Still many warmup request event after I set min resident instance

Question

My GAE/J application is set to use min and max idle instance both to 1:

Idle Instances: ( 1 – 1 ) 

To my understanding, there will be one and only one residence instance available there, so every time it try to serve a request, it will serve the traffic immediately(should be only one warmup request just after deployed).

But from the log I still can see quite a few warmup requests calling:

/_ah/warmup

before serving request.

Hope someone can explain, Thanks.

Answer 1

Idle instances are "reserve" instances (confusingly named Resident in the GAE console), only used during traffic increase while new instances are spun up. In steady traffic they are not be used.

So after a time of inactivity, when a new request arrives, it will be handled by an Idle instance, but a new normal instance will be spun up. Hence the warmup request.