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By “guarantee,” AWS doesn't mean that service disruptions or security breaches will never occur. Drives may stop spinning, major electricity systems may fail, and natural disasters may happen. But when something does go wrong, AWS will provide service credits to reimburse customers for their direct losses whenever uptimes fall below a defined threshold. Of course, that won't help you recover customer confidence or lost business.

The exact percentage of the guarantee will differ according to service. The service level agreement (SLA) rate for AWS EC2, for instance, is set to 99.99 percent—meaning that you can expect your EC2 instances, ECS containers, and EBS storage devices to be available for all but around four minutes of each month.

The important thing to remember is that it's not if things will fail but when. Build your applications to be geographically dispersed and fault tolerant so that when things do break, your users will barely notice.