This document describes what an instance template is, when to use an instance template, and what a deterministic instance template is. It also provides a comparison between regional and global instance templates.
An instance template is a convenient way to save a virtual machine (VM) instance's configuration that includes machine type, boot disk image, labels, startup script, and other VM properties.
You can use an instance template to do the following:
Create individual VMs.
Create VMs in a managed instance group (MIG).
Create reservations for VMs.
Create future reservations for VMs.
To learn how to create an instance template, see Create instance templates.
When to use instance templates
Use instance templates any time you want to quickly create VMs or reservations for VMs based off of a pre-existing VM property. If you want to create a group of identical VMs (a MIG), then you must create the MIG using an instance template.
How to update instance templates
Instance templates are designed to create VMs with identical configurations. You can't update instance templates after they're created. Instead, do one of the following:
Create a new instance template as follows:
Create VMs while overriding the instance template's properties.
Use deterministic instance templates to ensure identical VMs
Deterministic instance templates make explicitly clear the type of third-party services or apps to install on your VMs. This helps to ensure that your instance template always creates VMs with an identical configuration. For example, if your template has a startup script that fetches an app, you can specify the version of the app that you want in your template's startup script.
For more information, see Deterministic instance templates.
Regional and global instance templates
Instance templates are available both as regional and global resources. Unless you need to reuse an instance template across multiple regions, Google recommends using regional instance templates over global instance templates.
The following table provides a comparison between regional and global instance templates:
| Regional instance template | Global instance template | |
|---|---|---|
| Scope | You can use the template only in the template's region. | You can use the template in any region. |
| Reliability | Hardware errors are isolated to the template's region. | Hardware errors can impact any region where the template is used. |
| Use case |
|
Reuse your global instance template to create VMs, MIGs, and reservations across multiple regions. |
Use of zonal or regional resources in instance templates
In an instance template, you might specify zonal resources, which restricts the
use of that template to the zone where that resource resides. Similarly, if you
specify a regional resource in a global instance template, the template is
restricted to that region. For example, if you include a read-only Persistent Disk
from us-central1-a in your instance template, you can't use that template in
any other zone because that specific Persistent Disk exists only in zone
us-central1-a.
For more information about zonal resources, see Regions and zones.
Pricing
There is no additional charge for using instance templates. You are charged for the resources that you create based on the templates.
For Compute Engine pricing information, see Pricing.What's next
Create an instance template from scratch, based on an existing instance, or based on an existing instance template.
Use an instance template to create a VM.
Use an instance template to create a MIG.
Use an instance template to create a reservation for VMs.