GAE: instances count drops to 0 but min_instances is set to 1

Question

I'm using GAE standard with nodejs10.

I redeployed a new version of the app approx 9 hours ago with this conf (as shown in the UI under App Engine > Versions):

runtime: nodejs10
env: standard
instance_class: F1
handlers:
  - url: '.*'
    script: auto
  - url: '.*'
    script: auto
automatic_scaling:
  min_idle_instances: automatic
  max_idle_instances: automatic
  min_pending_latency: automatic
  max_pending_latency: automatic
  min_instances: 1
  max_instances: 3

And yet I noticed that the instances count dropped to 0:

Any idea why GAE doesn't keep 1 instance running?

Answer 1

GAE standard is autoscaled to zero instance by default. Whenever a url is matched by handlers according to the conf, an instance will be initiated and will continue to be live until there is no more url request for a period of time, so it will automatically rescale to zero if there is no more traffic for a period. Here is the instance state for my test:

automatic_scaling:
  min_instances: 2

And here is the traffic: