Some services (before I left) just did not have the sheer capacity to have Amazon.com as a customer at peak. The service teams just said "nope, sorry, you're going to kill us".

The requirements for prime day/black friday/cyber monday were mind-boggling.

That was quickly adjusted. I think like 3 years ago, Amazon publicly was saying that every day to AWS they were adding the same amount of computing power that was used to run Amazon.com when it was a $10 billion business. AWS is massively greater in scale than Amazon.com at this point.

Scaling services for many independent businesses is somewhat of a different challenge than “vertically” scaling for one large one like amazon.com.

I cannot confirm or deny that (since my memories are sorta fuzzy), but the real problem was not EC2 capacity. Many other AWS services were involved. :)

This is not right from an infra POV. You are just moving the problem (by team) but it doesn’t go away. Then you end up with duplication.

At the same time you are also giving the other team more flexibility to solve the problem, as they can optimize it for the Amazon.com use case and are not constrained to a generic solution that is optimized for the masses.

I'm a big fan of dogfooding. But given Amazon's ecommerce huge computing needs, maybe it would make sense to not burden the compute availability of your fledgling cloud offering with your ginormous system?

If built well your compute offering can scale with the resources you give it. If you have Amazon’s commerce platform running on a pool of computers, the team running commerce can give those computers to EC2 in exchange for an equivalent amount of EC2 quota. In theory it’s a bookkeeping operation.

There are plenty of good reasons why you wouldn’t run Amazon’s commerce on EC2, but I don’t think cloud availability is one of them.

FWIW 100% of the Amazon retail website is run on EC2 (and iirc it's 100% on spot instances too). (There are of course lots of internal services that still use non-ec2 capacity but fewer and fewer every day.)

note though it is not 100% AWS, ex they use Akamai instead of cloudfront

I don't think the spot thing is accurate. Amazon retail EC2 capacity is more like reserved instances.