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i want to expose my postgres instance to the world. to do that I need to setup a security group but i'm in a catch-22 situation. to create the security group I need the port number from the RDS instance, and to create the RDS instance I need the security group 🤷

This is maybe not helpful but: I have my security group created before I create(d) the RDS instance. In my case it was a permissions thing: The Security group was created by someone with appropirate rights on our aws account so were the subnets. and I just use SecurityGroup.get() to aquire it (even if no rds cluster is existant yet). So I think it is possible but what exactly you need to do I cannot say. 🙈

It think it might! I'm using the postgres module for now but that means I have to make the database public. I'm going to try your solution next.

You can also try to use ec2.NetworkInterfaceSecurityGroupAttachment, but there are some caveats if you already set security groups in another way