Terraform Small Utility Project

Step-01: Introduction

Current Problem:

• We are not able to create EC2 Instances in all the subnets of our VPC which are spread across all availability zones in that region

Approach to a Solution:

• We need to find a solution to say that our desired EC2 Instance Type example: t3.micro is supported in that availability zone or not
• In simple terms, give me the availability zone list in a particular region where by desired EC2 Instance Type (t3.micro) is supported

Why utility project?

• In Terraform, we should not go and try things directly in large code base.
• First try your requirements in small chunks and integrate that to main code base.
• We are going to do the same now.

Step-02: c1-versions.tf

• Hard-coded the region as we are not going to use any variables.tf in this utility project

# Provider Block
provider "aws" {
  region  = "us-east-1"
}

Step-03: c2-v1-get-instancetype-supported-per-az-in-a-region.tf

• We are first going to explore the datasource and it outputs

# Determine which Availability Zones support your instance type
aws ec2 describe-instance-type-offerings --location-type availability-zone  --filters Name=instance-type,Values=t3.micro --region us-east-1 --output table

Step-03-01: Review / Create the datasource and its output

# Datasource
data "aws_ec2_instance_type_offerings" "my_ins_type1" {
  filter {
    name   = "instance-type"
    values = ["t3.micro"]
  }
  filter {
    name   = "location"
    values = ["us-east-1a"]
    #values = ["us-east-1e"]    
  }
  location_type = "availability-zone"
}


# Output
output "output_v1_1" {
 value = data.aws_ec2_instance_type_offerings.my_ins_type1.instance_types
}

Step-03-02: Execute Terraform Commands

# Terraform Initialize
terraform init

# Terraform Validate
terraform validate

# Terraform Plan
terraform plan
terraform apply -auto-approve
Observation: 
1. Output should have the instance value `t3.micro` when `values = ["us-east-1a"]` in location filter
# Sample Output
output_v1_1 = toset([
  "t3.micro",
])

# Make a change
Switch the values in `location` filter to `values = ["us-east-1e"]` and test again with `terraform plan`

# Terraform Plan
terraform plan
terraform apply -auto-approve
Observation: 
1. Output should have the instance value empty `[]` when `values = ["us-east-1e"]` in location filter
# Sample Output
output_v1_1 = toset([])

Step-04: c2-v2-get-instancetype-supported-per-az-in-a-region.tf

• Using for_each create multiple instances of datasource and loop it with hard-coded availability zones in for_each

Step-04-01: Review / Create the datasource and its output with for_each

# Check if that respective Instance Type is supported in that Specific Region in list of availability Zones
# Get the List of Availability Zones in a Particular region where that respective Instance Type is supported
data "aws_ec2_instance_type_offerings" "my_ins_type2" {
  for_each = toset([ "us-east-1a", "us-east-1e" ])
  filter {
    name   = "instance-type"
    values = ["t3.micro"]
  }
  filter {
    name   = "location"
    values = [each.key]
  }
  location_type = "availability-zone"
}


# Important Note: Once for_each is set, its attributes must be accessed on specific instances
output "output_v2_1" {
 #value = data.aws_ec2_instance_type_offerings.my_ins_type1.instance_types
 value = toset([
      for t in data.aws_ec2_instance_type_offerings.my_ins_type2 : t.instance_types
    ])  
}

# Create a Map with Key as Availability Zone and value as Instance Type supported
output "output_v2_2" {
 value = { for az, details in data.aws_ec2_instance_type_offerings.my_ins_type2 :
  az => details.instance_types }   
}

Step-04-02: Execute Terraform Commands

# Terraform Plan
terraform plan
terraform apply -auto-approve
Observation: refer sample output
# Sample Output
output_v2_1 = toset([
  toset([
    "t3.micro",
  ]),
  toset([]),
])
output_v2_2 = {
  "us-east-1a" = toset([
    "t3.micro",
  ])
  "us-east-1e" = toset([])
}

Step-05: c2-v3-get-instancetype-supported-per-az-in-a-region.tf

Step-05-01: Add new datasource aws_availability_zones

• Get List of Availability Zones in a Specific Region

# Get List of Availability Zones in a Specific Region
# Region is set in c1-versions.tf in Provider Block
data "aws_availability_zones" "my_azones" {
  filter {
    name   = "opt-in-status"
    values = ["opt-in-not-required"]
  }
}

Step-05-02: Update for_each with new datasource

# Check if that respective Instance Type is supported in that Specific Region in list of availability Zones
# Get the List of Availability Zones in a Particular region where that respective Instance Type is supported
data "aws_ec2_instance_type_offerings" "my_ins_type" {
for_each=toset(data.aws_availability_zones.my_azones.names)
  filter {
    name   = "instance-type"
    values = ["t3.micro"]
  }
  filter {
    name   = "location"
    values = [each.key]
  }
  location_type = "availability-zone"
}

Step-05-03: Implement Incremental Outputs till we reach what is required

# Basic Output: All Availability Zones mapped to Supported Instance Types
output "output_v3_1" {
 value = { for az, details in data.aws_ec2_instance_type_offerings.my_ins_type :
  az => details.instance_types }   
}

# Filtered Output: Exclude Unsupported Availability Zones
output "output_v3_2" {
  value = { for az, details in data.aws_ec2_instance_type_offerings.my_ins_type :
  az => details.instance_types if length(details.instance_types) != 0 }
}

# Filtered Output: with Keys Function - Which gets keys from a Map
# This will return the list of availability zones supported for a instance type
output "output_v3_3" {
  value = keys({ for az, details in data.aws_ec2_instance_type_offerings.my_ins_type :
  az => details.instance_types if length(details.instance_types) != 0 }) 
}

# Filtered Output: As the output is list now, get the first item from list (just for learning)
output "output_v3_4" {
  value = keys({ for az, details in data.aws_ec2_instance_type_offerings.my_ins_type :
  az => details.instance_types if length(details.instance_types) != 0 })[0]
}

Step-05-04: Execute Terraform Commands

# Terraform Plan
terraform plan
terraform appy -auto-approve
Observation: refer sample output
1. In the final output you will only get the availability zones list in which `t3.micro` instance is supported
# Sample Output
output_v3_1 = {
  "us-east-1a" = toset([
    "t3.micro",
  ])
  "us-east-1b" = toset([
    "t3.micro",
  ])
  "us-east-1c" = toset([
    "t3.micro",
  ])
  "us-east-1d" = toset([
    "t3.micro",
  ])
  "us-east-1e" = toset([])
  "us-east-1f" = toset([
    "t3.micro",
  ])
}
output_v3_2 = {
  "us-east-1a" = toset([
    "t3.micro",
  ])
  "us-east-1b" = toset([
    "t3.micro",
  ])
  "us-east-1c" = toset([
    "t3.micro",
  ])
  "us-east-1d" = toset([
    "t3.micro",
  ])
  "us-east-1f" = toset([
    "t3.micro",
  ])
}
output_v3_3 = [
  "us-east-1a",
  "us-east-1b",
  "us-east-1c",
  "us-east-1d",
  "us-east-1f",
]
output_v3_4 = "us-east-1a"

Step-06: Clean-Up

# Terraform Destroy
terraform destroy -auto-approve

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