在「New Event Notifications for Amazon S3」這邊提到的重點：
The bucket owner (or others, as permitted by an IAM policy) can now arrange for notifications to be issued to Amazon Simple Queue Service (SQS) or Amazon Simple Notification Service (SNS) when a new object is added to the bucket or an existing object is overwritten.
也就是說，S3 上有變動時會透過 SQS 觸發後續行為。由於這是 AWS 自己提供的功能，解決了以往「上傳檔案後自己觸發 SQS」不能保證 atomic 的問題。
好像馬上就想到某個專案可以用到...