UDP Server on Appengine Flex? - google-app-engine

Question

I would like to have a service in appengine flexible that has a UDP server that takes incoming udp traffic on a given port and redirects it to another service in appengine standard that uses HTTPS.
It is my understanding that flex environment allows opening UDP listen sockets and indeed my application starts the server OK. However, I cannot make any traffic reach the UDP server.
I suspect the problem is a GAE or Docker configuration problem but I cannot find documentation or similar issues online to solve it. All Google documentation for appengine flexible is around HTTPS. So any guidance would be helpful. I have several questions that I believe relate to my understanding on Flexible Appengine, the VM and Docker:
Is flex appengine supposed to be used at all as a UDP server? lack
of documentation on UDP load balancing seems to indicate me no...
Any ideas if this is on the roadmap?
If supported, to which IP/URL should I direct my UDP traffic? Is it to my-project . appspot . com or to each of the individual VM instances (would seem like a bad idea since VMs are ephemeral)?
This is my current application
app.yaml
As you can see I forwarded my listen UDP port as explained here
runtime: python
env: flex
entrypoint: python main.py
runtime_config:
python_version: 2
network:
forwarded_ports:
- 13949/udp
service: udp-gateway
# This sample incurs costs to run on the App Engine flexible environment.
# The settings below are to reduce costs during testing and are not appropriate
# for production use. For more information, see:
# https://cloud.google.com/appengine/docs/flexible/python/configuring-your-app-with-app-yaml
manual_scaling:
instances: 1
resources:
cpu: 1
memory_gb: 0.5
disk_size_gb: 10
For the server I am using python SocketServer in threaded mode and I am keeping my main thread in an infinite loop in order not to exit the server.
I have also added a firewall rule in my GCP console:
{
"allowed": [
{
"IPProtocol": "udp",
"ports": [
"13949"
]
}
],
"creationTimestamp": "2018-02-24T16:39:24.282-08:00",
"description": "allow udp incoming on 13949",
"direction": "INGRESS",
"id": "4340622718970870611",
"kind": "compute#firewall",
"name": "allow-udp-13949",
"network": "projects/xxxxxx/global/networks/default",
"priority": 1000,
"selfLink": "projects/xxxxx/global/firewalls/allow-udp-13949",
"sourceRanges": [
"0.0.0.0/0"
]
}

Answer 1

So I ended up being able to answer my own questions (thanks SO for allowing me to put down my thoughts, it helps :))
Indeed, flex environment only features a load balancer for HTTPS, which means that even if it is possible to open UDP sockets, it is not meant to be used as an UDP server. I have not found any evidence, Google plans to add support for UDP/TCP load balancing for Appengine flex. The next service that offers UDP load balancing is Kubernetes Engine (and Compute Engine of course). So that is where I am headed now.
With the configuration described in the OP, I could make traffic reach my application by addressing individual instances' IP. However, this is not meant to be used in a production application since instances are ephemeral and also does not scale (would need to do my own load balancer which is out of the question),