Terraform Registry modules - Step-by-Step Execution

Process Flow - Terraform Registry modules

Start Terraform config

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Declare module block

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Specify source from Registry

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Terraform init downloads module

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Terraform plan applies module resources

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Terraform apply creates infrastructure

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End

Terraform config declares a module from the Registry, init downloads it, then plan and apply create resources.

Execution Sample

Terraform

module "example" {
  source = "terraform-aws-modules/vpc/aws"
  version = "3.14.2"
  name = "my-vpc"
  cidr = "10.0.0.0/16"
}

This code uses a VPC module from Terraform Registry to create a VPC with a given name and CIDR.

Process Table

Step	Action	Details	Result
1	Read config	Find module block with source terraform-aws-modules/vpc/aws	Module identified
2	Run terraform init	Download module version 3.14.2 from Registry	Module code downloaded locally
3	Run terraform plan	Evaluate module inputs (name, cidr)	Plan shows VPC resource to create
4	Run terraform apply	Apply plan to cloud provider	VPC created with specified name and CIDR
5	End	Infrastructure created as per module	Success

💡 Terraform apply completes, infrastructure matches module configuration

Status Tracker

Variable	Start	After init	After plan	After apply
module_source	undefined	terraform-aws-modules/vpc/aws	terraform-aws-modules/vpc/aws	terraform-aws-modules/vpc/aws
module_version	undefined	3.14.2	3.14.2	3.14.2
module_name	undefined	my-vpc	my-vpc	my-vpc
module_cidr	undefined	10.0.0.0/16	10.0.0.0/16	10.0.0.0/16
infrastructure_state	empty	module code downloaded	plan ready	VPC created

Key Moments - 3 Insights

Why do we run 'terraform init' before 'terraform plan' when using Registry modules?

'terraform init' downloads the module code from the Registry so Terraform knows what resources to create. Without init, plan cannot evaluate the module. See execution_table step 2.

What happens if the module version is not specified?

Terraform uses the latest version available in the Registry by default. Specifying version ensures consistent infrastructure. This is shown in variable_tracker for module_version.

How does Terraform know what resources the module will create?

Terraform reads the module code downloaded during init, then uses inputs to plan resources. This is shown in execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 2?

A Terraform reads the configuration file

B Terraform applies the infrastructure changes

C Terraform downloads the module code from the Registry

D Terraform creates the VPC resource

💡 Hint

Check execution_table row with Step 2 describing 'terraform init'

According to variable_tracker, what is the value of module_version after 'terraform init'?

A 3.14.2

B undefined

C latest

D my-vpc

💡 Hint

Look at variable_tracker row for module_version under 'After init' column

If the module source was changed to a local path, how would the execution_table step 2 change?

A Terraform would download the module from the Registry

B Terraform would skip downloading and use local code

C Terraform would fail because source is invalid

D Terraform would apply the plan immediately

💡 Hint

Consider what 'terraform init' does when source is local instead of Registry

Concept Snapshot

Terraform Registry modules allow reuse of community or official modules.
Declare a module block with source and optional version.
Run 'terraform init' to download module code.
Run 'terraform plan' and 'apply' to create resources.
Specifying version ensures consistent deployments.
Modules simplify infrastructure management.

Full Transcript

Terraform Registry modules let you use pre-built infrastructure code. You write a module block in your Terraform config with a source pointing to the Registry module. Running 'terraform init' downloads the module code locally. Then 'terraform plan' reads the module inputs and shows what resources will be created. Finally, 'terraform apply' creates the resources in your cloud account. Specifying a version locks the module version for stability. This process helps you reuse tested infrastructure components easily.