handling DDoS to hosted CDN cache

How often does your hosted CDN cache get DDoS'ed? I am curious how these get handled (especially when it would cause upstream/backbone congestion). Is this treated differently than DDoS to customers? Any experience to share on working with CDNs to solve these issues? Any CDN that provides good information/resources for mitigation (e.g. drop A traffic, ratelimit B traffic to x pps)? If the cache provides flowspec feed, how useful would it be? Yang