Autoscaling for a managed instance group automatically adjusts the number of VM instances based on the load, ensuring optimal resource utilization. Here’s how to enable autoscaling using GCP Console, gcloud CLI, and Terraform:

1. Using GCP Console:

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

b. Navigate to Compute Engine > Instance groups.

c. Click on the managed instance group you want to enable autoscaling for.

d. Click on the “Edit group” button at the top of the page.

e. Scroll down to the “Autoscaling” section and click on the “Turn on” button.

f. Configure the autoscaling settings, such as the scaling policy (CPU utilization, HTTP load balancing, or custom metric), target values, and minimum/maximum number of instances.

g. Click the “Save” button to enable autoscaling for the managed instance group.

2. Using gcloud CLI:

a. Run the following command to enable autoscaling for a managed instance group:

gcloud compute instance-groups managed set-autoscaling INSTANCE_GROUP_NAME \
    --max-num-replicas MAX_INSTANCES \
    --min-num-replicas MIN_INSTANCES \
    --zone ZONE \
    --target-cpu-utilization TARGET_CPU_UTILIZATION

Replace INSTANCE_GROUP_NAME, MAX_INSTANCES, MIN_INSTANCES, ZONE, and TARGET_CPU_UTILIZATION with appropriate values.

3. Using Terraform:

a. Modify your main.tf file to include the following resources:

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

resource "google_compute_instance_group_manager" "example" {
  name               = "example-instance-group"
  base_instance_name = "example-instance"
  zone               = "ZONE"
  version {
    instance_template = google_compute_instance_template.example.self_link
  }

  target_size  = "1"

  named_port {
    name = "http"
    port = "80"
  }

  autoscaling_policy {
    min_replicas = MIN_INSTANCES
    max_replicas = MAX_INSTANCES
    cpu_utilization {
      target = TARGET_CPU_UTILIZATION
    }
  }
}

Replace ZONE, MIN_INSTANCES, MAX_INSTANCES, and TARGET_CPU_UTILIZATION with appropriate values.

b. Run the following commands to apply the changes:

terraform init
terraform plan
terraform apply

This will enable autoscaling for the managed instance group based on the specified settings. The number of VM instances in the group will automatically adjust according to the load, ensuring optimal resource utilization.