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Terraform

Terraform remote-exec Multiple Commands & Best Practices

By Filip on 11/18/2024

Learn how to execute multiple commands on your remote instances using Terraform's remote-exec provisioner with practical examples and workarounds.

Terraform remote-exec Multiple Commands & Best Practices

Table of Contents

Introduction

Terraform's remote-exec provisioner is a powerful tool for running commands on your resources, but it's essential to understand its limitations. A key point is that remote-exec runs only once during a resource's creation. This means if you modify the commands within remote-exec after the resource is already up and running, Terraform won't automatically re-execute those commands on the existing resource.

Step-by-Step Guide

Terraform's remote-exec provisioner runs only once when a resource is created. If you modify the commands within remote-exec after the resource exists, Terraform won't re-run them automatically.

Let's say you have this initial setup:

resource "aws_instance" "example" {
 ami           = "ami-0c94855ba95c574c9"
 instance_type = "t2.micro"

  provisioner "remote-exec" {
    inline = ["echo 'Initial setup' > /tmp/setup.txt"]
  }
}

If you later change the inline command to:

 inline = ["echo 'Updated setup' > /tmp/setup.txt"]

Terraform won't update /tmp/setup.txt on the existing instance.

Here's why and how to handle such situations:

  • Idempotency: Terraform strives to be idempotent. It aims to bring your infrastructure to the desired state without repeating actions unnecessarily. Since the resource already exists, Terraform assumes the initial provisioning was successful and doesn't re-run it.

  • Resource Lifecycle: Provisioners are tightly coupled to the lifecycle of a resource. They primarily run during the creation phase.

Solutions:

  1. Resource Recreation: The most straightforward approach is to:

    • Delete the existing resource (terraform destroy targeting the specific resource).
    • Apply the changes (terraform apply) which will recreate the resource and execute the updated remote-exec commands.

  2. null_resource with Triggers: For more granular control, use the null_resource in conjunction with triggers:

    resource "null_resource" "setup" {
  triggers = {
    commands = filemd5("/path/to/your/setup/script.sh") 
  }

  provisioner "remote-exec" {
    inline = ["bash /path/to/your/setup/script.sh"]
  }
}
    • The triggers block ensures the remote-exec runs whenever the content of your script changes. The filemd5 function calculates the MD5 checksum of the script, triggering the null_resource to re-provision when the checksum changes.

Important Considerations:

  • State Management: Terraform relies heavily on its state file. Always back up your state file before making significant changes.
  • Alternatives to Provisioners: While provisioners are useful for simple tasks, consider configuration management tools like Ansible, Chef, or Puppet for more complex scenarios. These tools offer greater flexibility and are better suited for managing the ongoing state of your infrastructure.

Code Example

This code example illustrates the problem of Terraform's remote-exec not executing again when resources are updated. It presents two solutions: recreating the resource and using a null_resource. The first solution involves deleting and recreating the resource to trigger the remote-exec provisioner. The second solution utilizes a null_resource with a remote-exec provisioner and a trigger based on the script's hash. Modifying the script triggers the null_resource to re-run the remote-exec. The choice between these solutions depends on the specific requirements and trade-offs of the situation.

This example demonstrates the issue of remote-exec not re-running on resource updates and provides solutions using resource recreation and null_resource.

Initial Setup (main.tf):

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.0"
    }
  }
}

provider "aws" {
  region = "us-west-2" # Replace with your desired region
}

resource "aws_instance" "example" {
  ami           = "ami-0c94855ba95c574c9" # Replace with a suitable AMI ID
  instance_type = "t2.micro"

  provisioner "remote-exec" {
    inline = ["echo 'Initial setup' > /tmp/setup.txt"]
  }
}

1. Resource Recreation:

  • Run terraform apply to create the initial instance.
  • Modify: Change the inline command in aws_instance.example to: 
    inline = ["echo 'Updated setup' > /tmp/setup.txt"]
  • Run terraform apply. You'll notice Terraform doesn't update the file.
  • Solution:
    • Run terraform destroy (or target the specific instance).
    • Run terraform apply again. This recreates the instance, and the updated command runs.

2. Using null_resource (main.tf):

resource "aws_instance" "example" {
  ami           = "ami-0c94855ba95c574c9" # Replace with a suitable AMI ID
  instance_type = "t2.micro"
}

resource "null_resource" "setup" {
  provisioner "remote-exec" {
    connection {
      type        = "ssh"
      user        = "ubuntu" # Replace with your instance user
      private_key = file("~/.ssh/your-key.pem") # Replace with your private key path
      host        = aws_instance.example.public_ip
    }
    inline = ["bash /tmp/setup.sh"]
  }

  triggers = {
    script_hash = filemd5("/tmp/setup.sh") # Replace with your script path
  }
}
  • Create setup.sh: 
    #!/bin/bash
echo "Updated setup from script" > /tmp/setup.txt
  • Run terraform apply.
  • Modify: Make changes to setup.sh.
  • Run terraform apply again. The null_resource detects the script change and re-runs the remote-exec.

Remember:

  • Replace placeholders like AMI ID, region, user, and key path with your actual values.
  • This example uses SSH for the remote-exec connection. Adjust accordingly if you're using a different method.
  • The null_resource approach provides more control but adds complexity. Choose the solution that best suits your needs.

Additional Notes

This document explains that Terraform's remote-exec provisioner is not intended for ongoing configuration management. It runs only once when a resource is created, and subsequent changes to the remote-exec block won't be applied automatically.

Here are some additional points to consider:

Why this behavior is desirable:

  • Predictability: Terraform prioritizes predictable infrastructure changes. Re-running provisioners on every apply could lead to unexpected side effects, especially if those commands aren't idempotent.
  • Performance: Skipping already-executed provisioners speeds up Terraform runs, which is crucial for large infrastructures.

When to use remote-exec:

  • One-time setup: Ideal for tasks like installing packages, placing initial configuration files, or running scripts that bootstrap the resource.
  • Simple tasks: Suitable for straightforward commands where idempotency isn't a major concern.

When to avoid remote-exec:

  • Complex configurations: For intricate setups or ongoing management, use dedicated configuration management tools like Ansible, Chef, Puppet, or SaltStack.
  • Sensitive operations: Avoid running commands with irreversible consequences within remote-exec. A mistake during provisioning could lead to data loss or service disruption.

Alternatives to resource recreation:

  • Tainting resources (deprecated): While you could taint a resource to force re-provisioning, this approach is deprecated and not recommended due to potential inconsistencies.
  • Data sources: Use data sources to fetch information from resources after creation. This avoids the need to run commands via remote-exec for gathering data.

Best Practices:

  • Modularize your code: Use modules to encapsulate resources and their provisioning logic. This improves code organization and makes it easier to manage updates.
  • Version control your code: Track your Terraform code in a version control system (like Git) to manage changes effectively and roll back if needed.
  • Thoroughly test your code: Use Terraform's planning and validation features (terraform plan, terraform validate) to catch potential issues before applying changes.

By understanding the limitations and appropriate use cases of remote-exec, you can leverage Terraform effectively while maintaining a predictable and manageable infrastructure.

Summary

Problem: Terraform's remote-exec provisioner runs only once during resource creation. Modifying remote-exec commands after resource creation won't trigger re-execution, leaving your infrastructure in a potentially outdated state.

Why?

  • Idempotency: Terraform prioritizes idempotency, avoiding unnecessary actions on existing resources.
  • Resource Lifecycle: Provisioners are tightly bound to the resource creation phase.

Solutions:

  1. Resource Recreation:

    • Delete: Use terraform destroy to remove the existing resource.
    • Recreate: Apply changes with terraform apply, triggering remote-exec on the newly created resource.

  2. null_resource with Triggers:

    • Utilize the null_resource for more controlled re-provisioning.
    • Define triggers based on file changes (e.g., using filemd5) to automatically trigger remote-exec when your scripts are modified.
    resource "null_resource" "setup" {
  triggers = {
    commands = filemd5("/path/to/your/setup/script.sh") 
  }

  provisioner "remote-exec" {
    inline = ["bash /path/to/your/setup/script.sh"]
  }
}

Important Considerations:

  • State Management: Regularly back up your Terraform state file.
  • Alternatives: For complex scenarios, explore configuration management tools like Ansible, Chef, or Puppet for more robust and flexible infrastructure management.

Conclusion

Terraform's remote-exec provisioner, while useful for initial resource setup, has limitations due to its one-time execution nature. It runs only during resource creation and modifications to the remote-exec block won't apply to existing resources. This behavior stems from Terraform's emphasis on idempotency and predictable infrastructure management. For simple, one-time configurations like initial package installations or script executions, remote-exec proves beneficial. However, for complex, ongoing configuration management, dedicated tools like Ansible, Chef, or Puppet are recommended. When updates are needed, consider resource recreation by deleting and re-creating the resource, which triggers remote-exec on the new instance. Alternatively, for more granular control, leverage the null_resource with triggers based on file changes, ensuring remote-exec runs when your scripts are modified. Remember to manage your Terraform state file diligently and explore alternative approaches like data sources when fetching information from resources post-creation. By understanding these nuances and best practices, you can effectively utilize Terraform's remote-exec provisioner while maintaining a predictable and manageable infrastructure.

References

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