The section about calculating the availability of hard and redundant dependencies ignores the fact that systems often fail in tandem. For e.g. you might have a primary and secondary database in different AZs, each with 99.9% availability. This gives read operations a hypothetical uptime of 6-9s. But hidden SPOFs like operator error, VM infra failing, load balancer or other networking outages can make the 0.01% failure time overlap, blowing away your 6-9s dependency guarantee.
This is total submarine marketing by AWS. Im sure some very smart people with good intentions at AWS produced this, but the fact is they're not an independent academic providing objectively "the best" practices, they're a company trying to sell you something. They probably in full belief in their own abilities believe the AWS way of doing things is best, but frankly, I know as much as you do, or at least if you want to win me over you need to comprehensively prove it, not just preach from the hill.<p>TL:DR stop telling me you're doing better architectures in the same of selling me something
> "... For example, if a system makes use of two independent components, each with an availability of 99.9%, the resulting system availability is >99.999% ... "<p>This does not seem correct.