Declarative vs imperative IaC in Terraform - Hands-On Comparison

Declarative vs Imperative Infrastructure as Code with Terraform

📖 Scenario: You are working as a cloud engineer. Your team wants to understand the difference between declarative and imperative ways of managing cloud infrastructure. You will create a simple Terraform configuration to declare a cloud resource, then add a variable to configure it, then use a loop to create multiple resources, and finally complete the configuration with an output.

🎯 Goal: Build a Terraform configuration that declares an AWS S3 bucket, uses a variable to set the bucket name prefix, creates multiple buckets using a loop, and outputs the bucket names. This will demonstrate declarative Infrastructure as Code (IaC) principles.

📋 What You'll Learn

Use Terraform syntax to declare resources

Create a variable to configure bucket name prefix

Use a for_each loop to create multiple buckets

Output the list of created bucket names

💡 Why This Matters

🌍 Real World

Cloud engineers use Terraform to declare and manage cloud resources in a clear, repeatable way. This project shows how to write simple, reusable Terraform code.

💼 Career

Understanding declarative IaC with Terraform is essential for roles like Cloud Engineer, DevOps Engineer, and Infrastructure Developer.

1

Declare a single AWS S3 bucket resource

Write a Terraform resource block named aws_s3_bucket with the resource name example_bucket. Set the bucket attribute to the exact string "my-unique-bucket-001". This declares one S3 bucket.

Terraform

# Declare an aws_s3_bucket resource named example_bucket
# Your code here

Hint

Use resource "aws_s3_bucket" "example_bucket" { ... } and set bucket = "my-unique-bucket-001".

2

Add a variable for bucket name prefix

Create a Terraform variable block named bucket_prefix of type string. Set its default value to "my-unique-bucket". This variable will help configure bucket names.

Terraform

resource "aws_s3_bucket" "example_bucket" {
  bucket = "my-unique-bucket-001"
}

# Declare a variable named bucket_prefix with default "my-unique-bucket"
# Your code here

Hint

Use variable "bucket_prefix" { type = string default = "my-unique-bucket" }.

3

Use a for_each loop to create multiple buckets

Replace the single aws_s3_bucket resource with a resource named aws_s3_bucket and resource name multiple_buckets. Use for_each with the set {1, 2, 3}. Set the bucket attribute to the expression "${var.bucket_prefix}-${each.key}" to create three buckets with names like my-unique-bucket-1.

Terraform

resource "aws_s3_bucket" "example_bucket" {
  bucket = "my-unique-bucket-001"
}

variable "bucket_prefix" {
  type    = string
  default = "my-unique-bucket"
}

# Replace example_bucket with multiple_buckets using for_each {1, 2, 3}
# Set bucket = "${var.bucket_prefix}-${each.key}"
# Your code here

Hint

Use for_each = toset([1, 2, 3]) and bucket = "${var.bucket_prefix}-${each.key}".

4

Add an output for the bucket names

Create a Terraform output block named bucket_names. Set its value to a list of all bucket names created by aws_s3_bucket.multiple_buckets. Use the expression [for b in aws_s3_bucket.multiple_buckets : b.bucket].

Terraform

variable "bucket_prefix" {
  type    = string
  default = "my-unique-bucket"
}

resource "aws_s3_bucket" "multiple_buckets" {
  for_each = toset([1, 2, 3])
  bucket   = "${var.bucket_prefix}-${each.key}"
}

# Add output block named bucket_names with value [for b in aws_s3_bucket.multiple_buckets : b.bucket]
# Your code here

Hint

Use output "bucket_names" { value = [for b in aws_s3_bucket.multiple_buckets : b.bucket] }.

Practice

1. What is the main difference between declarative and imperative Infrastructure as Code (IaC)?

easy

A. Declarative IaC describes the desired end state, while imperative IaC specifies exact steps to reach it.

B. Declarative IaC requires manual commands, while imperative IaC is fully automatic.

C. Declarative IaC is only for cloud, imperative IaC is for on-premises.

D. Declarative IaC uses scripts, imperative IaC uses configuration files.

Solution

1. Step 1: Understand declarative IaC

   Declarative IaC focuses on describing the final desired state of infrastructure without specifying how to achieve it.

2. Step 2: Understand imperative IaC

   Imperative IaC lists the exact commands or steps needed to create or change infrastructure.

3. Final Answer:

   Declarative IaC describes the desired end state, while imperative IaC specifies exact steps to reach it. -> Option A

4. Quick Check:

   Declarative = final state, Imperative = steps [OK]

Hint: Declarative = what you want; imperative = how to do it [OK]

Common Mistakes:

• Confusing declarative with imperative steps
• Thinking declarative requires manual commands
• Assuming declarative only works for cloud

2. Which of the following Terraform code snippets correctly shows a declarative resource definition?

easy

A. create_instance("ami-123", "t2.micro")

B. resource "aws_instance" "web" { ami = "ami-123" instance_type = "t2.micro" }

C. if not instance_exists() then create_instance()

D. run_command("aws ec2 run-instances --image-id ami-123")

Solution

1. Step 1: Identify declarative syntax

   Terraform resource blocks declare the desired state, like the resource "aws_instance" "web" { ... } block.

2. Step 2: Compare with imperative styles

   Options B, C, and D show commands or logic steps, which are imperative, not declarative.

3. Final Answer:

   The Terraform resource block syntax is declarative. -> Option B

4. Quick Check:

   Terraform resource block = declarative [OK]

Hint: Terraform resource blocks are declarative descriptions [OK]

Common Mistakes:

• Choosing imperative command syntax as declarative
• Confusing function calls with resource declarations
• Ignoring the resource block structure

3. Given this Terraform snippet, what will Terraform do when you run terraform apply if the AWS instance does not exist yet?

resource "aws_instance" "example" {
  ami           = "ami-abc123"
  instance_type = "t2.micro"
}

medium

A. Terraform will prompt for manual commands to create the instance.

B. Terraform will delete any existing instances and do nothing else.

C. Terraform will create a new AWS instance with the specified AMI and instance type.

D. Terraform will throw a syntax error because the resource block is incomplete.

Solution

1. Step 1: Understand Terraform apply behavior

   Terraform compares the current state with the desired state defined in the resource block.

2. Step 2: Predict action if resource missing

   If the AWS instance does not exist, Terraform will create it to match the declared state.

3. Final Answer:

   Terraform will create a new AWS instance with the specified AMI and instance type. -> Option C

4. Quick Check:

   Missing resource = Terraform creates it [OK]

Hint: Terraform creates missing resources to match declared state [OK]

Common Mistakes:

• Thinking Terraform deletes resources by default
• Expecting manual commands during apply
• Assuming syntax error in valid resource block

4. You wrote this Terraform code but terraform apply fails with an error:

resource "aws_instance" "web" {
  ami = "ami-123"
  instance_type = t2.micro
}

What is the likely cause of the error?

medium

A. Terraform requires an explicit create command inside the resource block.

B. The resource name "web" is invalid and must be changed.

C. The AMI ID "ami-123" is not allowed in Terraform.

D. Missing quotes around the instance_type value causes a syntax error.

Solution

1. Step 1: Check syntax of resource attributes

   Terraform requires string values to be in quotes. The instance_type value is missing quotes.

2. Step 2: Identify error cause

   Missing quotes cause a syntax error, preventing Terraform from parsing the file.

3. Final Answer:

   Missing quotes around the instance_type value causes a syntax error. -> Option D

4. Quick Check:

   Strings need quotes in Terraform [OK]

Hint: Always quote string values in Terraform resource blocks [OK]

Common Mistakes:

• Ignoring missing quotes around strings
• Thinking resource names cause syntax errors
• Expecting commands inside resource blocks

5. You want to create a cloud infrastructure using Terraform declarative IaC. Which approach best fits this goal?

hard

A. Write a Terraform configuration file describing the desired resources and run terraform apply.

B. Write a script with commands to create each resource step-by-step manually.

C. Manually create resources in the cloud console, then import them into Terraform.

D. Use a shell script to run cloud CLI commands in order.

Solution

1. Step 1: Understand declarative IaC goal

   Declarative IaC means describing the final infrastructure state in configuration files.

2. Step 2: Identify Terraform usage

   Terraform uses configuration files to declare resources, then applies changes automatically.

3. Final Answer:

   Write a Terraform configuration file describing the desired resources and run terraform apply. -> Option A

4. Quick Check:

   Declarative IaC = config files + apply [OK]

Hint: Use Terraform config files, not manual scripts, for declarative IaC [OK]

Common Mistakes:

• Confusing declarative IaC with manual scripting
• Thinking manual cloud console use is declarative
• Using imperative CLI commands instead of config files