Automatically rolling out updates to instances in a MIG

This document describes how to automatically apply a new instance template to all or to a random subset of the virtual machine (VM) instances in a managed instance group (MIG).

When you configure an automated update, the MIG rolls out a new version of an instance template automatically to your specifications. You can control the speed of deployment, the level of disruption to your service, and the number of instances that the MIG updates. After you start the update, you do not need to provide additional input and the update completes on its own.

If you want to selectively apply a new template only to new or to specific instances in a MIG, or if you have a stateful MIG and need to apply stateful per-instance configurations, see Selectively updating instances in a MIG instead. To help you decide, see Choosing between automated and selective updates.

Before you begin

• If you want to use the API examples in this guide, set up API access.
• If you want to use the command-line examples in this guide, install the gcloud command-line tool.
• Create an instance template.

Limitations

• You cannot use an automated rolling update if your MIG has any stateful configuration. You can control updates and limit disruption by updating specific instances instead.
• If you use custom instance names and don't configure stateful disks or metadata, you can use automated updates. But, to preserve instance names, you must set the replacement method to RECREATE.

Starting a basic rolling update

A basic rolling update is an update that is gradually applied to all instances in a MIG until all instances have been updated. You can control various aspects of a rolling update, such as how many instances can be taken offline for the update, how long to wait between updating instances, whether the update affects all or just a portion of instances, and so on.

Here are things to keep in mind when making a rolling update:

• Updates are intent based. When you make the initial update request, the Compute Engine API returns a successful response to confirm that the request is valid, but that doesn't indicate that the update succeeded. You must check the status of the group to determine whether your update was deployed successfully.

• The Instance Group Updater API is a declarative API. The API expects a request to specify the desired post-update configuration of the MIG, rather than an explicit function call.

• Automated updates support up to two instance template versions in your MIG. This means that you can specify two different instance template versions for your group, which is useful for performing canary updates.

To start a basic rolling update where the update is applied to 100% of the instances in the group, follow the instructions below.

Permissions required for this task

To perform this task, you must have the following permissions:

• For zonal MIGs, all permissions required to call the instanceGroupManagers.patch method
• For regional MIGs, all permissions required to call the regionInstanceGroupManagers.patch method

Console

1. In the Cloud Console, go to the Instance groups page.

   Go to Instance groups

2. Select the MIG that you want to update.

3. At the top of the page, click Rolling update.

4. Under Template, click the drop down list and select the new template to update to.

5. For the target size, enter 100% to update all instances.

6. Under Update mode, select Proactive.

7. Click Update to start the update.

gcloud

Use the rolling-action start-update command.

gcloud compute instance-groups managed rolling-action start-update INSTANCE_GROUP_NAME \
    --version=template=INSTANCE_TEMPLATE_NAME
    [--zone=ZONE | --region=REGION]

Replace the following:

• INSTANCE_GROUP_NAME: the name of the MIG
• INSTANCE_TEMPLATE_NAME: the new instance template
• ZONE: for zonal MIGs, provide the zone
• REGION: for regional MIGs, provide the region

API

Call the patch method on a regional or zonal MIG resource.

For example, for a regional MIG, the following request shows the minimal configuration necessary for automatically updating 100% of the instances to the new instance template.

PATCH https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/instanceGroupManagers/INSTANCE_GROUP_NAME

{
  "instanceTemplate": "global/instanceTemplates/NEW_TEMPLATE",
  "updatePolicy": {
    "type": "PROACTIVE"
   }
}

After you make a request, you can monitor the update to know when the update has finished.

For advanced configurations, include other update options. If you don't specify otherwise, the maxSurge and maxUnavailable options default to 1 multiplied by the number of affected zones. This means that only 1 instance is taken offline in each affected zone, and the MIG creates only 1 additional instance per zone, during the update.

Configuring options for your update

For more complex updates, you can configure additional options. These options are described in the following sections.

Mode

For automated rolling updates, you must set the mode to proactive.

Alternatively, if an automated update is potentially too disruptive, you can choose to perform an opportunistic update. The MIG applies an opportunistic update only when you manually initiate the update on selected instances or when new instances are created. New instances can be created when you or another service, such as an autoscaler, resizes the MIG. Compute Engine does not actively initiate requests to apply opportunistic updates on existing instances.

For more information about automated versus selective updates, see Choosing between automated and selective updates.

Max surge

Use the maxSurge option to configure how many new instances the MIG can create above its targetSize during an automated update. For example, if you set maxSurge to 5, the MIG uses the new instance template to create up to 5 new instances above your target size. Setting a higher maxSurge value speeds up your update, at the cost of additional instances, which are billed according to the Compute Engine price sheet.

You can specify either a fixed number, or, if the group has 10 or more instances, a percentage. If you set a percentage, the Updater rounds up the number of instances if necessary.

If you don't set the maxSurge value, the default value is used. For zonal MIGs, the default for maxSurge is 1. For regional MIGs, the default is the number of zones associated with the group, by default 3.

maxSurge only works if you have enough quota or resources to support the additional resources.

This option is recognized only when configured with the REPLACE minimal action. It is not supported with the RESTART action setting.

Max unavailable

Use the maxUnavailable option to configure how many instances are unavailable at any time during an automated update. For example, if you set maxUnavailable to 5, then only 5 instances are taken offline for updating at a time. Use this option to control how disruptive the update is to your service and to control the rate at which the update is deployed.

This number also includes any instances that are unavailable for other reasons. For example, if the group is in the process of being resized, instances in the middle of being created might be unavailable. These instances count toward the maxUnavailable number.

You can specify a fixed number, or, if the group has 10 or more instances, a percentage. If you set a percentage, the Updater rounds down the number of instances, if necessary.

If you don't set the maxUnavailable value, the default value is used. For zonal MIGs, the default is 1. For regional MIGs, the default is the number of zones associated with the group, by default 3.

Note: If you set both maxSurge and maxUnavailable properties and both properties resolve to 0, the Updater automatically sets maxUnavailable=1, to ensure that the automated update can always proceed.

Minimum wait time

Note: minReadySec is only available in the beta Compute Engine API and might be deprecated in a future release.

Use the minReadySec option to specify the amount of time to wait before considering a new or restarted instance as updated. Use this option to control the rate at which the automated update is deployed. The timer starts when both of the following conditions are satisfied:

• The instance's status is RUNNING.
• If health checking is enabled, when the health check returns HEALTHY.

Note that for the health check to return healthy, the Updater waits for the following conditions:

1. Waits for up to the period of time specified by the MIG's autohealingPolicies.initialDelaySec value for the health check to return as HEALTHY.
2. Then, waits for the period of time specified by minReadySec.

If the health check doesn't return HEALTHY within the initialDelaySec, then the Updater declares the VM instance as unhealthy and potentially stops the update. While the VM instance is waiting for verification during the initialDelaySec and the minReadySec time period, the instance's currentAction is VERIFYING. However, the underlying VM instance status remains RUNNING.

If there are no health checks for the group, then the timer starts when the instance's status is RUNNING.

The maximum value for the minReadySec field is 3600 seconds (1 hour).

Minimal action

Use the minimal action option to specify the minimum action that an automated update must perform to update the instances in the group. For example, if you set REPLACE as the minimum action, then all instances that are affected are deleted and replaced with new instances, whether or not it is necessary.

Setting a minimal action guarantees that the Updater performs that action, at a minimum. However, if the Updater determines that the minimal action you specify is not enough to perform the update, it might perform a more disruptive action. For example, if you set RESTART as the minimal action, the Updater attempts to restart instances to apply the update. But, if you are changing the OS, which can't be done by restarting the instance, then the Updater replaces the instances in the group with new VM instances.

Possible actions are REPLACE or RESTART:

• RESTART. Restarts the instance by performing a stop and start request. If your update request requires that the instance be replaced to pick up the changes (for example, changing the image requires that the instance be deleted and replaced), the Updater performs a REPLACE.

• REPLACE. Deletes the existing instance and creates an instance from the target template. The Updater creates an instance with all new instance properties, such as new internal and external IP addresses.

The following diagram shows how these options affect your instances.

Replacement method

By default, when you proactively update a MIG, the group deletes your VM instances and swaps them with new instances with new names. If you need to preserve the names of your VM instances, use the replacementMethod option.

Preserving existing instance names might be useful if you have applications or systems that rely on using specific instance names. For example, some applications, like Memcached, rely on instance names because they don't have a discovery service; as a result, whenever an instance name changes, the application loses connection to that specific VM.

To preserve instance names, set the replacement method to RECREATE instead of SUBSTITUTE.

Valid replacementMethod values are:

• SUBSTITUTE (default). Replaces VM instances faster during updates because new VMs are created before old ones are shut down. However, instance names aren't preserved because the names are still in use by the old instances.

• RECREATE. Preserves instance names through an update. Compute Engine releases the instance name as the old VM is shut down. Then Compute Engine creates a new instance using that same name. To use this mode, you must set maxSurge to 0.

For more information, see Preserving instance names.

Additional update examples

Here are some command-line examples with common configuration options.

Perform a rolling update of all VM instances, but create up to 5 new instances above the target size at a time

gcloud compute instance-groups managed rolling-action start-update INSTANCE_GROUP_NAME \
    --version=template=NEW_TEMPLATE \
    --max-surge=5 \
    [--zone=ZONE | --region=REGION]

Perform a rolling update with at most 3 unavailable machines and a minimum wait time of 3 minutes before marking a new instance as available

gcloud compute instance-groups managed rolling-action start-update INSTANCE_GROUP_NAME \
    --version=template=NEW_TEMPLATE \
    --min-ready=3m \
    --max-unavailable=3 \
    [--zone=ZONE | --region=REGION]

Perform a rolling update of all VM instances, but create up to 10% new instances above the target size at a time

For example, if you have 1,000 instances and you run the following command, the Updater creates up to 100 instances before it starts to remove instances that are running the previous instance template.

gcloud compute instance-groups managed rolling-action start-update INSTANCE_GROUP_NAME \
    --version=template=NEW_TEMPLATE \
    --max-surge=10% \
    [--zone=ZONE | --region=REGION]

Canary updates

A canary update is an update that is applied to a subset of instances in the group. With a canary update, you can test new features or upgrades on a random subset of instances, instead of rolling out a potentially disruptive update to all your instances. If an update is not going well, you only need to roll back the subset of instances, minimizing the disruption for your users.

A canary update is the same as a standard rolling update, except that the number of instances that should be updated is less than the total size of the instance group. Like a standard rolling update, you can configure additional options to control the level of disruption to your service.

Starting a canary update

To initiate a canary update, specify up to two instance template versions, typically a new instance template to canary and the current instance template for the remainder of the instances. For example, you can specify that 20% of your instances be created based on NEW_INSTANCE_TEMPLATE while the rest of the instances continue to run on the OLD_INSTANCE_TEMPLATE. You can't specify more than two instance templates at a time.

You must always specify a target size (targetSize) for the canary version. You can't initiate a canary update if you omit the target size for the canary version. For example, if you specified that 10% of instances should be used for canarying, the remaining 90% are untouched and use the current instance template.

Permissions required for this task

To perform this task, you must have the following permissions:

• For zonal MIGs, all permissions required to call the instanceGroupManagers.patch method
• For regional MIGs, all permissions required to call the regionInstanceGroupManagers.patch method

Console

1. In the Cloud Console, go to the Instance groups page.

   Go to Instance groups

2. Select the managed instance group that you want to update.
3. At the top of the page, click Rolling update.
4. Click Add template and choose the new instance template to canary.
5. Under Target size, enter the percentage or fixed number of instances you want to use to canary the new instance template.
6. If you want, you can configure other update options.
7. Click Update to start the update.

gcloud

Use the rolling-action start-update command. Provide both the current template and the new template to explicitly express how many instances should use each template:

gcloud compute instance-groups managed rolling-action start-update INSTANCE_GROUP_NAME \
    --version=template=CURRENT_INSTANCE_TEMPLATE_NAME \
    --canary-version=template=NEW_TEMPLATE,target-size=SIZE \
    [--zone=ZONE | --region=REGION]

Replace the following:

• INSTANCE_GROUP_NAME: the instance group name.
• CURRENT_INSTANCE_TEMPLATE_NAME: the instance template that the instance group is currently running.
• NEW_TEMPLATE: the new template that you want to canary.
• SIZE: the number or percentage of instances that you want to apply this update to. You must apply the target-size property to the --canary-version template. You can only set a percentage if the group contains 10 or more instances.
• ZONE: for zonal MIGs, provide the zone.
• REGION: for regional MIGs, provide the region.

For example, the following command performs a canary update that rolls out example-template-B to 10% of instances in the group:

gcloud compute instance-groups managed rolling-action start-update example-mig \
    --version=emplate=example-template-A \
    --canary-version=template=example-template-B,target-size=10%

API

Call the patch method on a regional or zonal MIG resource. In the request body, include both the current instance template and the new instance template that you want to canary. For example:

PATCH https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/instanceGroupManagers/INSTANCE_GROUP_NAME

{
 "versions": [
  {
   "instanceTemplate": "global/instanceTemplates/NEW_TEMPLATE",
   "targetSize": {
    "[percent|fixed]": NUMBER|PERCENTAGE # Use `fixed` for a specific number of instances
   }
  },
  {
   "instanceTemplate": "global/instanceTemplates/CURRENT_INSTANCE_TEMPLATE_NAME"
  }
 ]
}

Replace the following:

• NEW_TEMPLATE: the name of the new template you want to canary.
• NUMBER|PERCENTAGE: the fixed number or percentage of instances to canary this update. You can only set a percentage if the group contains 10 or more instances. Otherwise, provide a fixed number.
• CURRENT_INSTANCE_TEMPLATE_NAME: the name of the current instance template that the group is running.

After you make a request, you can monitor the update to know when the update has finished.

Rolling forward a canary update

After running a canary update, you can decide whether you want to commit the update to 100% of the MIG or roll it back.

Console

1. In the Cloud Console, go to the Instance groups page.

   Go to Instance groups

2. Select the managed instance group that you want to update.
3. At the top of the page, click Rolling update.
4. Under Template, update the target size of the canary template to 100% to roll forward the template to all your instances. Alternatively, you can replace the primary template with the canary template and set the target size to 100%. Then, you can remove the second template field altogether.
5. Click Update to start the update.

gcloud

If you want to commit to your canary update, roll forward the update by issuing another rolling-action start-update command but set only the version flag and omit the --canary-version flag.

gcloud compute instance-groups managed rolling-action start-update INSTANCE_GROUP_NAME \
    --version=template=NEW_TEMPLATE \
    [--zone=ZONE | --region=REGION]

API

Call the patch method on a regional or zonal MIG resource. In the request body, specify the new instance template as a version and omit the earlier instance template from your request body. Omit the target size specification to roll out the update to 100% of instances. For example:

PATCH https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/instanceGroupManagers/INSTANCE_GROUP_NAME

{
"versions": [
   {
   "instanceTemplate": "global/instanceTemplates/NEW_TEMPLATE" # New instance template
   }
 ]
}

Monitoring updates

After you initiate an update, it can take some time for the update to finish rolling out to all affected instances. You can monitor the progress of an update by inspecting the group status or by checking the current actions on instances.

Group status

At the group level, Compute Engine populates a read-only field called status that contains a versionTarget.isReached flag flag and an isStable flag. You can use the gcloud tool or the Compute Engine API to access these flags. You can also use the Cloud Console to see the current and planned number of instances being updated.

Permissions required for this task

To perform this task, you must have the following permissions:

• compute.instanceGroupManagers.get on the MIG

Console

You can monitor a rolling update for a group by going to the group's details page.

1. In the Cloud Console, go to the Instance groups page.

   Go to Instance groups

2. Select the managed instance group that you want to monitor. The overview page for the group shows the template that each instance is using.
3. To view the details, click the Details tab.
4. Under Instance template, you can see the current and target number of instances for each instance template, as well as the update parameters.

gcloud

Use the describe command.

gcloud compute instance-groups managed describe INSTANCE_GROUP_NAME \
    [--zone=ZONE | --region=REGION]

You can also use the gcloud compute instance-groups managed wait-until command with the --version-target-reached flag to wait until status.versionTarget.isReached is set to true for the group:

gcloud compute instance-groups managed wait-until INSTANCE_GROUP_NAME \
    --version-target-reached \
    [--zone=ZONE | --region=REGION]

API

Call the get method on a regional or zonal MIG resource.

GET https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/instanceGroupManagers/INSTANCE_GROUP_NAME/get

Verifying whether an update rollout is complete

Verify whether the rollout of an update is complete by checking the value of the MIG's status.versionTarget.isReached field:

status.versionTarget.isReached set to true indicates that all VM instances have been or are being created using the target version.

status.versionTarget.isReached set to false indidates that at least one VM is not yet using the target version. Or, in the case of a canary update, false indicates that the number of VMs using a target version doesn't match its target size.

Note: To verify that the update rollout is complete and also that all VM instances in the group are running and healthy, check both the status.isStable and status.versionTarget.isReached flags.

Checking whether a managed instance group is stable

Verify that all instances in a managed instance group are running and healthy by checking the value of the group's status.isStable field.

status.isStable set to false indicates that changes are active, pending, or that the MIG itself is being modified.

status.isStable set to true indicates the following:

• None of the instances in the MIG are undergoing any type of change and the currentAction for all instances is NONE.
• No changes are pending for instances in the MIG.
• The MIG itself is not being modified.

Remember that the stability of a MIG depends on numerous factors because a MIG can be modified in numerous ways. For example:

• You make a request to roll out a new instance template.
• You make a request to create, delete, resize or update instances in the MIG.
• An autoscaler requests to resize the MIG.
• An autohealer resource is replacing one or more unhealthy instances in the MIG.
• In a regional MIG, some of the instances are being redistributed.

As soon as all actions are finished, status.isStable is set to true again for that MIG.

Current actions on instances

You can see the currentAction being performed and the status of each instance in a managed instance group with the gcloud command-line tool or the Compute Engine API.

Permissions required for this task

To perform this task, you must have the following permissions:

• compute.instanceGroupManagers.get on the group
• compute.instanceGroupManagers.list on the group

gcloud

gcloud compute instance-groups managed list-instances INSTANCE_GROUP_NAME \
    [--zone=ZONE | --region=REGION]

The command returns a list of instances in the MIG and their statuses and current actions. For example:

NAME               ZONE           STATUS   HEALTH_STATE  ACTION  INSTANCE_TEMPLATE  VERSION_NAME  LAST_ERROR
vm-instances-9pk4  us-central1-f                          CREATING  my-new-template
vm-instances-h2r1  us-central1-f  STOPPING                DELETING  my-old-template
vm-instances-j1h8  us-central1-f  RUNNING                 NONE      my-old-template
vm-instances-ngod  us-central1-f  RUNNING                 NONE      my-old-template

The HEALTH_STATE column appears empty unless you have set up health checking.

API

Call the listManagedInstances method on a regional or zonal MIG resource. For example:

GET https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/zones/ZONE/instanceGroupManagers/INSTANCE_GROUP_NAME/listManagedInstances

The call returns a list of instances for the MIG including each instance's instanceStatus and currentAction.

{
 "managedInstances": [
  {
   "instance": "https://www.googleapis.com/compute/v1/projects/example-project/zones/us-central1-f/instances/vm-instances-prvp",
   "id": "5317605642920955957",
   "instanceStatus": "RUNNING",
   "instanceTemplate": "https://www.googleapis.com/compute/v1/projects/example-project/global/instanceTemplates/example-template",
   "currentAction": "REFRESHING"
  },
  {
   "instance": "https://www.googleapis.com/compute/v1/projects/example-project/zones/us-central1-f/instances/vm-instances-pz5j",
   "currentAction": "DELETING"
  },
  {
   "instance": "https://www.googleapis.com/compute/v1/projects/example-project/zones/us-central1-f/instances/vm-instances-w2t5",
   "id": "2800161036826218547",
   "instanceStatus": "RUNNING",
   "instanceTemplate": "https://www.googleapis.com/compute/v1/projects/example-project/global/instanceTemplates/example-template",
   "currentAction": "REFRESHING"
  }
 ]
}

To see a list of valid instanceStatus field values, see Checking an instance's status.

If an instance is undergoing some type of change, its currentAction field is populated with one of the following actions to help you track the progress of the change. Otherwise, the currentAction field is NONE.

Possible currentAction values are:

• ABANDONING. The instance is being removed from the MIG.
• CREATING. The instance is in the process of being created.
• CREATING_WITHOUT_RETRIES. The instance is being created without retries; if the instance isn't created on the first try, the MIG doesn't try to replace the instance again.
• DELETING. The instance is in the process of being deleted.
• RECREATING. The instance is being replaced.
• REFRESHING. The instance is being removed from its current target pools and being readded to the list of current target pools (this list might be the same or different from existing target pools).
• RESTARTING. The instance is in the process of being restarted using the stop and start methods.
• VERIFYING. The instance has been created and is in the process of being verified.
• NONE. No actions are being performed on the instance.

Note: The pendingActions field is deprecated and is to be removed. Contact [e-mail] to leave feedback if your workload relies on this field.

Rolling back an update

There is no explicit command for rolling back an update to a previous version, but if you decide to roll back an update (either a fully committed update or a canary update), you can do so by making a new update request and passing in the instance template that you want to roll back to.

gcloud

For example, the following gcloud tool command rolls back an update as fast as possible. Replace OLD_INSTANCE_TEMPLATE with the name of the instance template you want to roll back to.

gcloud compute instance-groups managed rolling-action start-update INSTANCE_GROUP_NAME \
    --version=template=OLD_INSTANCE_TEMPLATE_NAME \
    --max-unavailable=100% \
    [--zone=ZONE | --region=REGION]

API

Call the patch method on a regional or zonal MIG resource.

In the request body, specify the earlier instance template as a version:

PATCH https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/instanceGroupManagers/INSTANCE_GROUP_NAME

{
  "updatePolicy":
  {
    "maxUnavailable":
    {
      "percent": 100
    }
  },
  "versions": [
    {
      "instanceTemplate": "global/instanceTemplates/OLD_INSTANCE_TEMPLATE_NAME" # Old instance template
    }
  ]
}

For a regional MIG with less than 10 instances, you must use a fixed value for maxUnavailable and set the value to the number of instances in the group.

The Updater treats a rollback request the same as a regular update request, so you can specify additional update options.

Caution: For canary updates, we strongly recommend fully rolling back canary updates before you try to canary another version. There is no way to selectively choose which instances are updated based on which instance template, and performing another canary update on top of an existing update can cause unintended consequences. As a best practice, you should fully roll back a canary update first and restart the canary process again from the beginning.

Stopping an update

There is no explicit method or command to stop an update. You can change an update from proactive to opportunistic, and if the group is not being resized by other services like autoscaler, the change to opportunistic effectively stops the update.

To change an update from proactive to opportunistic by using the gcloud tool, run the following command:

gcloud compute instance-groups managed rolling-action stop-proactive-update INSTANCE_GROUP_NAME \
    [--zone=ZONE | --region=REGION]

To stop the update completely after converting it from proactive to opportunistic, you can stop it by using these steps:

1. Make a request to determine how many instances have been updated:

   gcloud compute instance-groups managed list-instances INSTANCE_GROUP_NAME \
       [--zone=ZONE | --region=REGION]

   The gcloud tool returns a response that includes a list of instances in the group and their current statuses:

   NAME               ZONE           STATUS   HEALTH_STATE  ACTION    INSTANCE_TEMPLATE  VERSION_NAME  LAST_ERROR
   vm-instances-9pk4  us-central1-f  RUNNING  HEALTHY       NONE      example-new-template
   vm-instances-j1h8  us-central1-f  RUNNING  HEALTHY       NONE      example-old-template
   vm-instances-ngod  us-central1-f  STAGING  UNKNOWN       CREATING  example-new-template
   

   In this example, two instances have already been updated.

2. Next, make a request to perform a new update, but pass in the number of instances that have already been updated as the target size:

   gcloud compute instance-groups managed rolling-action start-update INSTANCE_GROUP_NAME \
       --version template=OLD_INSTANCE_TEMPLATE_NAME \
       --canary-version template=NEW_INSTANCE_TEMPLATE_NAME,target-size=2 \
       [--zone=ZONE | --region=REGION]

   To the Updater, this update appears complete, so no other instances are updated, effectively stopping the update.

Controlling the speed of a rolling update

By default, when you make an update request, the Updater performs the update as fast as possible. If you aren't sure you want to apply an update fully or are tentatively testing your changes, you can control the speed of the update by using the following methods.

1. Start a canary update rather than a full update.
2. Set a large minReadySec value. Setting this value causes the Updater to wait this number of seconds before considering the instance successfully updated and proceeding to the next instance.
3. Enable health checking to cause the Updater to wait for your application to start and to report a healthy signal before considering the instance successfully updated and proceeding to the next instance.
4. Set low maxUnavailable and maxSurge values. This ensures that only a minimal number of instances are updated at a time.
5. Selectively updating instances in a MIG instead of using an automated update.

You can also use a combination of these methods to control the rate of your update.

Performing a rolling replace or restart

Restarting performs the stop and start methods on the instances, while a replace actively deletes and creates instances.

A rolling restart or replace does not change anything else about the group, including the instance template. It only refreshes the instances in the group using the method you choose.

There are many reasons why you might want a rolling restart or a rolling replace. For example, you might want to refresh your VM instances from time to time for one of the following reasons:

• Clear up memory leaks.
• Restart your application so it can run from a fresh machine.
• Apply a periodic replace as a best practice to test your VMs.
• Update your VM's operating system image or rerun startup scripts to update your software.

Use the Cloud Console, the gcloud command-line tool, or the Compute Engine API to perform a restart or replace.

Console

1. In the Cloud Console, go to the Instance groups page.

   Go to Instance groups

2. Select the managed instance group that you want to update.
3. At the top of the page, click Rolling restart/replace.
4. Choose whether you want to restart or replace your instances.
5. Optionally, you can toggle configuration options like max surge, max unavailable options, and minimum wait time.
6. Click the Restart or Replace button to start the update.

gcloud

Use the restart command or replace command.

The following command replaces all instances in the MIG, one at a time:

gcloud compute instance-groups managed rolling-action replace INSTANCE_GROUP_NAME

The following command restarts each instance, one at a time:

gcloud compute instance-groups managed rolling-action restart INSTANCE_GROUP_NAME

You can further customize each of these commands with the same options available for updates (for example, maxSurge and maxUnavailable).

API

Call the patch method on a regional or zonal MIG resource.

In the updatePolicy.minimalAction field, specify either RESTART or REPLACE. In both cases, you must also provide the versions.instanceTemplate and versions.name properties to trigger the action.

For example, for a zonal MIG, the following request shows the minimal configuration necessary to automatically restart 100% of the instances.

PATCH https://compute.googleapis.com/compute/v1/projects/example-project/zones/ZONE/instanceGroupManagers/INSTANCE_GROUP_NAME

{
 "updatePolicy": {
  "minimalAction": "RESTART",
  "type": "PROACTIVE"
 },
 "versions": [
  {
   "instanceTemplate": "global/instanceTemplates/CURRENT_INSTANCE_TEMPLATE_NAME",
   "name": "v2"
  }
 ]
}

Additional replace/restart examples

Perform a rolling restart of all VMs, two at a time

This command restarts all VMs in the group, two at a time. Notice that no new instance template is specified.

gcloud compute instance-groups managed rolling-action restart INSTANCE_GROUP_NAME \
    --max-unavailable=2 \
    [--zone=ZONE | --region=REGION]

Perform a rolling restart of all VMs as quickly as possible

gcloud compute instance-groups managed rolling-action restart INSTANCE_GROUP_NAME \
    --max-unavailable=100% \
    [--zone=ZONE | --region=REGION]

Perform a rolling replace of all VMs as quickly as possible

gcloud compute instance-groups managed rolling-action replace INSTANCE_GROUP_NAME  \
    --max-unavailable=100% \
    [--zone=ZONE | --region=REGION]

Preserving instance names

If you need to preserve the names of your VM instances across an update, set the replacementMethod to RECREATE. You must also set maxUnavailable to be greater than 0 and maxSurge to be 0. Recreating instances instead of replacing them causes your update to take longer to complete, but the updated instances keep their names.

If you do not specify a replacement method, the MIG's current updatePolicy.replacementMethod value is used. If it's not set then the default value of substitute is used, which replaces VM instances with new instances that have randomly generated names.

gcloud

When issuing a rolling-action command, include the --replacement-method=recreate flag.

gcloud compute instance-groups managed rolling-action start-update INSTANCE_GROUP_NAME \
    --replacement-method=recreate \
    --version=template=NEW_TEMPLATE \
    --max-unavailable=5 \
    [--zone=ZONE | --region=REGION]

API

Call the patch method on a regional or zonal MIG resource. In the request body, include the updatePolicy.replacementMethod field:

PATCH /compute/v1/projects/PROJECT_ID/regions/REGION/instanceGroupManagers/INSTANCE_GROUP_NAME
{
    "updatePolicy": {
        "type": "PROACTIVE",
        "maxUnavailable": { "fixed": 5 },
        "replacementMethod": "RECREATE"
    },
    "versions": [ {
        "instanceTemplate": "global/instanceTemplates/NEW_TEMPLATE"
    } ]
}

After you make a request, you can monitor the update to know when the update has finished.

Updating a regional managed instance group

A regional MIG contains VM instances that are spread across multiple zones within a region, as opposed to a zonal MIG, which only contains instances in one zone. Regional MIGs let you distribute your instances across more than one zone to improve your application's availability and to support extreme cases where one zone fails or an entire group of instances stops responding.

Performing an update on a regional MIG is same as performing an update on a zonal MIG, with a few exceptions described below. When you initiate an update to a regional MIG, the Updater always updates instances proportionally and evenly across each zone. You cannot choose which instances in which zones are updated first nor can you choose to update instances in only one zone.

Differences between updating regional versus zonal MIGs

Regional MIGs have the following default update values:

• maxUnavailable=NUMBER_OF_ZONES
• maxSurge=NUMBER_OF_ZONES

NUMBER_OF_ZONES is the number of zones associated with the regional MIG. By default, the number of zones for a regional MIG is 3. But you might select a different number.

If you are using fixed numbers when specifying an update, the fixed number must be either 0 or equal to or greater than the number of zones associated with the regional MIG. For example, if the group is distributed across three zones, then you can't set maxSurge to 1 or to 2 because the Updater has to create an additional instance in each of the three zones.

Using a fixed number or a percentage in update requests

If you specify a fixed number in your update requests, the number you specify is divided by the number of zones in the regional MIG and distributed evenly. For example, if you specify maxSurge=10, then the Updater divides 10 across the number of zones in the region and creates instances based on that number. If the number of instances does not divide evenly across zones, the Updater adds the remaining instances to a random zone. So, for 10 instances across three zones, two of the zones get 3 instances and one zone gets 4 instances. The same logic is applied to the maxUnavailable and the targetSize parameters for canary updates.

You can specify a percentage only if your MIG contains 10 or more VM instances. Percentages are handled slightly differently depending on the situation:

• If you specify a percentage of VM instances for a canary update, the Updater attempts to distribute the instances evenly across zones. The remainder is rounded either up or down in each zone but the total difference isn't more than 1 VM instance per group. For example, for a MIG with 10 instances and a target size percentage of 25%, the update is rolled out to 2 to 3 VM instances.

• If you specify a percentage for update options like maxSurge and maxUnavailable, the percentages are rounded independently per zone.

What's next

• Learn about Getting info about MIGs and managed instances.
• Learn about Creating instance templates.
• Learn how to use image families and a rolling replace to update the OS image on all VMs in a MIG.