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Terraform

How to use one module’s variable in another module in terraform?

August 1, 2023 comments off

To get the output from one module to another module in Terraform, you can use the following steps:

  1. Define the output in the first module.
  2. Import the second module.
  3. Reference the output of the first module in the second module.

Here is an example of how to do this:

# First module
module "web_server" {
  source = "./web_server"

  output "instance_ip_addr" {
    value = aws_instance.web_server.public_ip
  }
}

# Second module
module "database" {
  source = "./database"

  instance_ip_addr = module.web_server.instance_ip_addr
}

In this example, the web_server module defines an output called instance_ip_addr. The database module imports the web_server module and then references the instance_ip_addr output in its own configuration.

When you run terraform apply, Terraform will create the resources defined in both modules. The database module will use the instance_ip_addr output from the web_server module to connect to the web server.

Here is an explanation of the code:

  • The source attribute in the module block specifies the location of the module definition. In this case, the module definition is located in the web_server directory.
  • The output block defines an output called instance_ip_addr. The value of the output is the public IP address of the web server instance.
  • The source attribute in the module block specifies the location of the module definition. In this case, the module definition is located in the database directory.
  • The instance_ip_addr attribute references the instance_ip_addr output from the web_server module.

You can use outputs to accomplish this. In Terraform, a module can also return values. Again, this is done using a mechanism you already know: output variables. Such as if there are 2 modules, kafka and cloudwatch.

In your kafka module, you could define an output that looks something like this:


output "instance_ids" {
  value = ["${aws_instance.kafka.*.id}"]
}

Case 1 – If you are calling kafka module from cloudwatch module by instantiated the module with something like:


module "kafka" {
  source = "./modules/kafka"
}

# You can then access that output as follows:
instances = ["${module.kafka.instance_ids}"]

Case 2 – If your modules are isolated from each other (i.e. your cloudwatch module doesn’t instantiate your kafka module), you can pass the outputs as variables between modules:


module "kafka" {
  source = "./modules/kafka"
}

module "cloudwatch" {
  source = "./modules/cloudwatch"
  instances = ["${module.kafka.instance_ids}"]
}

# Of course, your "cloudwatch" module would have to declare the instances variable.

Case 3 – You can add the ASG name as an output variable in /modules/services/webserver-cluster/outputs.tf as follows.

output "asg_name" {
  value       = aws_autoscaling_group.example.name
  description = "The name of the Auto Scaling Group"
}

You can access module output variables the same way as resource output attributes. The syntax is:

module.<MODULE_NAME>.<OUTPUT_NAME>
module.frontend.asg_name
Rajesh Kumar
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