Terraform Basics

Basics¶

Providers¶

After we write a terraform configuration file the first thing we have to do is to run terraform init.
- When we run terraform init in the directory containing the configuration files, terraform downloads and installs plugins for the providers used in the configuration.
- It shows the version of the plugin being installed.
terraform init is a safe command which can be run as many times as required without impacting the architecture.
There are 3 different tiers of providers:
- Official: Owned and maintained by hashicorp. This includes the major cloud providers like AWS, Azure and GCP.
- Verified: Owned and maintained by a 3rd party technology company.
- Community: Published and maintained by individual contributors.
Once we run terraform init the plugins are downloaded to a hidden directory .terraform/plugins in the directory containing the configuration files.
- We can inspect this to find all the plugins installed.
If the registry is not provided then terraform uses the default registry of registry.terraform.io to download the plugins.

By default terraform installs the latest version of the provider.

Terraform will consider any file with .tf extension for configuration changes.
A single configuration file can also have as many configuration blocks as we need.
We can have multiple providers in the same configuration directory.
- Each time we add a new provider in the configuration file we have to run terraform init

Input Variables¶

We can create variables in a variables.tf file.
To create a variable we use the following syntax

# variables.tf
variable "variable_name" {
    default = "some_default_value"
}

The advice is to use argument name for the variable name for the argument that the variable is being used.
To use variables we use var.variable_name
- When using variable we need not enclose the values with double quotes.
The variable block in terraform accepts 3 parameters: default, type and description.
- default parameter in our variable block is also optional.
  - If we don't have this field and use this variable then terraform will ask us to enter the value interactively when we run terraform apply
- description is optional but it is a good practice to describe what the variable is used for.
- type argument is also optional but when used it enforces the type of the variable.
  - The basic types that can be used are string, number, boolean and any
  - If type parameter is not specified then it is set to any by default.
  - Other complex types are list, map, tuple, object and set.

Complex type examples¶

list: all the elements should be of the same type
- We use indexes to access the value in a list.
map
set: difference between a set and a list is that a set cannot have duplicate terms
objects: it is a complex data type which can contain other data types
tuple: the difference between tuple and list is that we can use elements of different variable types.
- Variable to be passed to it should be exactly 3 in number.
- Adding additional elements or incorrect type will result in an error.
Using type constraints while declaring complex types

Using variables¶

We can make use of command line flags to provide variables for eg: terraform apply -var "filename=/root/pets.txt"
- We can pass as many variables with this method as required by passing the -var flag multiple times
- We can also make use of environment variables using
  - export TF_VAR_filename="/root/pets.txt"
  - export TF_VAR_content="We love pets!"
We can also use a variable definition file if we are dealing with a lot of variables.
- These files can be named anything but they should either end in .tfvars or .tfvars.json
- Files with the extensions are automatically loaded by terraform: terraform.tfvars, terraform.tfvars.json, *.auto.tfvars and *.auto.tfvars.json
If we use any other file name like variables.tfvars then we will have to pass it like a command line flag terraform apply -var-file variables.tfvars
We can use any of the options above to assign values to variables.

We use variables.tf to declare the variables to be used in the terraform files, we can declare their values there or use the above methods to assign values to variables.

If we use multiple ways to assign the values of variables then terraform follows a variable definition precedence to decide which value should be used.
- 4 is the highest priority
- Priority increases when you go down. Environment variables have the last preference.
- Variable filename will be assigned /root/best-pet.txt

Resource attributes¶

There are bound to be multiple resources that are dependent on each other.
Example:

resource "local_file" "pet" {
    filename = var.filename
    content = "My favorite pet is ${random_pet.my-pet.id}"
}

resource "random_pet" "my-pet" {
    prefix = var.prefix
    separator = var.separator
    length = var.length
}

In the above example I am using id returned by the random_pet resource.
- Every resource might have 0 or more output variables.
- random_pet resource has one output variable called id

Resource Dependencies¶

In the above example due to the dependency terraform knows to create random_pet resource first and then create the local_file resource.
When resources are deleted they are deleted in the reverse order.
- local_file first and then the random_pet
This type of dependency is known as the implicit dependency.
- We are not specifying which resources are dependent on what, terraform figures it out itself.
We can specify dependency explicitly using depends_on argument.
- This is necessary only when a resource relies on some other resource indirectly and it does not make use of a reference expression.

resource "local file" "pet" {
    filename = var.filename
    content = "My favorite pet is Mr.Cat"
    depends_on = [
        random_pet.my-pet
    ]
}

resource "random pet" "my-pet" {
    prefix = var.prefix
    separator = var.separator
    length = var.length
}

Output Variables¶

Output variables can be used to store the value of an expression in terraform.
- When we run terraform apply we see that the output variable is printed on the screen.
We can use terraform output command to use the value of the output variables.
- It will print the output of all the terraform output variables in all the configuration files.
- We can use it to print a specific output variable using terraform output pet-name

The best use of terraform output variables is when we want to quickly display details about a resource provision or to feed the output to other IaC tools.

It isn't meant for feeding output of one resource to another resource. This is done using resource attributes.

Last updated: 2023-01-03