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How is the service being able to answer the question "is there budget available for this action?" different from "is there authorization for this action?"

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One example - Google Cloud network egress charges aren’t known until up to ~2 days after they happen. Since they can be obscenely expensive (eg $0.23/GB), they can make budget computation difficult.

What is the cause of this delay?

I don't know for sure, but based on my knowledge as a user, I'd guess it could be something like delivering usage logs from points of presence in the CDN. PoPs can go offline regularly, they're highly dependent on other people's networks, 2 days might be the arbitrary line that has been drawn that gets enough of them in most circumstances, while not being too annoying for customers.

Authorisation is much more cacheable than a value that inherently changes every single time you check it.

Also authorisation revocation is relatively uncommon, which means you can have a fast-path for approval, and then push only the revoked key IDs to just frontend servers.

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