Building Immutable Infrastructure Without Containers

December 8, 2024 - 4 minutes read - 832 words

Immutable infrastructure is a concept that has revolutionized the way we think about deploying and maintaining systems. It ensures that once an instance of infrastructure is created, it is never modified. Instead, any changes require the creation of a new instance. This approach enhances consistency, reduces configuration drift, and simplifies rollback processes. While immutable infrastructure is often associated with Infrastructure as Code (IaC) and containers, it’s entirely possible—and sometimes necessary—to implement it with Virtual Machines (VMs) instead of containers.

Here I use chatgtp generate how to build immutable infrastructure using VMs, detailing the principles, tools, and practices needed to achieve container-like immutability.

Understanding Immutable Infrastructure

Immutable infrastructure is based on the idea of creating systems that cannot be altered after deployment. If updates or patches are needed, a new instance of the infrastructure is built and deployed, replacing the old one. The key benefits of this approach include:

Consistency: Eliminates configuration drift as changes occur in new builds rather than in-place. Reliability: Ensures predictable environments by avoiding "snowflake" servers with unique configurations. Simplified Rollbacks: Reverting to a previous version is as simple as redeploying the last stable build.

Challenges of Implementing Immutability with VMs

Containers inherently align with immutability because they package applications with their dependencies in isolated environments. VMs, however, present unique challenges:

Size: VMs are typically larger than containers, making build and deployment times longer.
State Management: Traditional VMs often store state on the instance, which contradicts the immutability principle.
Configuration Drift: Without strict controls, manual changes or updates can lead to inconsistencies. Despite these challenges, with the right tools and strategies, VMs can be made immutable.

Steps to Build Immutable VMs

1. Adopt Infrastructure as Code (IaC)

IaC is the backbone of immutable infrastructure. Tools like Terraform, Pulumi, or AWS CloudFormation allow you to define your infrastructure declaratively. The key is to ensure that every VM instance is provisioned based on a well-defined configuration.

Example: A Terraform configuration that provisions a VM:

resource "aws_instance" "web" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"

  tags = {
    Name = "Immutable-Web-Server"
  }
}

This configuration can be versioned, ensuring reproducibility.

2. Bake Immutable VM Images

Instead of configuring VMs post-launch, create fully-configured VM images that are ready to run. Tools like Packer by HashiCorp enable you to build machine images with all necessary software and configurations baked in.

Steps to create an immutable image with Packer:

Define a base image (e.g., an Ubuntu AMI for AWS).
Automate software installation and configuration during the image build.
Output the image as an artifact to be used during deployment.

Packer template example:

{
  "builders": [
    {
      "type": "amazon-ebs",
      "region": "us-west-2",
      "source_ami": "ami-12345678",
      "instance_type": "t2.micro",
      "ssh_username": "ubuntu",
      "ami_name": "immutable-web-server-{{timestamp}}"
    }
  ],
  "provisioners": [
    {
      "type": "shell",
      "inline": [
        "sudo apt-get update",
        "sudo apt-get install -y nginx",
        "sudo systemctl enable nginx"
      ]
    }
  ]
}

The output is a VM image with all necessary software and configurations applied, ready for deployment.

3. Use Stateless VM Design

For VMs to be truly immutable, they must not retain state. State should be externalized to databases, object storage, or other persistent layers. Avoid writing directly to local disks for anything that needs to persist beyond the VM’s lifecycle.

Databases: Use managed services like Amazon RDS or Azure SQL.
File Storage: Offload to S3, Azure Blob Storage, or similar services.
Session State: Store in Redis, Memcached, or other caching layers.

4. Automate Deployment and Rollbacks

Automate the deployment of your VMs using tools like Ansible, Chef, or cloud-native deployment pipelines (e.g., AWS CodePipeline, Azure DevOps). Treat deployments as ephemeral, replacing VMs entirely rather than updating them.

Blue-Green Deployment: Spin up a new version of the VM and redirect traffic to it after validation, decommissioning the old version.

Canary Deployment: Gradually route traffic to new VMs to monitor performance and stability.

5. Enforce Immutable Policies

To ensure immutability in practice:

Disable SSH Access: Prevent manual changes by disallowing SSH access.
Use Read-Only File Systems: Limit write permissions on the VM to prevent modifications.
Monitor Drift: Use tools like AWS Config, HashiCorp Sentinel, or custom scripts to detect and prevent configuration drift.

6. Implement Continuous Image Building

Regularly update and rebuild VM images to incorporate patches and updates. A CI/CD pipeline can automate this process.

Example CI/CD flow:

Trigger: A new application version or security update.
Build: Use Packer to bake a new VM image.
Test: Validate the image in a staging environment.
Deploy: Replace existing VMs with new ones using IaC.

Conclusion

While containers are a natural fit for immutable infrastructure, VMs can also be made immutable with thoughtful design and the right tooling. By leveraging IaC, baking VM images, externalizing state, and automating deployment, you can achieve many of the benefits of immutable infrastructure without adopting containers.

This approach is particularly valuable in environments where containers are not feasible due to technical, organisational, or compliance reasons. With VMs as immutable building blocks, you can enhance reliability, maintain consistency, and simplify infrastructure management—bringing you one step closer to a modern, resilient architecture.