I see no way to connect my app engine backends with Cloud CDN. Seems like it's only available when using Compute Engine directly.
2 Answers
No, that is not currently possible. Currently, Google Cloud CDN can be used only to serve content that originates from Cloud Storage buckets, Compute Engine instance groups, and Compute Engine network endpoint groups. I agree that it would make sense to support App Engine Standard, too!
Update
Currently in beta is Serverless NEGs (Network Endpoint Groups) which will allow you to use App Engine behind a HTTP(S) Load balancer and give you all the features provided with that including CDN
Original
Yes, App Engine will serve content via Google Cloud CDN as long as a cacheable response is returned.
Try setting the Cache-Control header to something like max-age=86400, public and ensuring a Set-Cookie header is not sent with the response
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