> The push subscription will just ramp up attempting to DoS your Cloud Run service

Interesting. My assumption would be that Cloud Run should quickly* spin up more containers to handle the spike and then spin them down again. So there would be no need for back pressure? Guess it depends on the scale? How big of a spike are we talking about? :)

*Let's say a few seconds

You typically have it configured with a cap on instances for cost reasons.

Even if you don't, if you have other services (like a database) downstream, you might not want it to scale infinitely as then you're simply DoS'ing the DB instead of the Cloud Run service.

Backpressure is really important for the resiliency of any distributed system, IMO.