Remote state data source in Terraform - Mini Project: Build & Apply

Using Remote State Data Source in Terraform

📖 Scenario: You are managing infrastructure with Terraform. You have one Terraform configuration that creates a virtual network. Another Terraform configuration needs to use the network's ID from the first configuration. To do this, you will use Terraform's remote state data source to access the network ID.

🎯 Goal: Build a Terraform configuration that reads the virtual network ID from a remote state stored in an S3 bucket and uses it to create a subnet.

📋 What You'll Learn

Create a Terraform backend configuration for remote state stored in an S3 bucket

Define a data source to read the remote state

Extract the virtual network ID from the remote state

Use the extracted network ID to create a subnet resource

💡 Why This Matters

🌍 Real World

In real projects, infrastructure is often split into multiple Terraform configurations. Using remote state data sources allows sharing information like network IDs between these configurations safely and cleanly.

💼 Career

Understanding remote state data sources is essential for cloud engineers and DevOps professionals to manage complex infrastructure deployments across teams and environments.

1

Configure Terraform backend for remote state

Write a terraform block to configure the backend to use an S3 bucket named my-terraform-state in region us-west-2 with key subnet/terraform.tfstate.

Terraform

# Your code here

Hint

The terraform block configures where Terraform stores its state remotely. Use the backend "s3" block with the exact bucket, key, and region values.

2

Define remote state data source

Add a data block named remote_network of type terraform_remote_state that points to the same S3 bucket my-terraform-state, key network/terraform.tfstate, and region us-west-2.

Terraform

terraform {
  backend "s3" {
    bucket = "my-terraform-state"
    key    = "subnet/terraform.tfstate"
    region = "us-west-2"
  }
}

# Your code here

Hint

The terraform_remote_state data source reads state from a remote backend. Use the same bucket, key, and region as the backend configuration.

3

Extract virtual network ID from remote state

Create a locals block with a value called vnet_id assigned the value of data.terraform_remote_state.remote_network.outputs.vnet_id to get the virtual network ID from the remote state outputs.

Terraform

terraform {
  backend "s3" {
    bucket = "my-terraform-state"
    key    = "subnet/terraform.tfstate"
    region = "us-west-2"
  }
}

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

# Your code here

Hint

Use a locals block with the exact name vnet_id and assign the remote state output value to it.

4

Create subnet resource using remote network ID

Add a resource block named azurerm_subnet called example_subnet that uses local.vnet_id as the virtual_network_id and has the name example-subnet with address prefix 10.0.1.0/24.

Terraform

terraform {
  backend "s3" {
    bucket = "my-terraform-state"
    key    = "subnet/terraform.tfstate"
    region = "us-west-2"
  }
}

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

locals {
  vnet_id = data.terraform_remote_state.remote_network.outputs.vnet_id
}

# Your code here

Hint

Use the azurerm_subnet resource with the exact name example_subnet. Set virtual_network_id to local.vnet_id and provide the required name and address prefix.

Practice

1.

What is the main purpose of using a terraform_remote_state data source?

easy

A. To read outputs from another Terraform state file

B. To store Terraform state remotely

C. To initialize Terraform backend configuration

D. To create new resources in a remote cloud

Solution

1. Step 1: Understand the role of terraform_remote_state

   This data source is designed to read outputs from a different Terraform state, allowing one configuration to use values from another.

2. Step 2: Differentiate from other Terraform features

   Storing state remotely or initializing backend are separate concepts; terraform_remote_state specifically reads outputs, not stores or initializes.

3. Final Answer:

   To read outputs from another Terraform state file -> Option A

4. Quick Check:

   Remote state data source = read outputs [OK]

Hint: Remote state data source reads outputs, not stores state [OK]

Common Mistakes:

• Confusing remote state data source with backend configuration
• Thinking it stores state instead of reading outputs
• Assuming it creates resources remotely

2.

Which of the following is the correct syntax to define a terraform_remote_state data source to read from an S3 backend?

data "terraform_remote_state" "example" {
  backend = "s3"
  config = {
    bucket = "my-bucket"
    key    = "path/to/terraform.tfstate"
    region = "us-west-2"
  }
}

easy

A. data "terraform_remote_state" "example" { backend = "local" config = { path = "my-bucket/path/to/terraform.tfstate" } }

B. resource "terraform_remote_state" "example" { backend = "s3" config = { bucket = "my-bucket" key = "path/to/terraform.tfstate" region = "us-west-2" } }

C. data "terraform_remote_state" "example" { backend = "s3" config = { bucket = "my-bucket" key = "path/to/terraform.tfstate" region = "us-west-2" } }

D. data "terraform_state" "example" { backend = "s3" config = { bucket = "my-bucket" key = "path/to/terraform.tfstate" region = "us-west-2" } }

Solution

1. Step 1: Identify correct resource type and backend

   The data source must be declared with data "terraform_remote_state" and backend set to "s3" for S3 remote state.

2. Step 2: Check config block correctness

   The config block must include bucket, key, and region keys correctly for S3 backend.

3. Final Answer:

   Correct syntax uses data source with backend "s3" and proper config -> Option C

4. Quick Check:

   Correct syntax = data + backend s3 + config keys [OK]

Hint: Use data "terraform_remote_state" with backend and config keys [OK]

Common Mistakes:

• Using resource instead of data block
• Wrong backend name like "local" instead of "s3"
• Misspelling data source name as "terraform_state"

3.

Given this Terraform code snippet, what will be the output of output "vpc_id"?

data "terraform_remote_state" "network" {
  backend = "local"
  config = {
    path = "../network/terraform.tfstate"
  }
}

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

medium

A. The VPC ID value from the remote state output named "vpc_id"

B. An error because local backend cannot be used with remote state data source

C. Null because outputs are not accessible from remote state

D. The entire remote state file content as a string

Solution

1. Step 1: Understand remote state data source with local backend

   The local backend can be used to read a remote state file stored locally, so the data source reads outputs from that file.

2. Step 2: Access the specific output value

   The output block references data.terraform_remote_state.network.outputs.vpc_id, which fetches the "vpc_id" output from the remote state.

3. Final Answer:

   The VPC ID value from the remote state output named "vpc_id" -> Option A

4. Quick Check:

   Remote state outputs accessible = VPC ID value [OK]

Hint: Remote state outputs accessed via data.terraform_remote_state.outputs [OK]

Common Mistakes:

• Assuming local backend is invalid for remote state data source
• Expecting entire state file instead of specific output
• Thinking outputs are inaccessible from remote state

4.

What is the likely cause of this error when using terraform_remote_state?

Error: Failed to read remote state

  on main.tf line 5, in data "terraform_remote_state" "example":
   5: data "terraform_remote_state" "example" {

Could not find state file at configured path

Given this snippet:

data "terraform_remote_state" "example" {
  backend = "local"
  config = {
    path = "../nonexistent/terraform.tfstate"
  }
}

medium

A. The backend "local" is not supported for remote state data source

B. The path to the remote state file is incorrect or file does not exist

C. Terraform version is outdated and cannot read remote state

D. The remote state file is corrupted and unreadable

Solution

1. Step 1: Analyze the error message

   The error says it could not find the state file at the configured path, indicating a missing or wrong file path.

2. Step 2: Check the config path in the data source

   The path is set to "../nonexistent/terraform.tfstate" which likely does not exist or is mistyped.

3. Final Answer:

   The path to the remote state file is incorrect or file does not exist -> Option B

4. Quick Check:

   Missing or wrong path causes file not found error [OK]

Hint: Check file path carefully for remote state data source [OK]

Common Mistakes:

• Assuming backend "local" is unsupported
• Blaming Terraform version without checking path
• Ignoring file existence and permissions

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 safely get vpc_id using remote state?

// In app/main.tf

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

resource "aws_instance" "app_server" {
  subnet_id = data.terraform_remote_state.network.outputs.vpc_id
  # other config
}

What is the best practice to ensure this setup works correctly?

hard

A. Use local backend in app to avoid remote state complexity

B. Run Terraform apply in app before network to create the state file

C. Copy the VPC ID manually into app variables instead of using remote state

D. Ensure the S3 bucket and key exist and the app project has read access to the state file

Solution

1. Step 1: Verify remote state backend accessibility

   The S3 bucket and key must exist with the network project's state file, and app must have permission to read it.

2. Step 2: Understand dependency order and security

   Network must be applied first to create the state file. Using remote state avoids manual copying and keeps data consistent.

3. Final Answer:

   Ensure the S3 bucket and key exist and the app project has read access to the state file -> Option D

4. Quick Check:

   Remote state requires accessible backend and permissions [OK]

Hint: Check backend existence and permissions before using remote state [OK]

Common Mistakes:

• Running app before network causing missing state file
• Manually copying outputs causing drift
• Using local backend ignoring remote state benefits