Terraform module output

Outputs are especially useful when working with modules, because they let one module expose values that another module (or the root configuration) can use.

Think of it like this:

• A module is a “black box”
• Outputs are the data it returns

🔹 Basic flow across modules

1. Child module defines outputs

Inside a module (e.g., modules/network/outputs.tf):

output "vpc_id" {
  value = aws_vpc.main.id
}

output "subnet_ids" {
  value = aws_subnet.public[*].id
}

This exposes the VPC ID and subnet IDs to whoever calls the module.

2. Root module accesses those outputs

In your main configuration:

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

You can now use the outputs like this:

module.network.vpc_id
module.network.subnet_ids

3. Pass outputs into another module

Now let’s say you have a compute module that needs a VPC:

module "compute" {
  source = "./modules/compute"

  vpc_id     = module.network.vpc_id
  subnet_ids = module.network.subnet_ids
}

👉 Here, the output of one module becomes the input of another.

🔹 Visual idea

[ network module ]
        │
        ▼ (outputs)
   vpc_id, subnet_ids
        │
        ▼
[ compute module ]

🔹 Inside the receiving module

The compute module must define variables:

variable "vpc_id" {}
variable "subnet_ids" {}

Now it can use them internally:

resource "aws_instance" "app" {
  subnet_id = var.subnet_ids[0]
}

🔹 Optional: Re-expose outputs at root

You can also “bubble up” values:

output "vpc_id" {
  value = module.network.vpc_id
}

🔹 Key takeaway

Outputs across modules enable:

• Loose coupling (modules don’t depend on internal details)
• Reusability (modules become plug-and-play)
• Clear data flow (inputs → resources → outputs → next module)