Terraform Outputs Across Modules: Linking Your Infrastructure the Right Way

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Understanding Outputs Recap

As covered in Blog 4, output blocks let you expose values from your config like instance IDs, IPs, or names.

Basic example:

output "instance_id" {
  value = aws_instance.web.id
} 

But the real power of outputs is unlocked when you start working with modules.

How Module Outputs Work

When a module defines an output, it can be accessed from the parent module like this:

Step 1: Inside the module

output "subnet_id" {
  value = aws_subnet.main.id
} 

Step 2: In the root module

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

output "public_subnet" {
  value = module.network.subnet_id
}

Hands-On Example: Sharing Subnet ID from VPC Module to EC2 Module

modules/vpc/outputs.tf

output "public_subnet_id" {
  value = aws_subnet.public.id
}

main.tf (Root Module)

module "vpc" {
  source = "./modules/vpc"
  cidr_block = "10.0.0.0/16"
}

module "ec2" {
  source     = "./modules/ec2"
  subnet_id  = module.vpc.public_subnet_id
}

modules/ec2/variables.tf

variable "subnet_id" {
  type = string
}

modules/ec2/main.tf

resource "aws_instance" "web" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  subnet_id     = var.subnet_id
}

This pattern ensures your infra layers are loosely coupled but logically connected.

📦 Best Practices for Output Management

✅ Name outputs clearly – reflect purpose, not just resource type
 ✅ Use outputs for IDs, IPs, metadata – avoid exposing full objects
 ✅ Reference only what you need – keep modules decoupled
 ✅ Use depends_on only if absolutely necessary – outputs already imply order
 ✅ Group outputs in one file (outputs.tf) – improves readability

⚠️ Common Pitfalls to Avoid

❌ Using entire resources as output (value = aws_instance.web)
 ❌ Referencing output that’s not yet created → causes apply failures
 ❌ Creating circular dependencies (Module A needs output from Module B and vice versa)
 ❌ Over-exposing internal values — keep modules as black boxes where possible

🛠️ Debugging Output Issues

If an output isn’t showing up:

• ✅ Check spelling in both module and parent reference

• ✅ Run terraform output to inspect available outputs

• ✅ Use terraform console to evaluate expressions

• ✅ Check whether the resource producing the value exists in the same apply phase

💡 Tip of the Day:

📚 Resources & References

1️⃣ Terraform Output Values
🔗 Docs
Official documentation for output blocks and usage.

2️⃣ Module Composition in Terraform
🔗 Guide
How to build clean input/output APIs for modules.

3️⃣ terraform output CLI Command
🔗 Docs
Inspect values after apply or for debugging.

4️⃣ Using Outputs in CI/CD
🔗 Blog
Pass outputs into pipelines, scripts, and next-stage Terraform runs.

5️⃣ Terraform Console for Debugging
🔗 Docs
Explore outputs, expressions, and values interactively.

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