Declarative vs imperative IaC in Terraform - Performance Comparison

Time Complexity: Declarative vs imperative IaC

O(n)

Understanding Time Complexity

We want to understand how the number of steps grows when using declarative or imperative infrastructure code.

Which approach takes more repeated actions as the infrastructure grows?

Scenario Under Consideration

Analyze the time complexity of creating multiple resources using declarative Terraform code.

resource "aws_instance" "example" {
  count         = var.instance_count
  ami           = "ami-123456"
  instance_type = "t2.micro"
}

This code creates a number of virtual machines equal to instance_count in a declarative way.

Identify Repeating Operations

Look at what happens repeatedly when this code runs.

• Primary operation: Creating each virtual machine resource via API calls.
• How many times: Once per instance, so as many times as instance_count.

How Execution Grows With Input

As you increase the number of instances, the number of API calls grows directly with it.

Input Size (n)	Approx. Api Calls/Operations
10	10
100	100
1000	1000

Pattern observation: The work grows evenly as you add more resources.

Final Time Complexity

Time Complexity: O(n)

This means the time to create resources grows directly with the number of resources you want.

Common Mistake

[X] Wrong: "Declarative code runs all steps only once no matter how many resources."

[OK] Correct: Each resource still requires its own API call and provisioning, so the total work grows with the number of resources.

Interview Connect

Understanding how resource count affects execution helps you explain infrastructure scaling clearly and confidently.

Self-Check

"What if we used a loop in imperative code to create resources one by one? How would the time complexity compare?"

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