Imperative vs Declarative IaC: Ansible and Terraform

Introduction

In the bustling world of DevOps and cloud computing, Infrastructure as Code (IaC) has quickly grown from an innovative idea to a fundamental part of the software development process. But have you ever wondered about the different approaches to IaC? Have you found yourself puzzled by terms like "imperative" and "declarative" IaC? Don't worry, this article will cover the basics using examples from both.

Understanding Infrastructure as Code (IaC)

IaC is a practice that involves managing and provisioning computer data centers through machine-readable definition files, instead of using interactive configuration tools. In simpler terms, it's like writing code to automate the process of setting up and managing your servers. It's like creating a recipe for your IT environment! When it comes to IaC, there are two fundamental styles: imperative and declarative. The key difference lies in the approach to defining the desired state of infrastructure. Imperative and declarative, while sounding like words pulled from an English literature class, are central concepts in IaC. Let's unravel them one by one.

Example with Ansible

Ansible is an excellent example of an imperative IaC tool. It uses YAML-based playbooks to define a sequence of tasks. Each task modifies the state of the system. Here's a basic example:

---
- name: Install and Manage Apache
  hosts: webservers
  tasks:
    - name: Ensure Apache is installed
      yum:
        name: httpd
        state: present
    - name: Ensure Apache service is running
      service:
        name: httpd
        state: started

In this Ansible playbook, after Apache is installed, another task starts the Apache service.

Declarative Infrastructure as Code

Contrary to imperative IaC, declarative IaC is all about the "what." You define the desired state, and the IaC tool figures out how to achieve it. You don't need to provide step-by-step instructions.

Example with Terraform

resource "aws_instance" "web" {
  ami           = "ami-0c94855ba95c574c8"
  instance_type = "t2.micro"

  vpc_security_group_ids = [aws_security_group.allow_http.id]

  tags = {
    Name = "WebServer"
  }
}

resource "aws_security_group" "allow_http" {
  name        = "allow_http"
  description = "Allow inbound traffic on port 80"

  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

This Terraform code snippet has two main parts: an EC2 instance resource and a Security Group resource. The [aws_security_group.allow_http.id] in the aws_instance block connects the security group with the EC2 instance, allowing incoming traffic on port 80, which is typically used for web servers.

Key Differences Between Imperative and Declarative IaC

As mentioned earlier, the main difference lies in the approach. Imperative IaC is procedural, focusing on how to reach the desired state. Declarative IaC, on the other hand, focuses on what the final state should be.

Flexibility and Complexity

Imperative IaC can offer more flexibility as it allows you to dictate each step. However, this can result in increased complexity. Declarative IaC, while less flexible, tends to be simpler and more manageable.

State Management

Declarative tools like Terraform maintain a state file that records the current state of infrastructure. This allows them to adjust only what's needed to reach the desired state. On the other hand, imperative tools like Ansible do not maintain state, leading to possible redundancy in operations.

Use Cases

Imperative IaC, with its detailed instructions, is often better suited for sequential tasks such as software deployments. Declarative IaC shines in defining and managing entire systems or environments due to its emphasis on the end state.

Choosing Between Imperative and Declarative IaC

Key Considerations

The choice between imperative and declarative IaC is largely dependent on your project's requirements, team skill set, and specific use cases. It's often a trade-off between control and complexity.

Conclusion

Both imperative and declarative IaC have their strengths and weaknesses. Choosing the right one can significantly influence the efficiency and reliability of your infrastructure management. Remember, the best tool is the one that fits your needs and helps you to deliver software faster and more reliably.