails open if shared state is unavailable.