Blue-green infrastructure pattern in Terraform - Mini Project: Build & Apply

Blue-green infrastructure pattern

📖 Scenario: You are managing a web application deployment on the cloud. To avoid downtime during updates, you want to use the blue-green deployment pattern. This means you have two identical environments: blue and green. Only one environment serves live traffic at a time. When you update, you deploy to the inactive environment, then switch traffic to it.

🎯 Goal: Create a Terraform configuration that defines two identical infrastructure environments named blue and green. Then add a variable to select which environment is active. Finally, configure a load balancer resource that routes traffic to the active environment only.

📋 What You'll Learn

Define two identical infrastructure resources named blue and green.

Add a variable called active_environment with allowed values "blue" and "green".

Use the active_environment variable to select which environment the load balancer routes traffic to.

The Terraform configuration must be valid and deployable.

💡 Why This Matters

🌍 Real World

Blue-green deployment is a common pattern to update applications without downtime by switching traffic between two identical environments.

💼 Career

Cloud engineers and DevOps professionals use this pattern to ensure smooth, reliable application updates in production.

1

Define blue and green infrastructure resources

Create two identical aws_instance resources named blue and green. Use the AMI ID "ami-12345678" and instance type "t2.micro" for both.

Terraform

# Your code here

Need a hint?

Define two aws_instance blocks with names blue and green. Use the same AMI and instance type for both.

2

Add active environment variable

Add a Terraform variable called active_environment with type string and allowed values ["blue", "green"]. Set the default value to "blue".

Terraform

resource "aws_instance" "blue" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

resource "aws_instance" "green" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

# Your code here

Need a hint?

Use a variable block with type = string, default = "blue", and a validation block to restrict values.

3

Select active instance using the variable

Create a local value called active_instance_id that uses a conditional expression to select aws_instance.blue.id if var.active_environment is "blue", otherwise aws_instance.green.id.

Terraform

resource "aws_instance" "blue" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

resource "aws_instance" "green" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

variable "active_environment" {
  type    = string
  default = "blue"
  validation {
    condition     = contains(["blue", "green"], var.active_environment)
    error_message = "active_environment must be either 'blue' or 'green'"
  }
}

locals {
  # Your code here
}

Need a hint?

Use a locals block with a conditional expression to pick the active instance ID.

4

Configure load balancer to route to active environment

Create an aws_lb_target_group_attachment resource named active_attachment that attaches the load balancer target group aws_lb_target_group.main.id to the instance ID from local.active_instance_id.

Terraform

resource "aws_instance" "blue" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

resource "aws_instance" "green" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

variable "active_environment" {
  type    = string
  default = "blue"
  validation {
    condition     = contains(["blue", "green"], var.active_environment)
    error_message = "active_environment must be either 'blue' or 'green'"
  }
}

locals {
  active_instance_id = var.active_environment == "blue" ? aws_instance.blue.id : aws_instance.green.id
}

resource "aws_lb_target_group_attachment" "active_attachment" {
  # Your code here
}

Need a hint?

Use aws_lb_target_group_attachment with target_group_arn, target_id, and port set to 80.