Connecting Google Cloud Platform's compute engine and app engine via VPC connector - postgresql

Question

I'd like to know in detail how to connect google compute engine virtual machine instance and app engine.
I've set up a virtual machine instance on Google compute engine, and my Postgres server is running there, following this tutorial: https://cloud.google.com/community/tutorials/setting-up-postgres
I've deployed my flask app under the same project on Google Cloud Platform, creating an app engine instance.
I searched on how to connect compute engine and app engine together, and it seems it should be possible through a VPC connector: connect Google App Engine and Google Compute Engine
This is what my VPC connector looks like:
Serverless VPC access
Name Network Region IP address range Min. throughput Max. throughput
connector-name default europe-west2 10.8.0.0/28 200 300
On my compute engine, I have my VM instance like so:
Name Zone Internal IP External IP
some-name europe-west2-c 10.154.0.2 (nic0) 34.89.113.193
On my flask app, I'm trying to connect to my remote DB like so:
db = PostgresqlExtDatabase(
"some-name", # databse name
user="postgres",
password="some-password",
host="10.154.0.2", # remote host internal ip
port=5432,
)
db.connect()
This is my app.yaml for the vpc access part, I've followed this reference: https://cloud.google.com/appengine/docs/standard/python/connecting-vpc#configuring
vpc_access_connector:
name: projects/some-name/locations/europe-west2/connectors/connector-name
If I understood correctly, if the VPC connector is present, I should just be able to connect using the internal IP address of my VM instance(this case, 10.154.0.2)?
The problem is, when the app is deployed for production, It is still complaining that it cannot connect:
2020-09-26 12:54:51 default[20200926t134815] Is the server running on host "10.154.0.2" and accepting
2020-09-26 12:54:51 default[20200926t134815] TCP/IP connections on port 5432?
If it's connected internally I assume I don't have to add that internal IP to firewall rules, although I did try that as well. As for firewall rules, I have allowed my local machine's IP address so I can connect to the remote Postgres server via PgAdmin.
I've actually tried External IP(34.89.113.193) as well although that doesn't make sense to me.
I'm a bit of a noob on networks and backend stuff in general, any help would be much appreciated.
UPDATED 1
This is my firewall rules:
Direction
Ingress, Egress
Action on match
Allow
Source filters
IP ranges
92.40.176.9/32
78.146.103.141/32
10.154.0.2
Protocols and ports
tcp:5432
Image for reference: Screenshot for the list of firewall rules

Answer 1

It turns out the firewall / postgres configurations were all ok, but because this VPC connector method was on beta, I needed to run:
gcloud beta app deploy
instead of the usual
gcloud app deploy.
This command then updated gcloud Beta Commands and prompted me to enable API:
API [appengine.googleapis.com] not enabled on project [742932836941]. Would you like to enable and retry (this will take a few minutes)? (y/N)?
After enabling this everything worked fine.

Answer 2

Per the information provided seems like both VPC firewall rules and the connector are well configured.
However, based on the messages
2020-09-26 12:54:51 default[20200926t134815] Is the server running on host "10.154.0.2" and accepting
2020-09-26 12:54:51 default[20200926t134815] TCP/IP connections on port 5432?
Seems like the VM or server using 10.154.0.2 is not accepting requests on port 5432 or the port has not been opened, you can use this site to do a port scan.
Based on the guide you followed to create PostgreSQL you are using Ubuntu as OS, therefore I suggest you open the port in ubuntu and see if the issue persists.