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Terraform Dynamic Expressions

Step-01: Introduction

  • Learn Dynamic Expressions in Terraform
  • Conditional Expression: A conditional expression uses the value of a bool expression to select one of two values. 
    # Example-1
condition ? true_val : false_val

# Example-2
var.a != "" ? var.a : "default-a"
  • Splat Expression: A splat expression provides a more concise way to express a common operation that could otherwise be performed with a for expression.
  • The special [*] symbol iterates over all of the elements of the list given to its left and accesses from each one the attribute name given on its right. 
    # With for expression
[for o in var.list : o.id]

# With Splat Expression [*]
var.list[*].id
  • A splat expression can also be used to access attributes and indexes from lists of complex types by extending the sequence of operations to the right of the symbol: 
    var.list[*].interfaces[0].name
aws_instance.example[*].id

Step-02: Review Terraform Manifests

c1-versions.tf

  • Added new random provider in required_providers block

c2-vairables.tf

  • Added new variables
  • availability_zones
  • name
  • team
  • high_availability

c3-security-groups.tf

  • Added common tags

c4-ec2-instance.tf

  • Added Random ID resource block
  • Added new locals block
  • Important Note: Inside locals block we can add conditional expressions as below. 
    # Create Locals
locals {
  #name = var.name
  name  = (var.name != "" ? var.name : random_id.id.hex)
  owner = var.team
  common_tags = {
    Owner = local.owner
    DefaultUser = local.name 
  }
}
  • Added Availability zone argument with count.index
  • We will discuss about following conditional expressions here 
    # In Locals Block: Conditional Expression
name  = (var.name != "" ? var.name : random_id.id.hex)

# In EC2 Resource Block: Conditional Expression
count = (var.high_availability == true ? 2 : 1)

# In EC2 Resource Block: count.index
availability_zone = var.availability_zones[count.index]

c5-outputs.tf

  • Added Splat expression [*] for all outputs
  • Added Common Tags and ELB DNS Name as new outputs

c6-ami-datasource.tf

  • No changes

c7-elb.tf

  • Added this new resource
  • We will be creating ELB only if "high_availability" variable value is true else it will not be created 
    # Create ELB if high_availability=true
# In ELB Block: Conditional Expression
count = (var.high_availability == true ? 1 : 0)

Step-03: Execute Terraform Commands

# Terraform Initialize
terraform init

# Terraform Validate
terraform validate

# Terraform Plan: When Variable values, high_availability = false and name = "ec2-user"
terraform plan
Observation: 
1) Plan will generate for only 1 EC2 instance and 2 Security Groups.
2) ELB Resource will not be created with these variable options


# Terraform Plan: When Variable values, high_availability = true and name = ""
terraform plan
1) Plan will generate for only 2 EC2 instance, 2 Security Groups and 1 ELB
2) ELB Resource will be created with these variable options
3) name value will be a random value known after terraform apply completed

# Terraform Apply
terraform apply -auto-approve

# Verify
1) Verify Outputs
2) Verify EC2 Instances & Security Groups & Common Tags
3) Verify ELB & Common Tags for ELB
4) Access App using ELB DNS Name

Step-04: Clean-Up

# Terraform Destroy
terraform destroy -auto-approve

# Clean-Up
rm -rf .terraform*
rm -rf terraform.tfstate*

# Uncomment and Comment right values in c2-variables.tf (Roll back to put ready for student demo)
high_availability = false 
name = "ec2-user"

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