Let's Learn: Terraform

A deep dive into Terraform. It's components, workflow, expressions, state data, functions, workspaces and more.

What is Terraform?

• Terraform is an Infrastructure Automation tool.

• It is Open Source and Vendor Agnostic.

• It is a Single binary compiled from Go.

• It follows Declarative syntax.

• It uses HashiCorp Configuration Language(HCL) or JSON.

• Deployment is push based.

Core Components

• Executable
• Can be downloaded from official Terraform webpage.

• Configuration files

• Provider plugins

• State data

Terraform Code Components

• It can be divided into 3 parts:
• Inputs - Terraform gets input by leveraging variable(s).
• Outputs - The data in the output block is printed to screen.
• Logic - Your IaC.

Object Types

• Providers

• Resources

• Data sources

Terraform Syntax

• HashiCorp uses Block syntax

      block_type "label" "name_of_label" {
          key = "value"

          nested_block{
              key = "value"
          }
      }

Workflow

• terraform init
• Looks for configuration files in current working directory and examines them. If they need any provider plugins, it will search in public Terraform registry unless any specified location provided.
• Terraform needs to store data of configuration somewhere as part of init process. A state data file will be created in current directory unless any specified backed it created or pointed.
• When init completes, infrastructure is ready to be deployed.

• terraform plan
• This step is optional.
• It will look current configuration i.e. contents of state data and determine the difference. Then it draws a plan for Target environment to match the desired configuration.

• terraform apply
• It will build our desired configuration in Target environment.

• terraform destroy
• It will bring down the infrastructure of desired configuration in Target environment.

Terrafrom variable

• Variable declaration

variable "name_of_variable" {}

• Variable definition

      variable "name_of_variable" {
          type = value
          description = "value"
          default = "value"
          sensitive = true or false
      }

• Example

variable "device_name" {
          type = string
          description = "Provides the Device name"
          default = "computer"
          sensitive = true
  }

• type - Data type

• Description - Context for the user. It will be displayed when the user encountered an error.

• default - This value will be considered if no value has been provided.

• sensitive - If it is 'true', data will not be printed in logs or terminal. If 'false', vice versa. Default sticks to 'false'.

• Referencing a variable

var.<name_of_variable>

• Example

var.device_name

• Hierarchy
• For variable(s) data or value, Terraform designed a precedence chart.
• Precedence
Environment variable(s) > .tfvars (or) .tfvars.json > .auto.tfvars (or) .auto.tfvars.json > -var-file flag > - var flag > CLI
 

Terraform Data Types

Primitive

• string

• number
• integer
• decimal

Boolean

• true

• false

Collection

• list
• Enclosed with [ ]
• Example

[1,2,3]
      ["Mobile", "Computer", "Laptop"]

Variable reference

• Example

 variable "list_of_device_types"{
     type = list(string)
     description = "Provides list of device types"
     default = ["Mobile","Computer","Laptop"]
 }

• Syntax

var.<name_of_variable>[<element_number>]

• Example
• var.list_of_device_types[0]
• For accessing first element in list. Likewise for other elements accessing.

Set

• Same syntax as list and do not hold duplicates.

Map

• Enclosed with { }

• Example

{
          India = "Delhi"
          USA = "Washigton"
          Canada = "Ottawa"
}

Variable reference

• Example

      variable "capitals"{
          type = map(string)
          description = "Provides captials for countries."
          default = {
              India = "Delhi"
              USA = "Washigton"
              Canada = "Ottawa"
          }
      }

• Syntax

      var.<name_of_variable>.<name_of_the_key>

      // OR

      var.<name_of_the_variable>["name_of_the_key"]

• Example
• var.capitals.India or var.capitals["India"]
• For accessing first element in map. Likewise for other elements accessing.

Structural

• type

• object

Terrafrom Locals

• Values provides in locals are computed inside the configuration and cannot be passed as parameter during runtime.

• Syntax

locals{
          key : "value"
      }

• Example

      locals{
          name_of_the_company = "ThunderWonder"
      }

• Referencing locals
 

Terraform Output

• Output values are extracting information from Terraform.

• Output is printed to terminal screen at the end of configuration execution.

• It also exposes values when a configuration is placed inside a module.

• Syntax

output "name_of_the_variable"{
          value = output_to_be_exposed
          description = string
          sensitive = true | false
      }

// Example

output "selected_device"{
          value = var.list_of_device_types[0]
          description = "Prints selected device(s).
      }

• First run terraform init.

• terraform validate will check for your syntax and logic.

• It will not guarantee for successful deployment.

Terraform State Data

• It stored in JSON format
• Important: It should not be modified. To work with State Data, we can use State commands.

• It contains Resource Mapping and metadata.

• It can be stored in:
• Local (Default, by name terraform.tfstate)
• Remote (Basically Cloud Providers)
• Ex: AWS, Azure, Terraform Cloud, etc.

• State Commands
• terraform state list
• To list all state resources.
• terraform state show <ADDRESS>
• To show a specific resource.
• Example :

terraform state show module web_app_s3.aws_s3_bucket.web_bucket

• terraform state mv SOURCE DESTINATION
• To move an item in state file.
• terraform state rm <ADDRESS>
• To remove an item in state file.
• Example:

terraform state rm module web_app_s3.aws_s3_bucket.web_bucket

• terraform state pull
• Output current state to stdout.
• terraform state push
• Update remote state from local.

Terraform Providers

• It contains Public & Private registries.

• Registries types are:
• Official
• Maintained by HashiCorp.
• Verified
• Maintained by Third Party and verified by HashiCorp.
• Community
• Maintained by Individuals in Community and not verified.

• Providers are Open Source and written in Go Lang.

• It has Resources and Data Sources.

• Providers are versioned.

Terraform Block Syntax

• It is used for configuring General Settings of Terraform Configuration like,
• Version
• Backend Settings for State Data
• Required Provider Plugins
• Provider metadata
• Experimental features

• Syntax

terraform{
          requried_providers {
              provider_name = {
                  source = "provider's_address"
                  version = "version_expression"
              }
          }
      }

// Example

terraform{
      requried_providers {
          aws = {
              source = "hashicorp/aws"
              version = "=3.0"
          }
      }
  }

Terraform Looping Constructs

• count
• Takes integer and used as iterator.
• index is used to keep track of count.
• Example

resource "aws_instance" "web_app_server"{
          count = 3
          tags = {
              Name = "web-app-Instance-${count.index}
          }
      }

Referencing
• Referring single instance

<resource_type><name_of_the_variable>[element].<attribute>

• Referring all the instances

		<resource_type><name_of_the_variable>[*].<attribute>

• for_each
• used to for iterating Map or set/list.
• Example

resource "aws_s3_bucket_object" "website_content"{
          for_each = {
              app_logo = "app-logo.png"
              main_page = "index.html"
          }
          key      = each.value
          source   = "${each.value}"
      }

• Dynamic block
• used to for iterating Map or set/list.
 
 

Terraform Inbuilt Functions

• Numeric Functions
• String Functions
• Collection Functions
• Encoding Functions
• Filesystem Functions
• Date and Time Functions
• Hash & Crypto Functions
• IP Network Functions
• Type Conversion Functions
• You can play around with in-built functions. Type terraform console and test it out.
• Example



• Check out Terraform Functions documentation for respective function methods, and it's usage.
• Note: Terraform does not support for User-defined functions.

Terraform FOR Expression

• Input type:
• List, Set, Tuple, Map or, Object.

• Result type:
• Tuple or Object

• Filtering is possible by leveraging if statement.

• Examples
• Tuple
• Syntax

[for item in items : tuple_element]

// Example

locals{
              devices = ["Mobile", "Laptop", "Tablet"]
      }

      [for device in local.devices : "Awesome-${device}"]

• Result



• Object
• Syntax

      {for key, value in map : obj_key => obj_value}

// Example

locals{
          type_of_device{
              Mobile = "compact"
              Laptop = "productive"
          }
      }

      {for key, value in local.type_of_device : key => upper(value)}

 
• Result



 

Terraform Workspaces

• Terraform creates a initial workspace for us called default.
• It cannot be deleted.
• We can create workspace(s) per our requirement and management.
• Management like
• State management
• Variable values
• Credential management, etc.
• workspace commands
• terraform workspace list - To show all existing workspaces.
• terraform workspace select <workspace_name> - To switch to desired workspace.
• terraform workspace new <workspace_name> - To create & switch to new workspace.
• terraform workspace delete <workspace_name> - To delete existing workspace.
• terraform workspace show - To print current working workspace.
• Example