Not every API operation belongs in the request/response cycle. Some tasks are simply too slow – generating a PDF report, running a video transcoding pipeline, hitting a third-party service with unpredictable latency, or processing a CSV upload row by row. When these operations are forced into synchronous endpoints, the problems compound. Connections are held open while work happens. Upstream timeouts fire arbitrarily. A slow downstream service degrades every consumer on your API. And if anything goes wrong partway through, the client has no reliable way to know the state of the operation. by Steve McDougall