> There's a difference between S3 API spec and what Amazon does with S3 - for isntance, the new CAS capabilities with Amazon are not part of the spec.

Sure, but those are S3 APIs and features that provided by AWS. We not talking about S3 spec, we're talking about s3 product.

You’re wrong. “Implementing the S3 API” means the spec, not an Amazon product.

We're talking about using AWS S3 and using something that implements S3 spec. Really not that hard to understand.

When customer wants to switch away from AWS S3, customer cares about AWS S3 feature coverage and not the spec.

Spec and features are intertwined. Customers who switch away from AWS S3 still want to use the same SDKs, libraries, etc that support S3 API. They don't want to rewrite their applications to use a new API. So then is it feature coverage or spec coverage?

It's both? Customer doesn't care if spec is 100% covered if feature that they are using in AWS S3 isn't supported.

Also, who is rewriting their application to change interaction with an object storage? People that directly use some S3 sdk all around the app should read a book on software engineering or at least a blog post.

Wrong again. Not only have I worked on DigitalOcean’s S3 implementation but I currently work on an open source product that targets S3 spec and can be used with any cloud provider and any other spec-compliant drop-in, like Garage.

All that means: you only used AWS S3 features that are in the S3 spec.