Google app engine can requests fail while shifting traffic to the new version?

[Sudhanshu Gaur]

I am just wondering that as when I deploy my NodeJs app to new version on App Engine, now the traffic gets shifted from old app version to new app version. Or when I manually shift traffic to some other version.

Then is there any chances that in between shifting the traffic to the new server, can some requests fail or is it 100 percent Safe by default?

[Kenworth (Google Cloud Platform)]

The answer is different depending on the App Engine environment. GAE Standard for example, supports gradual traffic migration wherein you can enable 'warmup request' to prepare the new instances before those instances serve any traffic.

In contrast, GAE Flex does not support gradual traffic migration which means user requests are sent to those instances before the instances get created. This results to delay/latency (not an outright fail) since the request has to wait until the instance gets up and running. It is possible that request may fail if the request timeouts before the instance is ready. See full article here.