Custom images in Compute Engine can be useful for various use cases, including:

1. Pre-installed software: Custom images can have pre-installed software or configurations, reducing the setup time and ensuring consistent deployments.
2. Consistent environments: Custom images help maintain consistent environments across development, staging, and production.
3. Faster deployment: Custom images can speed up the instance creation process by having the required dependencies and configurations already in place.
4. Template for similar instances: Custom images can be used as a template for creating multiple instances with the same configuration.

Here’s how to create and use a custom image using GCP Console, gcloud CLI, and Terraform:

1. Using GCP Console:

Create a custom image:

a. Go to the GCP Console: https://console.cloud.google.com/

b. Navigate to Compute Engine > Images.

c. Click on the “Create Image” button.

d. Enter a name and description for the image, and select the source disk or snapshot.

e. Configure any additional settings, such as encryption or location, and click the “Create” button.

Use the custom image to create a new instance:

a. Navigate to Compute Engine > VM instances.

b. Click on the “Create instance” button.

c. Fill in the required fields, and under “Boot disk,” click on the “Change” button.

d. In the “Custom images” tab, select the custom image you created earlier and click the “Select” button.

e. Finish configuring the instance and click the “Create” button.

2. Using gcloud CLI:

Create a custom image:

a. First, stop the instance if it’s running:

gcloud compute instances stop INSTANCE_NAME --zone ZONE

Replace INSTANCE_NAME and ZONE with appropriate values.

b. Create a custom image from the instance’s boot disk:

gcloud compute images create CUSTOM_IMAGE_NAME --source-disk INSTANCE_NAME --source-disk-zone ZONE --family FAMILY_NAME

Replace CUSTOM_IMAGE_NAME, INSTANCE_NAME, ZONE, and FAMILY_NAME with appropriate values.

Use the custom image to create a new instance:

gcloud compute instances create NEW_INSTANCE_NAME --image-family FAMILY_NAME --image-project PROJECT_ID --zone ZONE

Replace NEW_INSTANCE_NAME, FAMILY_NAME, PROJECT_ID, and ZONE with appropriate values.

3. Using Terraform:

Create a custom image:

Terraform does not provide a direct way to create a custom image from an existing instance. However, you can create a custom image from a disk snapshot:

resource "google_compute_snapshot" "example" {
  # ... existing configuration ...
}

resource "google_compute_image" "example" {
  name        = "example-image"
  source_snapshot = google_compute_snapshot.example.self_link
  family      = "FAMILY_NAME"
}

Replace FAMILY_NAME with an appropriate value.

Use the custom image to create a new instance:

resource "google_compute_instance" "example" {
  # ... existing configuration ...

  boot_disk {
    initialize_params {
      image = "projects/PROJECT_ID/global/images/family/FAMILY_NAME"
    }
  }
}

Replace PROJECT_ID and FAMILY_NAME with appropriate values.

b. Run the following commands to apply the changes:

terraform init
terraform plan
terraform apply

This will create a new Compute Engine instance using the custom image.