Immutable Infrastructure

Explain immutable infrastructure and its benefits. How does it differ from traditional server management?

Infrastructureintermediate

// interview question

Explain immutable infrastructure and its benefits. How does it differ from traditional server management?

Sample answer

Immutable infrastructure means never modifying running instances post-deployment. Instead of patching servers, you build new machine images with updates, deploy fresh instances, and destroy old ones. Benefits include: no configuration drift, consistent environments, easy rollbacks (previous image), simpler debugging (fewer variables), and better security (no SSH access needed).

Why this matters

The key principle is 'pets vs cattle' - treating infrastructure as disposable rather than precious. Traditional approaches SSH into servers and run updates, leading to drift over time where servers become unique snowflakes. Immutable infrastructure ensures every deployment is identical and reproducible.

Code examples

Packer AMI build

hcl

source "amazon-ebs" "app" {
  ami_name      = "app-{{timestamp}}"
  instance_type = "t3.small"
  region        = "us-east-1"
  source_ami_filter {
    filters = {
      name = "ubuntu/images/*ubuntu-jammy-22.04-amd64-server-*"
    }
    most_recent = true
    owners      = ["099720109477"]
  }
  ssh_username = "ubuntu"
}

build {
  sources = ["source.amazon-ebs.app"]

  provisioner "shell" {
    scripts = ["scripts/install_deps.sh"]
  }
}

Blue-green with new AMI

hcl

resource "aws_launch_template" "app" {
  name_prefix   = "app-"
  image_id      = var.ami_id  # New AMI for each release
  instance_type = "t3.small"

  lifecycle {
    create_before_destroy = true
  }
}

# ASG automatically replaces instances with new AMI
resource "aws_autoscaling_group" "app" {
  launch_template {
    id      = aws_launch_template.app.id
    version = "$Latest"
  }
  instance_refresh {
    strategy = "Rolling"
  }
}

Common mistakes to avoid

SSH-ing into production servers to make 'quick fixes'
Not versioning machine images properly
Building images too slowly, making deployments take too long

Likely follow-ups

How do you handle stateful applications with immutable infrastructure?
What tools do you use to build machine images?
How do you manage secrets in an immutable infrastructure setup?

Answer out loud first, then check yourself against the model answer.

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