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

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

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
}

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
}

Hint

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

Practice

1. What is the main purpose of the blue-green infrastructure pattern in Terraform deployments?

easy

A. To reduce infrastructure costs by using a single environment

B. To automate database backups during deployment

C. To increase the number of servers in a single environment

D. To avoid downtime by switching traffic between two identical environments

Solution

1. Step 1: Understand the blue-green pattern concept

   The blue-green pattern uses two identical environments to ensure zero downtime during updates.

2. Step 2: Identify the main goal in Terraform deployments

   Terraform manages these environments and switches traffic between them to avoid downtime.

3. Final Answer:

   To avoid downtime by switching traffic between two identical environments -> Option D

4. Quick Check:

   Blue-green pattern = avoid downtime [OK]

Hint: Remember: blue-green means two environments for zero downtime [OK]

Common Mistakes:

• Thinking it reduces costs by using one environment
• Confusing it with scaling servers in one environment
• Assuming it automates backups

2. Which Terraform resource is commonly used to switch traffic between blue and green environments in a blue-green deployment?

easy

A. aws_lb_listener_rule

B. aws_instance

C. aws_s3_bucket

D. aws_security_group

Solution

1. Step 1: Identify Terraform resources related to traffic routing

   Load balancer listener rules control how traffic is routed to target groups.

2. Step 2: Match resource to blue-green traffic switch

   The aws_lb_listener_rule resource allows switching traffic between blue and green target groups.

3. Final Answer:

   aws_lb_listener_rule -> Option A

4. Quick Check:

   Traffic switch uses listener rules [OK]

Hint: Traffic routing uses listener rules, not instances or buckets [OK]

Common Mistakes:

• Choosing aws_instance which manages servers, not traffic
• Selecting aws_s3_bucket which is for storage
• Picking aws_security_group which controls firewall rules

3. Given this Terraform snippet for blue-green deployment traffic switching:

resource "aws_lb_listener_rule" "blue" {
  listener_arn = aws_lb_listener.front_end.arn
  priority     = 10
  action {
    type             = "forward"
    target_group_arn = aws_lb_target_group.blue.arn
  }
  condition {
    path_pattern {
      values = ["/blue/*"]
    }
  }
}

resource "aws_lb_listener_rule" "green" {
  listener_arn = aws_lb_listener.front_end.arn
  priority     = 20
  action {
    type             = "forward"
    target_group_arn = aws_lb_target_group.green.arn
  }
  condition {
    path_pattern {
      values = ["/green/*"]
    }
  }
}

What happens when a user visits /green/home?

medium

A. Traffic is routed to the green target group

B. Traffic is routed to both blue and green target groups

C. Traffic is blocked by the load balancer

D. Traffic is routed to the blue target group

Solution

1. Step 1: Analyze path pattern conditions in listener rules

   The green listener rule matches paths starting with /green/* and forwards to the green target group.

2. Step 2: Match user request path to rules

   The request /green/home matches the green rule condition, so traffic goes to the green target group.

3. Final Answer:

   Traffic is routed to the green target group -> Option A

4. Quick Check:

   Path /green/* routes to green group [OK]

Hint: Match URL path to listener rule path pattern [OK]

Common Mistakes:

• Assuming default routing to blue group
• Thinking traffic is blocked without default rule
• Believing traffic splits between groups

4. You wrote this Terraform code to switch traffic in a blue-green setup but the traffic does not switch as expected:

resource "aws_lb_listener_rule" "blue" {
  listener_arn = aws_lb_listener.front_end.arn
  priority     = 10
  action {
    type             = "forward"
    target_group_arn = aws_lb_target_group.blue.arn
  }
  condition {
    host_header {
      values = ["blue.example.com"]
    }
  }
}

resource "aws_lb_listener_rule" "green" {
  listener_arn = aws_lb_listener.front_end.arn
  priority     = 10
  action {
    type             = "forward"
    target_group_arn = aws_lb_target_group.green.arn
  }
  condition {
    host_header {
      values = ["green.example.com"]
    }
  }
}

What is the likely problem?

medium

A. Target groups are not defined correctly

B. Host header condition is invalid for load balancers

C. Both listener rules have the same priority, causing conflict

D. Listener ARN is missing in one of the rules

Solution

1. Step 1: Check listener rule priorities

   Both rules have priority 10, which causes a conflict because priorities must be unique.

2. Step 2: Understand effect of priority conflict

   Load balancer cannot decide which rule to apply, so traffic routing fails or is unpredictable.

3. Final Answer:

   Both listener rules have the same priority, causing conflict -> Option C

4. Quick Check:

   Unique priorities required for listener rules [OK]

Hint: Listener rule priorities must be unique numbers [OK]

Common Mistakes:

• Ignoring priority uniqueness
• Assuming host_header condition is invalid
• Overlooking target group correctness

5. You want to implement a blue-green deployment in Terraform with minimal downtime. Which approach best achieves this?

hard

A. Deploy new version to green environment and keep routing traffic to blue until green is manually deleted

B. Deploy new version to green environment, test it, then update load balancer to route all traffic to green

C. Deploy new version directly to blue environment and restart all servers simultaneously

D. Deploy new version to blue environment and use DNS TTL to switch traffic slowly

Solution

1. Step 1: Understand blue-green deployment goals

   The goal is zero downtime by having two identical environments and switching traffic atomically.

2. Step 2: Evaluate deployment approaches

   Deploying to green, testing, then switching load balancer traffic ensures smooth transition without downtime.

3. Step 3: Compare other options

   Direct deploy with restart causes downtime; manual deletion delays switch; DNS TTL causes slow switch and possible downtime.

4. Final Answer:

   Deploy new version to green environment, test it, then update load balancer to route all traffic to green -> Option B

5. Quick Check:

   Blue-green = test new env, then switch traffic [OK]

Hint: Test new environment fully before switching traffic [OK]

Common Mistakes:

• Restarting servers causing downtime
• Delaying traffic switch by manual deletion
• Relying on DNS TTL for instant switch