Terraform_remote_state usage - Commands & Configuration

Introduction

When you work on infrastructure with Terraform, you often need to share information between different parts of your setup. Terraform remote state lets you save and access this shared information safely and reliably.

When you have multiple Terraform projects that need to share outputs like IP addresses or resource IDs.

When you want to keep your Terraform state files in a central place to avoid conflicts.

When you work in a team and want everyone to use the same infrastructure data.

When you want to separate infrastructure layers, like networking and applications, but connect them.

When you want to back up your Terraform state securely in cloud storage.

Config File - main.tf

main.tf

terraform {
  backend "s3" {
    bucket = "example-terraform-state"
    key    = "network/terraform.tfstate"
    region = "us-east-1"
  }
}

provider "aws" {
  region = "us-east-1"
}

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
  tags = {
    Name = "example-vpc"
  }
}

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


# Another Terraform configuration to use remote state

terraform {
  required_version = ">= 1.0"
}

provider "aws" {
  region = "us-east-1"
}

data "terraform_remote_state" "network" {
  backend = "s3"
  config = {
    bucket = "example-terraform-state"
    key    = "network/terraform.tfstate"
    region = "us-east-1"
  }
}

resource "aws_subnet" "app_subnet" {
  vpc_id     = data.terraform_remote_state.network.outputs.vpc_id
  cidr_block = "10.0.1.0/24"
  tags = {
    Name = "app-subnet"
  }
}

The first part configures Terraform to store its state file in an S3 bucket under the path network/terraform.tfstate. It creates a VPC and outputs its ID.

The second part shows how another Terraform project can read that VPC ID from the remote state using terraform_remote_state data source, then create a subnet inside that VPC.

Commands

This command initializes Terraform, setting up the backend to store state remotely in S3 and downloading necessary plugins.

Terminal

terraform init

Expected Output

Initializing the backend...



Successfully configured the backend "s3"!



Terraform has been successfully initialized!



You may now begin working with Terraform. Try running "terraform plan" to see

any changes that are required for your infrastructure. All Terraform commands

should now work.

This command applies the Terraform configuration, creating the VPC and saving the state remotely in S3.

Terminal

terraform apply -auto-approve

Expected Output

aws_vpc.main: Creating...

aws_vpc.main: Creation complete after 3s [id=vpc-0abcd1234efgh5678]



Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

→

-auto-approve - Skip manual approval to apply changes immediately

Initialize the second Terraform project that will use the remote state to read the VPC ID.

Terminal

terraform init

Expected Output

Initializing the backend...



Successfully configured the backend "s3"!



Terraform has been successfully initialized!



You may now begin working with Terraform. Try running "terraform plan" to see

any changes that are required for your infrastructure. All Terraform commands

should now work.

Apply the second Terraform configuration, which creates a subnet inside the VPC using the remote state output.

Terminal

terraform apply -auto-approve

Expected Output

aws_subnet.app_subnet: Creating...

aws_subnet.app_subnet: Creation complete after 2s [id=subnet-0abcd1234efgh5678]



Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

→

-auto-approve - Skip manual approval to apply changes immediately

Key Concept

If you remember nothing else from this pattern, remember: Terraform remote state lets separate projects share infrastructure data safely by storing state files in a central place.

Common Mistakes

Not initializing Terraform before applying changes

Terraform needs to set up the backend and download plugins before it can work properly.

Always run 'terraform init' before 'terraform apply' or 'terraform plan'.

Using different backend configurations between projects

If backend settings like bucket or key differ, Terraform cannot find the remote state and will fail.

Ensure all projects accessing the remote state use the exact same backend configuration.

Trying to access outputs from remote state before applying the first project

The remote state file does not exist yet, so Terraform cannot read outputs.

Apply the first Terraform project to create resources and save outputs before referencing them remotely.

Summary

Configure Terraform backend to store state remotely in S3 for safe sharing.

Use 'terraform_remote_state' data source to read outputs from another Terraform project.

Run 'terraform init' before applying to set up backend and plugins.

Apply the first project to create resources and save state remotely.

Apply the second project to use remote outputs and create dependent resources.

Practice

1. What is the main purpose of using terraform_remote_state in Terraform?

easy

A. To create new resources in the cloud

B. To store Terraform state files locally on your machine

C. To safely share outputs from one Terraform project with another

D. To run Terraform commands faster

Solution

1. Step 1: Understand the role of terraform_remote_state

   The terraform_remote_state data source is used to access outputs from another Terraform state, enabling sharing data between projects.

2. Step 2: Differentiate from other Terraform functions

   It does not store state locally, create resources, or speed up commands; it only reads remote state outputs.

3. Final Answer:

   To safely share outputs from one Terraform project with another -> Option C

4. Quick Check:

   terraform_remote_state shares outputs safely [OK]

Hint: Remember: remote_state reads outputs from other projects [OK]

Common Mistakes:

• Thinking it stores state locally
• Confusing it with resource creation
• Assuming it speeds up Terraform commands

2. Which of the following is the correct syntax to declare a terraform_remote_state data source in Terraform?

easy

A. data "terraform_remote_state" "example" { backend = "s3" config = { bucket = "mybucket" key = "state.tfstate" region = "us-east-1" } }

B. resource "terraform_remote_state" "example" { backend = "s3" config = { bucket = "mybucket" key = "state.tfstate" region = "us-east-1" } }

C. variable "terraform_remote_state" { backend = "s3" config = { bucket = "mybucket" key = "state.tfstate" region = "us-east-1" } }

D. output "terraform_remote_state" { backend = "s3" config = { bucket = "mybucket" key = "state.tfstate" region = "us-east-1" } }

Solution

1. Step 1: Identify correct resource type for remote state

   The terraform_remote_state is declared as a data source, not a resource, variable, or output.

2. Step 2: Check syntax structure

   data "terraform_remote_state" "example" { backend = "s3" config = { bucket = "mybucket" key = "state.tfstate" region = "us-east-1" } } correctly uses data "terraform_remote_state" "example" with backend and config blocks.

3. Final Answer:

   data "terraform_remote_state" "example" { backend = "s3" config = { bucket = "mybucket" key = "state.tfstate" region = "us-east-1" } } -> Option A

4. Quick Check:

   terraform_remote_state is a data source [OK]

Hint: Use 'data' block, not 'resource' for remote_state [OK]

Common Mistakes:

• Using resource instead of data
• Declaring as variable or output
• Missing backend or config blocks

3. Given this Terraform snippet accessing remote state outputs:

data "terraform_remote_state" "network" {
  backend = "s3"
  config = {
    bucket = "net-state"
    key    = "network/terraform.tfstate"
    region = "us-west-2"
  }
}

output "vpc_id" {
  value = data.terraform_remote_state.network.outputs.vpc_id
}

What will be the output value of vpc_id if the remote state has vpc_id = "vpc-123abc"?

medium

A. null

B. "vpc-000000"

C. Error: output not found

D. "vpc-123abc"

Solution

1. Step 1: Understand remote state output access

   The code reads the remote state from S3 bucket and accesses the output named vpc_id.

2. Step 2: Match output value from remote state

   The remote state has vpc_id = "vpc-123abc", so the output will be exactly this string.

3. Final Answer:

   "vpc-123abc" -> Option D

4. Quick Check:

   Remote output vpc_id = "vpc-123abc" [OK]

Hint: Output matches remote state's output value exactly [OK]

Common Mistakes:

• Assuming output is null if not declared locally
• Confusing output with resource ID
• Expecting error if output exists remotely

4. You wrote this Terraform code to read remote state:

data "terraform_remote_state" "app" {
  backend = "s3"
  config = {
    bucket = "app-state"
    key    = "app/terraform.tfstate"
    region = "us-east-1"
  }
}

output "subnet_id" {
  value = data.terraform_remote_state.app.outputs.subnet_id
}

But Terraform shows error: Could not read state file. What is the most likely cause?

medium

A. The output name subnet_id is misspelled in the remote state

B. The S3 bucket or key does not exist or is inaccessible

C. You used resource block instead of data block

D. Terraform version is too old to support remote state

Solution

1. Step 1: Analyze error message

   "Could not read state file" usually means Terraform cannot find or access the remote state file in S3.

2. Step 2: Check configuration and permissions

   Verify the S3 bucket name, key path, and AWS permissions are correct and accessible.

3. Final Answer:

   The S3 bucket or key does not exist or is inaccessible -> Option B

4. Quick Check:

   State file access error means bucket/key issue [OK]

Hint: Check bucket/key existence and permissions first [OK]

Common Mistakes:

• Assuming output name typo causes state read error
• Confusing data block with resource block error
• Blaming Terraform version without checking config

5. You have two Terraform projects: network creates a VPC and outputs vpc_id. app needs to use that vpc_id. How should you configure app to use terraform_remote_state to get vpc_id from network stored in an S3 backend?

hard

A. In app, declare a data "terraform_remote_state" "network" block with backend "s3" and config matching network S3 bucket, key, and region, then access data.terraform_remote_state.network.outputs.vpc_id

B. In app, copy the vpc_id value manually from network outputs and hardcode it

C. In app, declare a resource "terraform_remote_state" "network" block with backend "s3" and config matching network

D. In app, use terraform_remote_state without specifying backend or config

Solution

1. Step 1: Understand cross-project state sharing

   To share outputs, app must declare a data "terraform_remote_state" block with backend and config matching network's S3 backend.

2. Step 2: Access the output properly

   Then app can access vpc_id via data.terraform_remote_state.network.outputs.vpc_id.

3. Final Answer:

   In app, declare a data "terraform_remote_state" "network" block with backend "s3" and config matching network S3 bucket, key, and region, then access data.terraform_remote_state.network.outputs.vpc_id -> Option A

4. Quick Check:

   Use data block with correct backend config to share outputs [OK]

Hint: Use data block with matching backend config to share outputs [OK]

Common Mistakes:

• Hardcoding output values instead of referencing remote state
• Using resource block instead of data block
• Omitting backend or config details