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This page describes when and why to create deterministic instance
templates. Deterministic instance templates make explicitly clear
the type of third-party services or apps to install on
your instances when the instance template is deployed. By creating deterministic
instance templates, you minimize ambiguity and unexpected behavior from your
instance templates.
Why create deterministic instance templates
In general, we recommend that the properties of your instance template be as
explicit and deterministic as possible. If you employ startup
scripts in your instance templates that install or use third-party services,
make sure that these scripts provide explicit information, such as the
version of app to install. Compute Engine can only rely on
information defined in the template and has no control over referenced
third-party services. If your template is too vague, your instance template
might behave unexpectedly.
For example, consider the following command to create an instance template with
a startup script that installs apache2 and uses a file that is hosted on an
external server:
There are two potential issues with this startup script:
The script does not explicitly define which version of apache2 to install,
and relies on the current version available in theaptrepository.
The script relies on a file hosted on a third-party that isn't versioned and
could have been changed since the last time the instance template was used.
If you use anautoscaler, a non-deterministic
instance template can cause your autoscaler to add new instances to amanaged instance groupwith a different
configuration, such as a different version of apache2.
Similarly, if you applied this template to a managed instance group,updated
the group to a different templateservice, and then decided to roll back to the previous template, you might end
up with instances that use a different version of apache2 or index.php file
than before the update because your instances would always fetch the most recent
version at startup.
Avoiding ambiguous or unexpected instance template behavior
To avoid unexpected template behavior, use the following methods:
Usecontainer-optimized imagesor
Docker, with Docker tags. For example, we recommend that you assign new tags
for every new build of your Docker image, and use these tags in your instance
templates instead of the default latest tag. For a container-optimized image,
you can explicitly reference a particular build of your image in your manifest
file. The example below uses Docker image "myimage" at version tagged with
"version_2_1_3":
Create a custom imageto use as the image for the template. This is preferable to startup scripts
because it guarantees that every instance is the same. Startup scripts might
have different results after distribution package updates. Use startup scripts
in your instance templates for prototyping and rapid development, and use
custom images when you are ready to deploy production-quality services.
If you do use startup scripts, consider updating your scripts to be
deterministic. For example, create a new version of the previous template, and
specify a deterministic startup script as follows:
where "version_2_1_3" is a subdirectory containing PHP scripts for the
version 2.1.3 of your service.
When specifying an instance template—for example when you are creating or
updating a managed instance group—Google recommends that you specify the
template's ID value instead of its name value. Although both values are valid,
the ID is unique, which means that the instance template that you specify is
the one that is used when creating VMs from that instance template. Using an
ID instead of a name helps to mitigate potential security vulnerabilities—for
example,TOCTOUvulnerabilities, where an attacker can delete a template and recreate it with
the same name prior to its use.
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-09-02 UTC."],[[["\u003cp\u003eDeterministic instance templates ensure clarity on the third-party services or apps installed on instances during deployment, minimizing ambiguity.\u003c/p\u003e\n"],["\u003cp\u003eUsing explicit versioning in startup scripts, such as specifying the exact version of Apache2 to install, helps avoid unexpected behavior due to changes in third-party services.\u003c/p\u003e\n"],["\u003cp\u003eNon-deterministic templates can cause autoscalers to deploy instances with varying configurations, such as different software versions, potentially leading to inconsistencies.\u003c/p\u003e\n"],["\u003cp\u003eTo maintain consistency and predictability, use container-optimized images with specific Docker tags, or create custom images instead of relying heavily on startup scripts.\u003c/p\u003e\n"],["\u003cp\u003eWhen using startup scripts, make them deterministic by specifying exact versions of software and files, like \u003ccode\u003eapache2=2.2.20-1ubuntu1\u003c/code\u003e, and versioned directories for hosted files.\u003c/p\u003e\n"]]],[],null,["# Deterministic instance templates\n\n*** ** * ** ***\n\nThis page describes when and why to create deterministic instance\ntemplates. Deterministic instance templates make explicitly clear\nthe type of third-party services or apps to install on\nyour instances when the instance template is deployed. By creating deterministic\ninstance templates, you minimize ambiguity and unexpected behavior from your\ninstance templates.\n\nWhy create deterministic instance templates\n-------------------------------------------\n\nIn general, we recommend that the properties of your instance template be as\nexplicit and deterministic as possible. If you employ startup\nscripts in your instance templates that install or use third-party services,\nmake sure that these scripts provide explicit information, such as the\nversion of app to install. Compute Engine can only rely on\ninformation defined in the template and has no control over referenced\nthird-party services. If your template is too vague, your instance template\nmight behave unexpectedly.\n\nFor example, consider the following command to create an instance template with\na startup script that installs apache2 and uses a file that is hosted on an\nexternal server:\n**Note:** This is just an example snippet pointing to a non-existent server at 108.59.87.185. Copying this example directly fails when the script attempts to connect to 108.59.87.185. Instead, replace the last line with your own server information, if applicable. \n\n gcloud compute instance-templates create example-template-with-startup \\\n --image-family debian-9 \\\n --image-project debian-cloud \\\n --metadata startup-script='#! /bin/bash\n sudo apt install -y apache2\n scp myuser@108.59.87.185:index.php /var/www/'\n\nThere are two potential issues with this startup script:\n\n- The script does not explicitly define which version of apache2 to install, and relies on the current version available in the `apt` repository.\n- The script relies on a file hosted on a third-party that isn't versioned and could have been changed since the last time the instance template was used.\n\nIf you use an [autoscaler](/compute/docs/autoscaler), a non-deterministic\ninstance template can cause your autoscaler to add new instances to a\n[managed instance group](/compute/docs/instance-groups) with a different\nconfiguration, such as a different version of apache2.\n\nSimilarly, if you applied this template to a managed instance group, [updated\nthe group to a different template](/compute/docs/instance-groups/updating-migs)\nservice, and then decided to roll back to the previous template, you might end\nup with instances that use a different version of apache2 or index.php file\nthan before the update because your instances would always fetch the most recent\nversion at startup.\n\nAvoiding ambiguous or unexpected instance template behavior\n-----------------------------------------------------------\n\nTo avoid unexpected template behavior, use the following methods:\n\n- Use [container-optimized images](/compute/docs/containers/container_vms) or\n Docker, with Docker tags. For example, we recommend that you assign new tags\n for every new build of your Docker image, and use these tags in your instance\n templates instead of the default latest tag. For a container-optimized image,\n you can explicitly reference a particular build of your image in your manifest\n file. The example below uses Docker image \"myimage\" at version tagged with\n \"version_2_1_3\":\n\n version: v1beta2\n containers:\n - name: simple-echo\n image: myimage:version_2_1_3\n [ rest of your manifest file ]\n\n- [Create a custom image](/compute/docs/images/create-delete-deprecate-private-images)\n to use as the image for the template. This is preferable to startup scripts\n because it guarantees that every instance is the same. Startup scripts might\n have different results after distribution package updates. Use startup scripts\n in your instance templates for prototyping and rapid development, and use\n custom images when you are ready to deploy production-quality services.\n\n- If you do use startup scripts, consider updating your scripts to be\n deterministic. For example, create a new version of the previous template, and\n specify a deterministic startup script as follows:\n\n gcloud compute instance-templates create example-template-with-startup-2-1-3 \\\n --image-family debian-9 \\\n --image-project debian-cloud \\\n --metadata startup-script='#! /bin/bash\n sudo apt install -y apache2=2.2.20-1ubuntu1\n scp myuser@108.59.87.185:version_2_1_3/index.php /var/www/'\n\n where \"version_2_1_3\" is a subdirectory containing PHP scripts for the\n version 2.1.3 of your service.\n- When specifying an instance template---for example when you are creating or\n updating a managed instance group---Google recommends that you specify the\n template's ID value instead of its name value. Although both values are valid,\n the ID is unique, which means that the instance template that you specify is\n the one that is used when creating VMs from that instance template. Using an\n ID instead of a name helps to mitigate potential security vulnerabilities---for\n example,\n [TOCTOU](https://en.wikipedia.org/wiki/Time_of_check_to_time_of_use_vulnerability)\n vulnerabilities, where an attacker can delete a template and recreate it with\n the same name prior to its use.\n\n To view the ID of an instance template, see\n [Get information about an instance template](/compute/docs/instance-templates/get-list-delete-instance-templates#get_information_about_an_instance_template).\n\nWhat's next\n-----------\n\n- [Create an instance template](/compute/docs/instance-templates/create-instance-templates).\n- [Manage your instance templates](/compute/docs/instance-templates/get-list-delete-instance-templates)."]]
