# 💻 Day-4: Compute Engine in GCP (VMs, SSH, Images, Firewall)

## 🎯 Goal
To learn how to provision virtual machines in GCP, access them securely, run startup automation, install software like NGINX, and create reusable custom images — all through a DevOps lens.

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## 🧱 What is Compute Engine?

- Compute Engine is GCP’s **IaaS** (Infrastructure as a Service) that lets you launch **virtual machines** (VMs) on demand.
- VMs are customizable by CPU, RAM, OS, boot disk, and networking.
- Supports **automated provisioning**, **startup scripts**, **auto-healing**, and **custom images**.

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## ⚙️ Key Concepts

| Concept           | Description |
|------------------|-------------|
| **Instance**      | A VM (virtual server) that runs in a GCP zone |
| **Zone**          | A single data center in a GCP region |
| **Machine Type**  | Defines CPU, RAM (e.g., `e2-micro`, `n1-standard-1`) |
| **Boot Disk**     | Persistent disk attached to VM with OS |
| **Startup Script**| Script that runs when the VM starts |
| **Firewall Rule** | Controls allowed traffic to/from VM |

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## 🧪 Hands-On Lab

### 🛠️ Launch a VM and Connect via SSH

    gcloud compute instances create devops-vm \
      --zone=us-central1-a \
      --machine-type=e2-micro \
      --image-family=ubuntu-2004-lts \
      --image-project=ubuntu-os-cloud \
      --tags=http-server

    # SSH into the instance
    gcloud compute ssh devops-vm --zone=us-central1-a

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### 🛠️ Manually Install NGINX on Ubuntu VM

Once inside the VM via SSH, run the following:

    sudo apt update
    sudo apt install -y nginx
    sudo systemctl enable nginx
    sudo systemctl start nginx

To test:

    curl http://localhost

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### 🛠️ Install NGINX Automatically via Startup Script

    gcloud compute instances create nginx-vm \
      --zone=us-central1-a \
      --machine-type=e2-micro \
      --image-family=ubuntu-2004-lts \
      --image-project=ubuntu-os-cloud \
      --metadata startup-script='#! /bin/bash
    apt update
    apt install -y nginx
    systemctl enable nginx
    systemctl start nginx' \
      --tags=http-server

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### 🛠️ Enable Firewall Rule for HTTP Access

    gcloud compute firewall-rules create allow-http \
      --allow tcp:80 \
      --target-tags=http-server \
      --description="Allow HTTP traffic" \
      --direction=INGRESS \
      --priority=1000 \
      --network=default

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### 🛠️ Create a Custom Image from Existing VM

    # Stop the VM before creating image
    gcloud compute instances stop nginx-vm --zone=us-central1-a

    # Create image from disk
    gcloud compute images create nginx-custom-image \
      --source-disk=nginx-vm \
      --source-disk-zone=us-central1-a

    # Launch a new VM from custom image
    gcloud compute instances create nginx-from-image \
      --zone=us-central1-a \
      --machine-type=e2-micro \
      --image=nginx-custom-image \
      --tags=http-server

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## 🧠 Real-World DevOps Use Case

> You want to:
> - Automatically install NGINX during VM creation
> - Expose the app to the internet
> - Reuse that base image in CI/CD pipelines

✅ Use **startup scripts** for provisioning  
✅ Enable **firewall rules** for public access  
✅ Use **custom images** to standardize deployments

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## ✅ Key Takeaways

- Compute Engine is GCP’s IaaS offering for running VMs.
- Use **startup scripts** for configuration-as-code.
- Create **custom images** to reuse standardized OS setups.
- Don’t forget to set **firewall rules** for external access.
- DevOps Engineers should automate VM creation using CLI or Terraform.

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