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Terraform Resource Block

Terraform Resource Block

Terraform Beginners Guide

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Venkatesh K
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8 min read

In this article, we are going to understand

  1. What is Terraform resource block

  2. Understand Terraform basic commands

    • terraform init

    • terraform validate

    • terraform plan

    • terraform apply or terraform apply -auto-approve

    • terraform destroy or terraform destroy -auto-approve

  3. Understand terraform resource behavior on executing terraform basic commands

Terraform Resource Block

  • A Terraform Resource is a fundamental unit used to model and manage infrastructure components.

  • Each resource block describes one or more infrastructure objects that you want to create, modify, or manage.

    • Example: s3 bucket, ec2 instance, RDS instance, Security Group, VPC

  • resource Syntax

      resource "type" "name" {
      argument1 = "value1"
      argument2 = "value2"
      ......... = "......"
      ......... = "......"
      argumentn = "valuen"
  }

  • Example of a Resource Block:

      resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  tags = {
  Name = "Linux"
  }
  }

    • resource: The keyword to start a resource block.

    • aws_instance:

      • The resource type, which defines what you're creating (e.g., an AWS EC2 instance).

      • Resource type determines the kind of infrastructure object it manages and what arguments and other attributes the resource supports

    • example:

      • The resource name is a unique identifier within your configuration.

      • It is a Local name ("example").

      • The name is used to refer to this resource from elsewhere in the same Terraform module but has no significance outside that module's scope.

    • ami, instance_type and tags: Arguments or the Configuration parameters specific to the resource you're defining.

Terraform Resource Behaviors

Terraform resource behaviors refer to,

  • How Terraform manages and interacts with resources in your infrastructure.

  • These behaviors determine how resources are

    • created

    • destroyed

    • updated

  1. Create :

    • Terraform attempts to create resources in your target infrastructure based on your configuration.

    • Terraform creates resources that exist in the configuration but are not associated(present) with a real infrastructure object in the state

  2. Destroy :

    • Destroys resources that exist in the state/infra but no longer exist in the configuration.

    • Removing a resource from your Terraform configuration leads to the planned destruction of that resource in the infrastructure.

  3. Update in-place :

    • update the resources whose arguments have changed

    • Terraform detects differences between the desired state in your configuration and the current state in the infrastructure. It plans and applies changes to update resources accordingly.

  4. Destroy and re-create :

    • Terraform will destroy and re-create resources whose arguments have changed but which cannot be updated in-place due to remote API limitations

    • Example: Changing the Availability zone of an AWS EC2 instance

  5. Dependency Management :

    • Terraform ensures dependent resources are created or updated before resources that rely on them to avoid issues.

  6. Concurrency Control :

    • Terraform manages resource operation concurrency to prevent conflicts and ensure consistency.

  7. State Management :

    • Terraform maintains a state file that records the current state of the infrastructure, which is used to plan and apply updates.

Understanding Terraform Resource Behavior with Example

  • Let's Create an AWS EC2 instance and understand Terraform Resource(EC2) Behavior

    1. Create a Terraform provider block

       terraform {
 required_providers {
     aws = {
         source = "hashicorp/aws"
         version = "~-> 5.0"
     }
 }
 }

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

     default_tags {
         tags = {
         terraform = "yes"
         project = "terraform-learning"
         }
     }  
 }

    2. Create Resource (EC2) block

       resource "aws_instance" "example" {
 ami           = "ami-0df435f331839b2d6"
 instance_type = "t2.micro"

 tags = {
     Name = "Linux2023"
     Owner = "Venkatesh"
 }
 }

  • Let's Execute Terraform commands to understand resource behavior

    1. terraform init : Initialize terraform

      • The terraform init command is used to initialize a Terraform configuration.

      • It sets up the necessary components and dependencies for Terraform to manage your infrastructure.

      • Downloading Providers and Modules: On executing terraform init command terraform downloads the plugin for provider (in our case AWS) under .terraform folder in the current directory

      • Locking and Tracking Dependencies: Terraform also creates a terraform.lock.hcl file to track provider and module versions.

    2. terraform validate : Validate terraform code

      • Validate the configuration files in a directory

      • Validate runs checks that verify whether a configuration is syntactically valid and internally consistent

      • Primarily useful for general verification of reusable modules, including correctness of attribute names and value types.

      • It is safe to run this command automatically

      • Validation requires an initialized working directory (terraform init) with any referenced plugins and modules installed

      • Example :

        • terraform validate with error statements to correct

        • In the above case, resource should be aws_instance and not aws_ec2 hence terraform validate will show an error

        • terraform validate with no errors

    3. terraform fmt : format terraform code

      • The terraform fmt command is used to automatically format Terraform configuration files, promoting consistent coding style and improved readability.

      • Formatting: It standardizes the indentation, line breaks, and element ordering in your configuration files.

      • Syntax Rules: Enforces Terraform's syntax rules, ensuring proper indentation and formatting

      • Comments: Doesn't alter comments but ensures they are consistently styled.

      • It reformats files without changing the content or functionality of your configuration and is saved to run the command anytime

      • when terraform fmt is executed, it lists the files that it formatted, if no files are listed it means all files are well formatted.

    4. terraform plan : Review the terraform plan

      • The terraform plan command is used to create an execution plan for your Terraform configuration, showing the changes Terraform will make to your infrastructure.

      • Plan Infrastructure Changes: Compares the current state of your infrastructure to the desired state and plans the necessary changes.

      • Output Summary: Provides a human-readable summary of the planned changes, including creation, modification, and destruction of resources.

      • Detailed Information: Offers detailed information about planned changes, resource modifications, and dependencies.

      • Validation: Validates (terraform validate) your configuration for syntax errors and inconsistencies before applying changes.

      • Dry Run: terraform plan is a "dry run" command; it shows proposed changes without applying them.

      • It's safe to execute this command

      • terraform plan will require you to have your AWS Credentials set and you can connect/access your AWS infrastructure. You will receive the below error when AWS cred are not set
        Error: No valid credential sources found

      • setup AWS Credentials using your preferred methods

      • Example of terraform plan

    5. terraform apply: Create Resources by terraform

      • terraform apply is like pressing the "execute" button for your Terraform configuration.

      • It tells Terraform to create, update, or delete resources in your infrastructure based on your configuration.

      • Be very cautious before you provide approval for terraform apply command as it modifies your infrastructure

      • Let's understand terraform apply in more detail:

        1. Execution: Terraform analyzes your configuration and the current state of your infrastructure to identify the differences between the desired state and the actual state.

        2. Changes: Terraform takes action to make the actual state match the desired state, which can involve creating, updating, or deleting resources.

        3. User Confirmation: Before making any changes, Terraform shows you a summary of what it's about to do. It can be over-ridden with auto-approve

        4. User Approval: You must confirm by typing "yes" when prompted to ensure you're aware of the changes.

        5. Execution: Once you confirm, Terraform executes the changes, and you can see the progress in real-time.

        6. Completion: After applying the changes, Terraform provides a summary of what was created, updated, or deleted. It also updates the state file with the current state of your infrastructure.

      • Example of terraform apply

      • After you type yes to terraform apply prompt, terraform will start creating resources mentioned in the plan

      • You should also be able to check on your AWS Console resource (EC2) being created

      • Once terraform completes the execution you should be able to check on your AWS Console resource (EC2) successfully created.

6. terraform destroy: destroy or delete Resources

  • terraform destroy is like the "off" switch for your Terraform-managed infrastructure.

  • It tells Terraform to tear down and delete all the resources in your infrastructure that were created or managed by Terraform.

  • Let's understand terraform destroy in more detail:

1. Execution: Terraform analyzes your configuration and the current state of your infrastructure, just like terraform apply.

2. Resource Destruction: However, instead of creating or updating resources, terraform destroy focuses on destroying and deleting them.

3. User Confirmation: Similar to terraform apply, it shows you a summary of what it's about to destroy. It can be overridden with auto-approve.

4. User Approval: You must confirm by typing "yes" when prompted, ensuring you're aware of the resources that are going to be deleted.

5. Execution: Once you confirm, Terraform executes the destruction, and you can see the progress in real-time.

6. Completion: After the resources are destroyed, Terraform provides a summary of what was deleted.

  • Example of terraform destroy

  • After you type yes to terraform destroy prompt, terraform will start destroying resources mentioned in the plan

  • You should also be able to check on your AWS Console resource (EC2) being shutting down and getting ready for termination

  • Once terraform completes the execution you should be able to check on your AWS Console resource (EC2) is successfully terminated.

terraform apply -auto-approve and terraform destroy -auto-approve

  • The -auto-approve flag is an option that can be added to the terraform apply command to skip the confirmation step.

  • When you use terraform apply -auto-approve or terraform destroy -auto-approve, Terraform will not ask for your confirmation and will immediately apply the changes described in the execution plan.

  • This can be useful for automation, scripting, or CI/CD pipelines where manual confirmation is not possible

  • However, be cautious when using -auto-approve in production environments, as it can lead to unintended changes if the execution plan is not thoroughly reviewed.

  • Always review the execution plan carefully before using -auto-approve to ensure that the changes are as expected and won't cause any issues in your infrastructure.

Let's connect and explore Terraform and AWS.

References :

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