Queues and Publish/Subscribe

A service that waits for every other service to finish is as slow as the slowest of them, and fails when any of them fails. The topic starts from that difference between waiting and handing off, then covers the two ways to hand off: a queue, where each message goes to one worker, and publish/subscribe, where each message goes to every subscriber. Each comes with an example straight after it, a burst of uploads worked down by four workers and one order event read by three services, before a comparison puts the two side by side. A gotcha covers the duplicate that every reliable queue eventually delivers, and practice sorts four real flows into a queue or a topic.