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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?

Chris Williams
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user13871033
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1 Answers1

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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.

Chris Williams
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  • But can i have separate alb and ec2 module which using they common alb and common ec2 modules . And in ec2 module i will have aws_lb_target_group_attachment without recreating alb? – user13871033 Jul 07 '20 at 18:10
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    Yes you can do this :) – Chris Williams Jul 07 '20 at 18:11
  • I often see modules where the target group is created along with the EC2 instances instead of the ALB, and it always feels like a mistake compared to this method. Changes to listeners, rules, and target groups always seem to take time to propagate, and the LB serves 502s in the meantime. Making the changes via membership in target groups is the way to go. – Indigenuity Dec 21 '21 at 19:41