Referring to variables in another terraform file you can't modify

Question

I'm using a tool named kops that generates a terraform file to set up some infrastructure for kubernetes. After that, we want to use terraform to create parts of our infrastructure specific to our application. e.g., a queue, a proxy, elasticache, etc.

The terraform file that kops generates has a lot of information in it that I'd like to refer to when creating the queue/proxy/elasticache. e.g., the subnet ranges to use, the cidr blocks, the availability zones, etc. But, I don't want to modify the kops generated terraform file because whenever there's a kops upgrade, I'll have to re-generate it then re-modify it.

The terraform file that kops generates doesn't provide any output variables. I could append my queue/proxy/elasticache configurations to the bottom of the file that kops generates. Then I'd be able to refer to the kops generated variables. But I consider this to be a modification to the kops generated file and would like to avoid this for the reasons above.

How can I make my custom terraform reference the parts of a generated terraform file?

Answer 1

If there are no output variables in the generated terraform files and you do not want to change them, how about using data sources?

https://www.terraform.io/docs/configuration/data-sources.html

Data sources allow data to be fetched or computed for use elsewhere in Terraform configuration. Use of data sources allows a Terraform configuration to build on information defined outside of Terraform, or defined by another separate Terraform configuration.