Immutable infrastructure concept in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is immutable infrastructure?

Immutable infrastructure means once a server or resource is created, it is never changed or updated. Instead, if changes are needed, a new resource is created and the old one is replaced.

beginner

Why is immutable infrastructure beneficial?

It reduces errors and inconsistencies because you avoid changing live systems. It also makes deployments predictable and easier to roll back by replacing resources instead of modifying them.

intermediate

How does Terraform support immutable infrastructure?

Terraform creates and manages infrastructure as code. When you change configuration, Terraform can replace resources instead of updating them in place, supporting immutable infrastructure practices.

beginner

What is a common real-life example of immutable infrastructure?

Using containers like Docker is a real-life example. Instead of changing a running container, you build a new container image and deploy it, replacing the old one.

beginner

What happens to the old infrastructure in immutable infrastructure approach?

The old infrastructure is destroyed or decommissioned after the new one is deployed, ensuring only the new, updated resources are running.

What does immutable infrastructure avoid?

A Using code to manage infrastructure

B Changing live servers directly

C Creating new servers

D Replacing old resources

Immutable infrastructure avoids changing live servers directly by replacing them instead.

In Terraform, how is immutable infrastructure usually implemented?

A By updating resources in place

B By ignoring configuration changes

C By replacing resources when configuration changes

D By manually editing servers

Terraform replaces resources when configuration changes to support immutable infrastructure.

Which of these is a benefit of immutable infrastructure?

A More manual updates

B Increased configuration drift

C Unpredictable deployments

D Easier rollback

Immutable infrastructure makes rollback easier by replacing resources instead of modifying them.

What happens to old resources in immutable infrastructure?

A They are destroyed after replacement

B They remain running alongside new ones

C They are updated

D They are ignored

Old resources are destroyed after the new ones are deployed to keep infrastructure clean.

Which technology is a common example of immutable infrastructure?

A Docker containers

B Manual server updates

C Traditional virtual machines

D Direct database edits

Docker containers are often used as immutable infrastructure by replacing containers instead of changing them.

Explain the concept of immutable infrastructure and why it is useful.

Think about how you would update a server without changing it directly.

Describe how Terraform helps implement immutable infrastructure.

Consider how Terraform manages resources when you change your code.

Practice

1. What does the term immutable infrastructure mean in Terraform?

easy

A. Replacing resources instead of modifying them in place

B. Changing resources directly without replacement

C. Manually updating resources outside Terraform

D. Using mutable variables to configure resources

Solution

1. Step 1: Understand the definition of immutable infrastructure

   Immutable infrastructure means you do not change existing resources but replace them entirely when updates are needed.

2. Step 2: Compare options with this definition

   Only Replacing resources instead of modifying them in place describes replacing resources instead of modifying them, which matches the concept.

3. Final Answer:

   Replacing resources instead of modifying them in place -> Option A

4. Quick Check:

   Immutable infrastructure = Replace, not modify [OK]

Hint: Immutable means replace, not change existing resources [OK]

Common Mistakes:

• Thinking mutable means immutable
• Confusing manual updates with Terraform-managed changes
• Assuming resources are changed in place

2. Which Terraform lifecycle argument helps implement immutable infrastructure by creating new resources before destroying old ones?

easy

A. ignore_changes

B. prevent_destroy

C. create_before_destroy

D. replace_triggered_by

Solution

1. Step 1: Identify lifecycle arguments related to resource replacement

   Terraform lifecycle has arguments like create_before_destroy, prevent_destroy, ignore_changes, and replace_triggered_by.

2. Step 2: Match argument to immutable infrastructure behavior

   Create_before_destroy ensures new resource is created before old one is destroyed, supporting immutable infrastructure.

3. Final Answer:

   create_before_destroy -> Option C

4. Quick Check:

   Lifecycle create_before_destroy = create new before delete old [OK]

Hint: Use create_before_destroy to replace safely [OK]

Common Mistakes:

• Confusing prevent_destroy with create_before_destroy
• Using ignore_changes which skips updates but doesn't replace
• Misunderstanding replace_triggered_by purpose

3. Given this Terraform resource snippet with lifecycle:

resource "aws_instance" "example" {
  ami           = "ami-123456"
  instance_type = "t2.micro"

  lifecycle {
    create_before_destroy = true
  }
}

What happens when you change instance_type from t2.micro to t2.small?

medium

A. Terraform creates a new instance first, then destroys the old one

B. Terraform ignores the change due to lifecycle rules

C. Terraform destroys the old instance first, then creates a new one

D. Terraform updates the existing instance in place

Solution

1. Step 1: Understand create_before_destroy lifecycle effect

   This setting tells Terraform to create the new resource before destroying the old one to avoid downtime.

2. Step 2: Apply this to instance_type change

   Changing instance_type requires replacement, so Terraform creates new instance first, then destroys old.

3. Final Answer:

   Terraform creates a new instance first, then destroys the old one -> Option A

4. Quick Check:

   create_before_destroy means create new before destroy old [OK]

Hint: create_before_destroy means new resource first [OK]

Common Mistakes:

• Assuming in-place update happens
• Thinking old resource is destroyed before new is ready
• Believing lifecycle ignores changes

4. You wrote this Terraform lifecycle block:

lifecycle {
  create_before_destroy = false
}

But you want to implement immutable infrastructure with zero downtime. What is the problem?

medium

A. Terraform will create duplicate resources without destroying old ones

B. Terraform will not replace resources at all

C. Terraform will ignore lifecycle block and update in place

D. Resources will be destroyed before new ones are created, causing downtime

Solution

1. Step 1: Analyze create_before_destroy false effect

   When false, Terraform destroys old resource before creating new one, causing downtime.

2. Step 2: Match with immutable infrastructure goal

   Immutable infrastructure aims for zero downtime by creating new resource first, so false breaks this goal.

3. Final Answer:

   Resources will be destroyed before new ones are created, causing downtime -> Option D

4. Quick Check:

   False create_before_destroy = destroy first, downtime risk [OK]

Hint: False create_before_destroy causes downtime risk [OK]

Common Mistakes:

• Thinking false means no replacement
• Assuming lifecycle block is ignored
• Believing duplicates are created without destroy

5. You want to deploy a web server with immutable infrastructure using Terraform. Which combination of lifecycle settings and resource management best supports this goal?

hard

A. Use ignore_changes on all attributes to prevent updates

B. Use create_before_destroy = true and avoid manual changes outside Terraform

C. Use prevent_destroy = true to stop any resource replacement

D. Manually update resources and disable Terraform lifecycle rules

Solution

1. Step 1: Identify lifecycle settings that enable immutable infrastructure

   Create_before_destroy = true ensures new resource is ready before old is removed, supporting immutable infrastructure.

2. Step 2: Consider resource management best practices

   Avoid manual changes outside Terraform to keep infrastructure consistent and manageable.

3. Final Answer:

   Use create_before_destroy = true and avoid manual changes outside Terraform -> Option B

4. Quick Check:

   Immutable infrastructure = create_before_destroy + Terraform-only changes [OK]

Hint: Combine create_before_destroy with Terraform-only changes [OK]

Common Mistakes:

• Using prevent_destroy which blocks replacement
• Ignoring changes disables updates, not replacement
• Manual changes cause drift and errors