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Merge pull request #1919 from terraform-providers/paddy_image_docs

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Add docs about image families and instance templates.
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website/docs/r/compute_instance_template.html.markdown
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@ -108,6 +108,61 @@ With this setup Terraform generates a unique name for your Instance
Template and can then update the Instance Group manager without conflict before
destroying the previous Instance Template.
## Deploying the Latest Image
A common way to use instance templates and managed instance groups is to deploy the
latest image in a family, usually the latest build of your application. There are two
ways to do this in Terraform, and they have their pros and cons. The difference ends
up being in how "latest" is interpreted. You can either deploy the latest image available
when Terraform runs, or you can have each instance check what the latest image is when
it's being created, either as part of a scaling event or being rebuilt by the instance
group manager.
If you're not sure, we recommend deploying the latest image available when Terraform runs,
because this means all the instances in your group will be based on the same image, always,
and means that no upgrades or changes to your instances happen outside of a `terraform apply`.
You can achieve this by using the [`google_compute_image`](../d/datasource_compute_image.html)
data source, which will retrieve the latest image on every `terraform apply`, and will update
the template to use that specific image:
```tf
data "google_compute_image" "my_image" {
family = "debian-9"
project = "debian-cloud"
}
resource "google_compute_instance_template" "instance_template" {
name_prefix = "instance-template-"
machine_type = "n1-standard-1"
region = "us-central1"
// boot disk
disk {
initialize_params {
image = "${data.google_compute_image.my_image.self_link}"
}
}
}
```
To have instances update to the latest on every scaling event or instance re-creation,
use the family as the image for the disk, and it will use GCP's default behavior, setting
the image for the template to the family:
```tf
resource "google_compute_instance_template" "instance_template" {
name_prefix = "instance-template-"
machine_type = "n1-standard-1"
region = "us-central1"
// boot disk
disk {
initialize_params {
image = "debian-cloud/debian-9"
}
}
}
```
## Argument Reference