Why Plan and apply separation in pipelines in Terraform? - Purpose & Use Cases

The Big Idea

What if you could see all cloud changes before they happen and avoid costly mistakes?

The Scenario

Imagine you are managing your cloud setup by running all commands directly on your computer or server. You try to update your infrastructure by typing commands one by one, mixing planning and applying changes without clear steps.

The Problem

This manual way is slow and risky. You might apply changes before knowing their impact, causing unexpected errors or downtime. It's easy to forget steps or apply changes in the wrong order, leading to confusion and mistakes.

The Solution

Using separate pipeline stages for planning and applying changes helps you see what will happen before making any real updates. This clear separation makes your process safer, more predictable, and easier to manage.

Before vs After

✗ Before

terraform apply

✓ After

terraform plan
terraform apply

What It Enables

This approach lets you catch problems early and confidently update your cloud setup without surprises.

Real Life Example

Think of it like checking a recipe before cooking. Planning is reading the recipe and gathering ingredients, applying is actually cooking the meal. Separating these steps avoids mistakes and wasted effort.

Key Takeaways

Manual cloud updates can cause errors and downtime.

Separating plan and apply steps makes changes safer and clearer.

This method helps catch issues before they affect your system.

Practice

1. What is the main benefit of separating terraform plan and terraform apply in pipelines?

easy

A. It speeds up the deployment by combining steps

B. It skips the need for state files

C. It automatically fixes errors during apply

D. It allows you to review changes before applying them

Solution

1. Step 1: Understand the purpose of terraform plan

   This command shows what changes Terraform will make without applying them.

2. Step 2: Understand the purpose of terraform apply

   This command applies the changes to the infrastructure based on the plan.

3. Final Answer:

   It allows you to review changes before applying them -> Option D

4. Quick Check:

   Plan shows changes first = B [OK]

Hint: Plan shows changes, apply makes them live [OK]

Common Mistakes:

• Thinking plan applies changes
• Believing apply skips state files
• Assuming steps run faster combined

2. Which command correctly saves a Terraform plan to a file named myplan.tfplan?

easy

A. terraform apply -out=myplan.tfplan

B. terraform save myplan.tfplan

C. terraform plan -out=myplan.tfplan

D. terraform plan save myplan.tfplan

Solution

1. Step 1: Identify the correct syntax for saving a plan

   The correct command uses terraform plan -out=filename to save the plan.

2. Step 2: Check each option

   terraform plan -out=myplan.tfplan matches the correct syntax exactly.

3. Final Answer:

   terraform plan -out=myplan.tfplan -> Option C

4. Quick Check:

   Plan with -out saves file = C [OK]

Hint: Use 'plan -out=' to save plans, not apply [OK]

Common Mistakes:

• Using apply instead of plan to save
• Using incorrect command like 'save'
• Missing the '-out=' flag

3. Given these commands run in order:
terraform plan -out=planfile
terraform apply planfile
What happens when you run terraform apply planfile?

medium

A. Terraform applies exactly the changes saved in planfile

B. Terraform ignores planfile and creates a new plan

C. Terraform applies changes but may differ from planfile

D. Terraform throws an error because planfile is not a valid command

Solution

1. Step 1: Understand the role of the saved plan file

   The plan file contains the exact changes Terraform will apply.

2. Step 2: Understand how terraform apply planfile works

   This command applies the saved plan exactly, without recalculating changes.

3. Final Answer:

   Terraform applies exactly the changes saved in planfile -> Option A

4. Quick Check:

   Apply saved plan = exact changes [OK]

Hint: Apply with plan file applies exact saved changes [OK]

Common Mistakes:

• Thinking apply recalculates plan
• Assuming apply ignores plan file
• Believing apply with plan file causes errors

4. You run terraform apply myplan.tfplan but get an error saying the plan file is invalid. What is the most likely cause?

medium

A. You forgot to run terraform init before apply

B. The plan file was created with a different Terraform version

C. You used terraform plan without -out flag

D. The plan file is empty because no changes were detected

Solution

1. Step 1: Identify compatibility issues with plan files

   Plan files are version-specific and may be invalid if Terraform versions differ.

2. Step 2: Check other options

   Running init is required but usually causes different errors; missing -out means no plan file saved; empty plan files do not cause invalid file errors.

3. Final Answer:

   The plan file was created with a different Terraform version -> Option B

4. Quick Check:

   Version mismatch causes invalid plan file error [OK]

Hint: Check Terraform versions match when using saved plans [OK]

Common Mistakes:

• Assuming missing init causes invalid plan file error
• Thinking empty plan files cause invalid file errors
• Confusing missing -out with invalid plan file

5. You want to implement a pipeline that separates planning and applying Terraform changes. Which sequence of commands ensures you can review the plan before applying exactly those changes?

hard

A. terraform plan -out=planfile && terraform apply planfile

B. terraform apply && terraform plan -out=planfile

C. terraform plan && terraform apply

D. terraform apply -out=planfile && terraform apply planfile

Solution

1. Step 1: Identify correct order for plan and apply separation

   First, run terraform plan -out=planfile to save the plan and review it.

2. Step 2: Apply the saved plan exactly

   Then, run terraform apply planfile to apply the reviewed changes exactly.

3. Final Answer:

   terraform plan -out=planfile && terraform apply planfile -> Option A

4. Quick Check:

   Plan with -out then apply saved plan = A [OK]

Hint: Plan with -out then apply saved plan file [OK]

Common Mistakes:

• Applying before planning
• Not saving plan with -out
• Using apply -out which is invalid