Which scaling type should you use?

You are deploying an application to App Engine. You want the number of instances to scale based on request rate. You need at least 3 unoccupied instances at all times.

Which scaling type should you use?
A . Manual Scaling with 3 instances.
B . Basic Scaling with min_instances set to 3.
C . Basic Scaling with max_instances set to 3.
D . Automatic Scaling with min_idle_instances set to 3.

Answer: D

Explanation:

Reference:

https://cloud.google.com/appengine/docs/standard/python/how-instances-are-managed

https://cloud.google.com/appengine/docs/standard/go/config/appref

"App Engine calculates the number of instances necessary to serve your current application traffic based on scaling settings such as target_cpu_utilization and target_throughput_utilization. Setting min_idle_instances specifies the number of instances to run in addition to this calculated number. For example, if App Engine calculates that 5 instances are necessary to serve traffic, and min_idle_instances is set to 2, App Engine will run 7 instances (5, calculated based on traffic, plus 2 additional per min_idle_instances)."

Automatic scaling creates dynamic instances based on request rate, response latencies, and other application metrics. However, if you specify the number of minimum idle instances, that specified number of instances run as resident instances while any additional instances are dynamic.

Ref: https://cloud.google.com/appengine/docs/standard/python/how-instances-are-managed