A is correct

no proxy needed

People referencing "VPC Network Peering does not provide transitive routing. For example, if VPC networks net-a and net-b are connected using VPC Network Peering, and VPC networks net-a and net-c are also connected using VPC Network Peering, VPC Network Peering does not provide connectivity between net-b and net-c." the question states that cloud sql is running in project B. Which means the instance is already part of the VPC in project B, so with Network Peering workers from A can definitely access data in B. No proxy is needed.

Why so many people are voting for D? There's no need for a proxy, the peering is enough to allow network traffic between subnets.

Proxy? no, it is not necessary.. A

https://cloud.google.com/sql/docs/mysql/connect-multiple-vpcs

A - The requirement for a proxy is un-necessary: https://cloud.google.com/sql/docs/mysql/private-ip#multiple_vpc_connectivity

Option D. Source: https://cloud.google.com/sql/docs/mysql/private-ip#multiple_vpc_connectivity

Option D

the reason : Cloud SQL supports private IP addresses through private service access. When you create a Cloud SQL instance, Cloud SQL creates the instance within its own virtual private cloud (VPC), called the Cloud SQL VPC. Enabling private IP requires setting up a peering connection between the Cloud SQL VPC and your VPC network.

Using VPC Network Peering, Cloud SQL implements private service access internally, which allows internal IP addresses to connect across two VPC networks regardless of whether they belong to the same project or organization. However, since VPC Network Peering isn't transitive, it only broadcasts routes between the two VPCs that are directly peered. If you have an additional VPC, it won't be able to access your Cloud SQL resources using the connection set up with your original VPC.

D. Set up VPC Network Peering between Project A and Project B. Create a Compute Engine instance without external IP address in Project B on the peered subnet to serve as a proxy server to the Cloud SQL database.

Option D is the most aligned to best practices for me

Option D is the correct answer. The reason is you cannot access cloud sql or alloydb instances from a peered vpc connection as they will be hosted in service project not in Project B. The VPC Peering doesn't give transitive routing so accessing cloud sql directly is not possible without a proxy vm. https://cloud.google.com/vpc/docs/vpc-peering#spec-general

D is the correct solution. To allow the Dataflow workers in Project A to connect to the private Cloud SQL instance in Project B, you need to set up VPC Network Peering between the two projects. Then create a Compute Engine instance without external IP in Project B on the peered subnet. This instance can serve as a proxy server to connect to the private Cloud SQL instance. The Dataflow workers can connect through the peered network to the proxy instance, which then connects to Cloud SQL. This allows accessing the private Cloud SQL instance without going over the public internet. Option A would allow access but still goes over the public internet. Option B and C would not work since the Cloud SQL instance does not have a public IP address. So D is the right approach to resolve the connection issue while keeping the data private.

VPC Network Peering allows for the connection of two VPC networks so that they can communicate internally as if they were part of the same network.

Secure Private Communication: Establishes a direct, private connection between the VPCs, eliminating exposure to the public internet. Ensures data confidentiality and integrity.