How to manage terraform for multiple repos

You can use terraform remote_state for this. It lets you read the output variables from another terraform state file.

Lets assume you save your state files remotely on s3 and you have your website.tfstate and server.tfstate file. You could output your hosted zone ID of your route53 zone as hosted_zone_id in your website.tfstate and then reference that output variable directly in your server state terraform code.

data "terraform_remote_state" "website" {
  backend = "s3"

  config {
    bucket = "<website_state_bucket>"
    region = "<website_bucket_region>"
    key = "website.tfstate"
  }
}

resource "aws_route53_record" "www" {
  zone_id = "${data.terraform_remote_state.website.hosted_zone_id}"
  name    = "www.example.com"
  type    = "A"
  ttl     = "300"
  records = ["${aws_eip.lb.public_ip}"]
}

Note, that you can only read output variables from remote states. You cannot access resources directly, as terraform treats other states/modules as black boxes.

Update

As mentioned in the comments, terraform_remote_state is a simple way to share explicitly published variables across multiple states. However, it comes with 2 issues:

1. Close coupling between code components, i.e., producer of the variable cannot change easily.
2. It can only be used by terraform, i.e., you cannot easily share those variables across different layers. Configuration tools such as Ansible cannot use .tfstate natively without some additional custom plugin/wrapper.

The recommended HashiCorp way is to use a central config store such as Consul. It comes with more benefits:

1. Consumer is decoupled from the variable producer.
2. Explicit publishing of variables (like in terraform_remote_state).
3. Can be used by other tools.

A more detailed explanation can be found here.

An approach I've used in the past is to have a single repo for all of the Infrastructure.

An alternative is to have 2 separate tf configurations, each using remote state. Config 1 can use output variables to store any arns/ids as necessary.

Config 2 can then have a remote_state data source to query for the relevant arns/ids.

E.g.

# Declare remote state
data "terraform_remote_state" "network" {
  backend = "s3"
  config {
    bucket = "my-terraform-state"
    key    = "network/terraform.tfstate"
    region = "us-east-1"
  }
}

You can then use output values using standard interpolation syntax ${data.terraform_remote_state.network.some_id}