Terraform try() with data blocks

terraform-try-with-data-blocks.txt

In Terraform, the data block is used to fetch existing resources, but if the resource doesn’t exist, it can result in an error. To handle such cases gracefully, you can use workarounds or implement conditional logic to prevent the error from causing a failure in your Terraform run.

Options for Error Handling

1. Use the try() Function

The try() function can catch errors and provide a default value if a data source lookup fails. This approach works if the data block supports an attribute that might return null or is optional.

data "aws_vpc" "example" {
  filter {
    name   = "tag:Name"
    values = ["non-existent-vpc"]
  }
}

output "vpc_id" {
  value = try(data.aws_vpc.example.id, "default-vpc-id")
}

In this example, if the data block fails, try() returns the default value "default-vpc-id" instead of causing an error.

2. Conditionally Fetch Resources

If you can predict whether the resource might exist, use a variable or condition to determine whether the data block should be executed.

variable "fetch_vpc" {
  default = false
}

data "aws_vpc" "example" {
  count = var.fetch_vpc ? 1 : 0
  filter {
    name   = "tag:Name"
    values = ["non-existent-vpc"]
  }
}

output "vpc_id" {
  value = length(data.aws_vpc.example) > 0 ? data.aws_vpc.example[0].id : "default-vpc-id"
}

Here:
	•	The count is controlled by the fetch_vpc variable.
	•	If the VPC doesn’t exist, the default-vpc-id is returned.

3. Use the ignore_errors Meta-Argument (Indirect)

Although Terraform doesn’t natively support ignore_errors for data blocks, you can work around it by using modules or scripting to conditionally fetch data. You might also consider wrapping Terraform in automation scripts (e.g., using Bash or Python) to handle these errors more flexibly.

4. Custom External Script to Validate Resource

You can use the external data source to check if a resource exists, and based on the result, proceed accordingly.

data "external" "check_vpc" {
  program = ["python3", "check_vpc.py"]
  query = {
    vpc_name = "non-existent-vpc"
  }
}

output "vpc_id" {
  value = data.external.check_vpc.result["vpc_id"]
}

The script check_vpc.py should return a JSON response with the resource’s ID or a default value if the resource doesn’t exist.

5. Use depends_on with Resource Dependency

If a data block depends on a resource that might not exist, you can manage its behavior using depends_on and handle missing dependencies explicitly in your Terraform logic.

Key Points
	•	Not all data sources return empty results if a resource doesn’t exist; many will fail, requiring careful error handling.
	•	Graceful handling can involve providing default values, conditional logic, or pre-validation with external tools.
	•	Combine Terraform’s intrinsic functions (try, length) with robust logic to manage edge cases effectively.




You can use the count meta-argument in Terraform to conditionally create resources based on the output of a data block. This is a common pattern for cases where the existence of a resource (determined by a data block) dictates whether or not another resource should be created.

Example: Conditional Resource Creation Based on Data Block

Here’s how you can implement this:

Scenario: Check if a VPC exists and create a new one only if it doesn’t exist.

# Data block to fetch an existing VPC
data "aws_vpc" "example" {
  filter {
    name   = "tag:Name"
    values = ["my-vpc"]
  }
}

# Local variable to check if the VPC exists
locals {
  vpc_exists = length(try(data.aws_vpc.example.id, [])) > 0
}

# Resource block to create a new VPC conditionally
resource "aws_vpc" "new_vpc" {
  count             = local.vpc_exists ? 0 : 1
  cidr_block        = "10.0.0.0/16"
  enable_dns_support = true
  enable_dns_hostnames = true

  tags = {
    Name = "my-vpc"
  }
}

output "new_vpc_id" {
  value = aws_vpc.new_vpc.*.id
}

Explanation
	1.	data Block:
	•	The data "aws_vpc" "example" block attempts to fetch a VPC by its name.
	2.	Local Variable:
	•	local.vpc_exists determines if the data block returned any results. The try() function ensures that Terraform doesn’t error out if the data block fails.
	3.	count Meta-Argument:
	•	The count in resource "aws_vpc" "new_vpc" is set to 0 if the VPC exists (local.vpc_exists is true), or 1 if it doesn’t.
	4.	Output:
	•	The output provides the ID of the newly created VPC (or an empty list if no VPC was created).

Adjusting for Other Scenarios

Example: Creating Subnets Only if a Specific VPC Exists

# Fetch VPC by name
data "aws_vpc" "example" {
  filter {
    name   = "tag:Name"
    values = ["existing-vpc"]
  }
}

# Create subnets only if the VPC exists
resource "aws_subnet" "example" {
  count = length(try(data.aws_vpc.example.id, [])) > 0 ? 2 : 0

  vpc_id            = data.aws_vpc.example.id
  cidr_block        = cidrsubnet("10.0.0.0/16", 8, count.index)
  availability_zone = element(data.aws_availability_zones.available.names, count.index)

  tags = {
    Name = "example-subnet-${count.index}"
  }
}

In this example:
	•	Subnets are created only if the specified VPC exists.
	•	The count dynamically controls how many subnets are created based on the availability of the VPC.

Key Points
	•	Use the count meta-argument to control resource creation dynamically.
	•	Combine try() and length() to