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Terraform Basics

When we run the terraform init command in a directory contains configuration file, terraform downloads the plugin and install them for the providers mentioned (AWS/Azure/GCP/Ali) in the configuration file.

Terraform plugins are available in Hashicorp registry https://registry.terraform.io for all the providers. There are three different providers.

  1. Official Providers : Owned and maintained by Hashicorp
  2. Partner Providers : Owned and maintained by third party company that has partner provider access with Hashicorp
  3. Community Providers : Published and maintained by individual contributors of Hashicorp community

All plugins are downloaded into <working directory>/.terraform directory. When we download the plugins (during terraform init command), this will tell you where the plugins are from.

<Hostname>/<Organisation namespace>/<Provider name>

Whereas,

Hostname is optional. If it is not mentioned, then it is default value (registry.terraform.io)

Organisation namespace is Hashicorp/Partner Provider/Community Provider

Provider name is AWS/Azure/GCP/Local etc.,

Configuration Directory

Terraform considers the file with .tf extension is configuration file. A single configuration file can have any number of configuration details. Also, we can have any number of configuration files as well in a directory. There are few other common configuration files created in a directory.

Filename Purpose
main.tf main configuration file
variables.tf Contains variable declaration
outputs.tf Contains output from resources
provider.tf Contains provider definitions

NOTE: Let us assume that there are two configurations defined in the configuration file. Both have different plugins and one of the plugin is already installed and other one is not yet installed In that case, if you run terraform plan / terraform apply command will yield an error. This is because whenever we add a resource for a provider that has not been used so far in the configuration directory, we have to initialize the directory by running terraform init command.

Multiple Providers

You can create a configuration file which contains multiple providers like below.

multi_providers_file.png

multi_providers_file_result.png

Input Variables

For re-usability purpose, we can use variables. To do so, we can create variable.tf file and update all variables in it. In general, whatever arguments we are passing in block, we can use variable for those arguments. Remember, both variable configuration file and your main configuration file should be in same directory.

Syntax 

variable <VARIABLE_NAME> {
    default = "<value>"
}

Example

resource "local_sensitive_file" "games" {
    filename = "/Users/satheeshpandian.j/Documents/Sats/Learning/Terraform/names.txt"
    content = "I am Satheesh and this is terraform update example"
}
In the above code snippet, we can use variable for filename and content.

variable "filename" {
    default = "/Users/satheeshpandian.j/Documents/Sats/Learning/Terraform/names.txt"
}

variable "content" {
    default = "I am Satheesh and this is terraform update example"
}

Now, our main configuration file "main.tf" needs to be updated like below

resource "local_sensitive_file" "games" {
    filename = var.filename
    content = var.content
}

In case you want to change the content or filename, you just need to update it in variable.tf configuration file. You DO NOT touch main configuration file at all.

Variable Types

In variable block, there are different parameters used.

variable <VARIABLE_NAME> {
    default = <value>
    type = <ANY/STRING/NUMBER/BOOL/LIST/MAP/OBJECT/TUPLE>
    description = <description of the value>
}

Whereas,

type and description are optional. If you don't use type parameter, it will consider as any by default. If you use type parameter, then it will make sure that default value should have the same type.

List

List will be used with its index and index starts from 0 

variable "names" {
    default = ["Satheesh", "Pandian", "Jeganathan", "Bangalore", "Karnataka"]
    type = list
    description = "List of names"
}


resource "random_pet" "players" {
    firstname = var.names[1]
}

We can enforce the list values using STRING/NUMBER/BOOL.

variable "names" {
    default = ["Satheesh", "Pandian", "Jeganathan", "Bangalore", "Karnataka"]
    type = list(string)
    description = "List of names"
}
This will ensure that all the default values should be string. It will throw an error if you pass the number or boolean value.

variable "names" {
    default = [5, 7, 8, 10, 23]
    type = list(number)
    description = "List of numbers"
}

terraform_list_number_file.png

terraform_list_number_file_variable.png

terraform_list_number_file_variable_result.png

This will ensure that all the default values should be number. It will throw an error if you pass the string or boolean value.

terraform_list_variables_invalid.png

terraform_list_variable_invalid_error.png

Map

Map will be in key/value format. 

variable "names" {
    default = {
        "firstname" = "Satheesh"
        "lastname" = "Pandian"
    }
    type = map
    description = "map of names"
}

resource "random_pet" "players" {
    lastname = var.names["lastname"]
}

We can enforce the map values using STRING/NUMBER/BOOL.

variable "names" {
        default = {
        "firstname" = "Satheesh"
        "lastname" = "Pandian"
    }
    type = map(string)
    description = "Map of names"
}
This will ensure that all the default values should be string. It will throw an error if you pass the number or boolean value.

variable "names" {
        default = {
        "favourite" = 5
        "birthdate" = 10
        "anniversary" = 23
    }
    type = map(number)
    description = "Map of numbers"
}

Sets

This is similar to list except that list can have duplicate elements whereas sets won't allow duplicate elements.

terraform_sets_number_file.png

Duplication example

terraform_sets_number_file_duplicate.png

terraform_sets_number_file_error.png

Object

Object is a custom variable type. You can have combination all other variable type using object.

variable "player" {
     default = {
        "name" = "Satheesh"
        "age" = 20
        "favourite_place" = ["Bangalore", "Chennai", "Mumbai"]
        "working_location" = {
            "state" = "Karnataka"
            "city" = "Bangalore"
        }
     }
     type = object({
        "name" = string
        "age" = number
        "favourite_place" = list(string)
        "working_location" = map(string)    
     })
    description = "Map of numbers"
}
terraform_object_main.png

terraform_object_main_plan.png

terraform_object_main_plan_result.png

Tuples

This is similar to list except that tuples can have all variable type.

variable "names" {
    default = [5, 7, "Satheesh", true, 4]
    type = tuple([number, number, string, bool, number])
    description = "Tuple example"
}

Ways to use variable in terraform

  1. We can create variable.tf file and put all the variables and then use it in main.tf. You can refer all the above cases for this type of usage.

  2. During runtime via terminal (Interactive Mode):

    To do this, we need to create a variable.tf file. But we don't need to update any variable, just keep at empty. 

    variable "filename" {
}
variable "content" {
}
    terraform_ways_two_main.png

terraform_ways_two_main_result.png

  1. Via command line arguments

    To do this, we need to create a variable.tf file. But we don't need to update any variable, just keep at empty. 

    variable "filename" {
}
variable "content" {
}
    terraform_ways_three_result.png

  2. Via environmental variables

    To do this, we need to create a variable.tf file. But we don't need to update any variable, just keep at empty. 

    variable "filename" {
}
variable "content" {
}

terraform_ways_four_result.png

  1. Via variable definition files

    Variables are loaded from terraform.tfvars file. This filename can be anything, but the extension should be terrform.tfvars or terraform.tfvars.json or .auto.tfvars or .auto.tfvars.json. Then it will be autoloaded. In case, if you have variable definition file named as satheesh.tfvars, then you need to pass this via command line arguments.

terraform plan -var-file satheesh.tfvars

To do this, we need to create a variable.tf file. But we don't need to update any variable, just keep at empty.

variable "filename" {
}
variable "content" {
}


filename="./test.txt"
content="This is Satheesh"
terraform_ways_five_file.png

terraform_ways_five_file_result.png

Variable Definition Precedence

Highest priority to lowest is as follows: 1. command line arguments (Ex : -var "filename=./test.txt") 2. .auto.tfvars or .auto.tfvars.json (* -> can be anything. If there are more than one file, then it will be applicable by alphabetical order) 3. terraform.tfvars or terraform.tfvars.json 4. environmental variables

Resource Attributes

We want to get the value from one configuration (output of it) and pass it to another configuration, this can be achieved.

terraform_resource_attribute_file.png

terraform_resource_attribute.png

Here, local_file resource depends on random_pet resource. So, terraform will create the resource for random_pet first and then local_file. While destroying, local_file will be deleted first and then random_pet. This will be taken care by terraform itself. This is known as implicit dependency.

We can specifically mention the dependency in the configuration file like below. This is called as explicit dependency. 

 resource "local_file" "my-file" {
       filename = "./test.txt"
       content = "This is Satheesh"
       depends_on = [random_pet.my-pet-name.id]
}

resource "random_pet" "my-pet-name" {
       prefix = "Dr"
       separator = "."
       length = 2
}

Output Variable

Output variables can be used to store the value of an expression. This can be passed into another configuration file as an input.

Syntax

output <OUTPUT_VARIABLE> {
   value = <output value from the configuration file>
}

Example

 resource "local_file" "my-file" {
       filename = "./test.txt"
       content = "This is ${random_pet.my-pet-name.id}"
}

resource "random_pet" "my-pet-name" {
       prefix = "Dr"
       separator = "."
       length = 2
} 

output "pet-name" {
   value = random_pet.my-pet-name.id
   description = "This is output value of random_pet and passed as an input to local_file"
}

terraform_output_result.png

terraform_output_result_file.png

terraform output command will print all the output variable in the configuration file.

Assume that in the configuration file, there are many output variables. You would like to see only one output variable among 5. In that case, run the below command

terraform output <variable_name>

Example: terraform output pet-name

terraform_output_variable.png