Tightly coupled systems fail together: if a web tier calls an order processor directly and the processor slows down or crashes, the web tier backs up and users see errors too. Decoupling puts a durable buffer between components so each can scale, deploy and fail independently. Amazon Simple Queue Service (SQS) and Amazon Simple Notification Service (SNS) are the two classic building blocks, and the exam expects you to know how each behaves under failure.
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