I have a simple webcam app used by a maximum of maybe 3 people at any time, 
which also handles requests from a batch process initiated from my house 
which uploads a new image to my app every 15 seconds via HTTP call to my 
app's frontend.  My goal is to get the app running on just a single frontend 
instance at all times, making this as close to a free app as possible, but 
this is proving much more difficult than I thought it would.

When no one is connected to my app, all the uploads go to a single instance. 
 All the requests complete with around 200ms latency.

However, as soon as one user accesses the main page of my app, a new 
instance spins up.  *This in spite of the fact that a browser request to my 
app typically completes in well under a second, and Min Pending Latency is 
set to 15 seconds.*

What is it that would make the scheduler think that one instance won't 
handle both sets of requests, when the Min Pending Latency is set so high 
and none of the requests come anywhere near this threshold? One theory: I 
remember reading on these forums a while back, under a topic regarding 
keeping an instance "always on", that the scheduler has strategies to 
prevent you from keeping an instance "always on" by constantly pinging it. 
My 15-second periodic upload is similar to a ping in this sense.  I don't 
intend anything nefarious, but the scheduler wouldn't know this, and maybe 
is just trying to close a loophole that someone else could exploit.

I'd use a backend to handle the image uploads, but I need it running 24 
hours and a 24-hour backend isn't free.

I don't think this would work as a "free" app by Google's billing terms 
(which I believe would restrict me to a single frontend by default), as it's 
likely I'll go over quota on datastore operations farily regularly.

Any ideas on how I can keep my app as cheap as possible without sacrificing 
functionality?

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