Regarding your first question. As documented here [1] Your app Engine 
should be subscribed to receive push requests. A Pub/Sub subscription can 
be configured to send messages as an HTTP POST request to a URL which can 
be your App Engine Standard URL. On App Engine Standard you have at least 
two options managing App Scalation. If your App Scales to Zero, the HTTP 
Post from Pub/Sub will immediately "Wake up" your instance and execute the 
logic you have programmed. Take into account that "Waking up" your App 
Engine Standard instance would take some warm up time. You can also 
configure your app.yaml [2] to not scale to zero, and have a 
min_idle_instance value of 2 or 3 to handle the pubsub requests without 
warm up time involved. This is up to you. 

Regarding your second question.It is encouraged by Google to use the 
Product Client Libraries which have the Application Default Credentials 
included, for you not to struggle with authentication using Google's 
products. So yes, Start up your instance with the Standard libraries and 
use the product client libraries needed. 

[1] https://cloud.google.com/pubsub/docs/push
[2] https://cloud.google.com/appengine/docs/standard/python/config/appref
On Monday, September 30, 2019 at 11:42:28 AM UTC-5, Nicholas Swekosky wrote:
>
> I'm considering migrating from AppEngine Flex to AppEngine Standard and I 
> have significant experience with both. I have 2 quick questions regarding 
> the migration and figured would post them here. Any insight would be 
> appreciated.
>
> 1. Let's say I have an AppEngine Standard service who's sole job is to 
> pull from a PubSub subscription and do some work. Let's say some time has 
> gone by and no work has been posted to that PubSub topic so my worker has 
> scaled to 0. If something new has been posted to that PubSub topic, will 
> that sandbox AppEngine Standard worker automatically scale to 1 to begin 
> working again?
>
> 2. Let's say I have a service on AppEngine Flex that I'm considering 
> migrating to AppEngine Standard. My existing service uses Google Cloud's 
> golang libraries and I'm considering the amount of work it would take to 
> migrate to the AppEngine Standard client libraries. For simplicity, let's 
> assume I'm only using AppEngine, PubSub, OAuth, & Datastore. Could I only 
> use the AppEngine Standard libraries to start the instance and use the 
> Google Cloud libraries for PubSub, OAuth, & Datastore?
>
> Please let me know if more detail is needed for these 2 questions. Thanks 
> so much!
>

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