Terraform 409 Conflict Errors: Understand & Resolve

Estimated reading time: 7 minutes

Last updated on November 8th, 2024 at 02:49 pm

Terraform 409 conflict errors occurred typically due to an existing resource or concurrent operations that don’t match with the current state of resources.

What is a Terraform 409 Conflict?

Terraform 409 Conflict error occurs when a request can’t be completed due to a conflict with the current Terraform state.

You might wonder why there is conflict in the resource state.

Well, In Terraform. these conflict errors arise during the terraform apply phase when it attempts to create or modify the resource that is already exists or concurrently accessed by others.

Let’s understand the common cause for the Terraform 409 Conflict errors in detail.

Common Causes of Terraform 409 Conflict Errors

There are two main reasons for the Terraform 409 Conflict errors.

1. Existing Resources
2. Concurrent Operations

1. Existing Resources

The most common cause for the Terraform 409 Conflict error is trying to create the resource that already exists.

For example, if you try to create an AWS S3 bucket with a name that is already present, Terraform will throw a 409 conflict error.

resource "aws_s3_bucket" "example" {
  bucket = "my-unique-bucket-name"
  acl    = "private"
}

If you run the above Terraform configuration, you will face the 409 conflict error if my-unique-bucket-name already exists.

2. Concurrent Operations

Another cause for the Terraform 409 conflict errors is running the concurrent operation which is an attempt to modify the same resources.

If two terraform apply commands are run simultaneously and trying to modify the same resource at the same time, leads to a conflict.

Let’s say you have the following Terraform configuration that manages an AWS EC2 instance:

resource "aws_instance" "example" {
  ami           = "ami-0c35b569cs5afe1f0"
  instance_type = "t3.micro"

  tags = {
    Name = "prod-instance"
  }
}

If two terraform apply commands are executed with the same configuration, both commands will try to create the or if the resource exists, modify the prod-instance at the same time.

This can result in the 409 conflict error, as concurrent modification of the same resource is not allowed.

Troubleshooting Terraform 409 Conflict Errors

Identify the Conflict

For troubleshooting a 409 conflict error, start by examining the Terraform logs and the error messages you see when you run the terraform apply command.

Terraform logs provide important clues to start the investigation about what caused the 409 conflict.

terraform apply | tee apply.log

This command will output the Terraform logs to the apply.log file. Review the apply.log to identify the conflicting resources.

Checking for Existing Resource

After checking the logs for the clues, you should check if the resource you’re trying to create/modify already exists.

You can use the provider console or CLI to check the same.

For example, you can check if the S3 bucket exists:

aws s3 ls | grep my-unique-bucket-name

If the bucket with the my-unique-bucket-name name already exists you can do that in the output.

From now on, you need to decide whether you want to import the resource to Terraform or delete it.

Managing State Files

If your Terraform state file is not accurate and out of sync, Terraform might attempt to recreate the existing resource and cause conflicts.

So remember that accurate and up-to-date state files are crucial.

terraform state list
terraform state rm <resource_name>

You above command as a reference point to manage and clean up state files.

Best Practices to Avoid Terraform 409 Conflict Errors

Resource Naming Conventions

Define the policy to adopt the unique and consistent naming convention for the resources to eliminate the Terraform 409 conflict errors.

You can utilize the variable to make the naming template for your resource to have a unique name.

resource "aws_s3_bucket" "example" {
  bucket = "${var.project_name}-${var.environment}-bucket"
  acl    = "private"
}

You should check out the:

Top 5 Terraform Module Versioning Best Practices

Sequential Operations

Well, it’s not best practice to run the sequential operation but for the same resource and same timing, it’s mandatory to avoid the concurrent operation.

More on how to avoid the issue with the sequential operation is state management.

State Management

Regularly maintain the Terraform state file and always use the remote state storage that supports the locking mechanism to prevent concurrent modification of the same resource.

If you’re not aware of Terraform Consul, check the official documentation for using the Consul as a backend provider.

You can also use the AWS S3 as a remote state management solution:

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

Implementing Retries

You can use the Terraform retry mechanism to handle the errors. Configure retry settings in your Terraform configuration within the provider.

provider "aws" {
  region = "us-west-2"
  max_retries = 3
}

Automation with Custom Script

You can automate the checks and resolution for the most common 409 conflict by using the automation scripts

#!/bin/bash

bucket_name="my-unique-bucket-name"
if aws s3 ls "s3://$bucket_name" 2>&1 | grep -q 'NoSuchBucket'
then
  echo "Bucket does not exist, creating..."
  aws s3 mb "s3://$bucket_name"
else
  echo "Bucket already exists."
fi

Real-World Examples and Case Studies

1. AWS S3 Bucket Conflicts

The Terraform encountered 409 conflicts when attempting to create the S3 buckets that already existed.

resource "aws_s3_bucket" "example" {
  bucket = "existing-bucket-name"
  acl    = "private"
}

If the bucket “existing-bucket-name” already exists, running the terraform apply will result in the 409 conflict error.

Implement the pre-check script to verify the existence of the S3 bucket before applying changes.

Pre-check Script:

#!/bin/bash

bucket_name="existing-bucket-name"
if aws s3 ls "s3://$bucket_name" 2>&1 | grep -q 'NoSuchBucket'
then
  echo "Bucket does not exist, creating..."
  aws s3 mb "s3://$bucket_name"
else
  echo "Bucket already exists, skipping creation."
fi

Terraform Import Command:

You can use the pre-check along with the Terraform import command to avoid the 409 conflicts and ensure the bucket does not exist or import to manage within the Terraform.

terraform import aws_s3_bucket.example existing-bucket-name

2. Azure Resource Group Conflicts

Concurrent terraform apply commands lead to conflict in Azure Resource Groups.

resource "azurerm_resource_group" "example" {
  name     = "default-resource-group"
  location = "West US"
}

As mentioned running multiple instances of the same command can cause the error.

You can eliminate a similar issue in the feature with the resource locking mechanism to avoid it.

Sequential Apply Script:

#!/bin/bash

lock_name="terraform-apply-lock"
lock_id=$(az lock create --name $lock_name --resource-group default-resource-group --lock-type CanNotDelete --output tsv --query id)

terraform apply -input=false -auto-approve

az lock delete --ids $lock_id

By using the Azure’s lock mechanism, you can ensure that only one terraform apply the command could modify the resource group at any time.

Conclusion

Terraform 409 conflict errors are caused by the existing resources or concurrent operations.

Troubleshooting includes identifying the conflict, checking for the existing resources, and managing the accurate and up-to-date state file.

Best practices include unique naming conventions, sequential operations, and remote state management.

Further Reading

Terraform Where to Store Secrets: Best Practices and Solutions

Top 5 Terraform Module Versioning Best Practices