The SQS we saw on Day 1 was a queue model — "decouple two components in time, but each message is consumed once and disappears." Yet in real systems it's far more common for a single event — "an order came in" — to need to be known independently by six different places: payment, inventory, shipping, email, search index, and data warehouse. In a queue, once one consumer takes a message, no other consumer can see it, so to solve this scenario with queues you'd have to create six queues and have the producer send six times. Every time a new consumer is added, the producer's code has to change — and this is the very definition of the "tightly coupled module" Edsger Dijkstra pointed out in 1968.