Terraform For Loops, Lists, Maps and Count Meta-Argument

Step-00: Pre-requisite Note

• We are using the default vpc in us-west-2 region

Step-01: Introduction

• Terraform Meta-Argument: Count
• Terraform Lists & Maps
  • List(string)
  • map(string)
• Terraform for loops
  • for loop with List
  • for loop with Map
  • for loop with Map Advanced
• Splat Operators
  • Legacy Splat Operator .*.
  • Generalized Splat Operator (latest)
  • Understand about Terraform Generic Splat Expression [*] when dealing with count Meta-Argument and multiple output values

Step-02: variables.tf - Lists and Maps

# AWS EC2 Instance Type - List
variable "instance_type_list" {
  description = "EC2 Instance Type"
  type = list(string)
  default = ["t3.micro", "t3.small"]
}


# AWS EC2 Instance Type - Map
variable "instance_type_map" {
  description = "EC2 Instance Type"
  type = map(string)
  default = {
    "dev" = "t3.micro"
    "qa"  = "t3.small"
    "prod" = "t3.large"
  }
}

Step-04: ec2securitygroups.tf and ami-datasource.tf

• No changes to both files

Step-05: ec2instance.tf

# How to reference List values ?
instance_type = var.instance_type_list[1]

# How to reference Map values ?
instance_type = var.instance_type_map["prod"]

# Meta-Argument Count
count = 2

# count.index
  tags = {
    "Name" = "Count-Demo-${count.index}"
  }

Step-06: outputs.tf

• for loop with List
• for loop with Map
• for loop with Map Advanced

# Output - For Loop with List
output "for_output_list" {
  description = "For Loop with List"
  value = [for instance in aws_instance.myec2vm: instance.public_dns ]
}

# Output - For Loop with Map
output "for_output_map1" {
  description = "For Loop with Map"
  value = {for instance in aws_instance.myec2vm: instance.id => instance.public_dns}
}

# Output - For Loop with Map Advanced
output "for_output_map2" {
  description = "For Loop with Map - Advanced"
  value = {for c, instance in aws_instance.myec2vm: c => instance.public_dns}
}

# Output Legacy Splat Operator (latest) - Returns the List
output "legacy_splat_instance_publicdns" {
  description = "Legacy Splat Expression"
  value = aws_instance.myec2vm.*.public_dns
}  

# Output Latest Generalized Splat Operator - Returns the List
output "latest_splat_instance_publicdns" {
  description = "Generalized Splat Expression"
  value = aws_instance.myec2vm[*].public_dns
}

Step-07: Execute Terraform Commands

# Terraform Initialize
terraform init

# Terraform Validate
terraform validate

# Terraform Plan
terraform plan
Observations:
1) play with Lists and Maps for instance_type

# Terraform Apply
terraform apply -auto-approve
Observations:
1) Two EC2 Instances (Count = 2) of a Resource myec2vm will be created
2) Count.index will start from 0 and end with 1 for VM Names
3) Review outputs in detail (for loop with list, maps, maps advanced, splat legacy and splat latest)

Step-08: Terraform Comments

• Single Line Comments - # and //
• Multi-line Commnets - Start with /* and end with */
• We are going to comment the legacy splat operator, which might be decommissioned in future versions

# Output Legacy Splat Operator (latest) - Returns the List
/* output "legacy_splat_instance_publicdns" {
  description = "Legacy Splat Expression"
  value = aws_instance.myec2vm.*.public_dns
}  */

Step-09: Clean-Up

# Terraform Destroy
terraform destroy -auto-approve

# Files
rm -rf .terraform*
rm -rf terraform.tfstate*