How organize terraform modules for common load balancer

Question

I have AWS ALB that using for multiple EC2 instances. In Terraform i have alb module which already created load balancer and listeners, target groups for that EC2 instances.

I want attach ec2 instance to target group . I did this in EC2 instance module which got ALB instace id from load balancer module outputs.

When i perform terraform apply command in ec2 instance module terraform wants to crete new ALB but its already created and using. Terraform state in ec2 module don't know about ALB but wants to perform code from this module to get outputs

How can i organize modules using common ALB for multiple EC2 instances without recreating ALB for every new EC2 instance?

Answer 1

The way in which you should do this is the following:

• Create a module for ALB, in your output.tf output the target group arn that you would like your Ec2 instance(s) to use
• Create a module for EC2, include a aws_lb_target_group_attachment resource. Add a variable for the target group
• In your main.tf create the load balancer via the ALB module first, then call the EC2 module for each instance you want to use, for the target group argument reference the ALB module name and its output variable for the target group.

An example main.tf including only the relevant parts

module "my_alb_module" {
   .....
}

module "my_ec2_module" {
    target_group_arn = module.my_alb_module.target_group
}

As a point of best practice if you have components like a load balancer and ec2, try to keep them in separate modules if you want to support reusability between resources.

Whilst not a direct example checkout the code from this GitHub repository, it illustrates how values can be exported between modules.