Health checks and autohealing of group instances

Instance Groups regularly runs health checks for instances in your instance group. If an instance stopped or an app is taking too long to respond, Instance Groups tries to heal the instance by either restarting it or creating a new one, depending on the recovery policy. The instances will be auto-healed based on the deployment policy.

Types of health checksTypes of health checks

To ensure autohealing, Instance Groups performs two types of health checks:

• Instance operability check.
• Application health check on the instance.

Do not confuse these checks with those the load balancer performs if an instance group is integrated with Yandex Network Load Balancer or Yandex Application Load Balancer. The load balancer health check affects only the deployment process: when the instance gets the OPENING_TRAFFIC status at startup, Instance Groups will wait until its status turns to HEALTHY in the load balancer. After that, Instance Groups will stop monitoring the instance status in the load balancer. You can also set up instance autohealing for health checks from the load balancer. For more information, see Health checks from load balancers.

Instance operability checkInstance operability check

Instance Groups checks the instance status in Compute Cloud every few seconds. If an instance stops or throws an error (STOPPED, ERROR, or CRASHED status), Instance Groups will try to restart the instance or create a new one, depending on the recovery policy. The instances will be healed based on the deployment policy.

Application health check on the instanceApplication health check on the instance

This check will detect if the app running on your instance has frozen, terminated, or is taking too long to respond. You can enable the application health check when creating or editing an instance.

If you enable this check, Instance Groups will poll the application status on the instance at preset intervals while the instance group is in the ACTIVE status.

Recommendations for instance groups with a load balancerRecommendations for instance groups with a load balancer

If your instance group is integrated with Network Load Balancer or Application Load Balancer, use less strict settings for checks in Instance Groups than those for the load balancer health checks (to learn more about health checks, see the Network Load Balancer or Application Load Balancer documentation). The load balancer distributes load to the app, while Instance Groups only monitors its performance.

For example, if you set the response timeout of 1 second in the load balancer, you should set 30 seconds in Instance Groups. If the application does not respond for 3 to 5 seconds, it might not be able to handle the current traffic. If it does not respond for more than 30 seconds, it is probably not working at all and you need to heal your instance.

Settings for application health checksSettings for application health checks

You can set up health checks for an app running on an instance in the management console or describe it in the group's YAML specification to provide it through the Yandex Cloud CLI or API.

You can specify the settings when creating or updating a group.

Below, you can find the descriptions of the fields with the settings from the YAML specification and their respective fields in the management console.

health_checks_spec:
  health_check_specs:
    - interval: 2s
      timeout: 1s
      unhealthy_threshold: 2
      healthy_threshold: 2
      http_options:
        port: 8081
        path: "/_hz"
    - interval: 2s
      timeout: 1s
      unhealthy_threshold: 2
      healthy_threshold: 2
      tcp_options:
        port: 8080
  max_checking_health_duration: 25s

The fields and options in the management console are located under Health checks on the instance group creation and editing pages.

Autohealing featuresAutohealing features

Autohealing and deployment policiesAutohealing and deployment policies

To autoheal instances, Instance Groups may restart them or create new ones. The healing method is defined by the deployment policies.

• Creating new VMs
  Instance Groups will create new instances to replace those that have failed the check, provided the deployment policy permits exceeding the group's target size. You use the max_expansion parameter to set the maximum number of instances by which the target size of the group can be exceeded. The possible values range from 0 to 100. In this case, Instance Groups will first create a new instance, wait until it passes all checks, and then delete the instance that failed the check.

  When bringing the number of instances in the group to the target value, instances created under the max_expansion quota can remain in the group, while those that existed in the group before may be deleted, even if they passed all checks.

• Restarting a VM
  Instance Groups will restart instances that have failed health check if the deployment policy permits reducing the target group size. You can use the max_unavailable parameter to set the maximum number of instances you can make unavailable at the same time. The acceptable values range from 0 to 100. Instance Groups will try not to exceed this value during autohealing.

  This restriction does not apply to instances with theCRASHED, ERROR, and STOPPED statuses, because these mean that the instance is already unavailable and must be restarted immediately.

If you set both max_expansion and max_unavailable, Instance Groups will use both autohealing methods.

A fully booted instance with the RUNNING_ACTUAL or RUNNING_OUTDATED status is considered running in terms of the max_unavailable quota, even if it did not pass the health check. To stop such an instance automatically, you need to have unused max_unavailable quota.

Additional instances are created under the max_expansion quota only if the max_unavailable quota is not enough. In other words, restarting an instance takes priority over creating a new one.

Let’s assume you specified max_expansion = 1 and max_unavailable = 1. When one of the instances fails the check, Instance Groups will restart this instance. If another instance fails the check at the same time, Instance Groups will not be able to restart it because the max_unavailable parameter is exceeded. Instead, it will begin to create a new instance while restarting the first one.

To limit the autohealing and deployment speed, you can also set:

• Maximum number of instances deployed simultaneously in the max_creating parameter. This includes instances with the CREATING and STARTING statuses.

  The possible values range from 0 to 100. 0 means any number of instances within the allowed range.

• Maximum number of instances undeployed simultaneously in the max_deleting parameter. This includes instances in the STOPPING status, since Instance Groups always stops instances before deleting them.

  The possible values range from 0 to 100. 0 means any number of instances within the allowed range.

Changing instance status during autohealingChanging instance status during autohealing

Instance Groups will not try to heal an instance if it is no longer needed.

For example, if all 10 instances out of 10 in a group are unavailable and max_unavailable = 3, Instance Groups will restart the first three instances. If the remaining seven instances become operational again in the meantime, Instance Groups will not restart them.

If max_expansion = 3, Instance Groups will start creating three new instances. The old instances will not be deleted until the new ones are created. If all instances in an instance group become operational again during the creation process, Instance Groups will cancel the creation of new instances.

When bringing the number of instances in the group to the target value, instances created under the max_expansion quota can remain in the group, while those that existed in the group before may be deleted, even if they are operational.

Healing while updating instance configurationsHealing while updating instance configurations

Instance healing has a higher priority than instance configuration update.

Let's assume you have a group of 100 instances with max_unavailable = 1. When you update the instance configuration in an instance group, Instance Groups will restart the instances one by one, updating their configuration.

If one of the instances fails the application health check at that point, Instance Groups will put it first in the restart queue.

Healing on instance group size changeHealing on instance group size change

When the target size of an instance group is reduced, the instances that failed the check, if any, will be deleted first.

If you increase the instance group's target size, new instances will be created in parallel with the instances that had failed the check, if allowed by max_creating and max_expansion:

Let's assume two out of four instances in an instance group failed the application health check. At that point, the target size of the instance group has increased to six instances. You have two instances to create and another two to heal.

If max_expansion = 1 and max_creating is not set, Instance Groups will start creating three instances in parallel: two as part of group expansion, and one as part of autohealing.

Autohealing preemptible instancesAutohealing preemptible instances

Preemptible instances can only be autohealed if there is enough computing resources in the availability zone. If the resources are insufficient, Instance Groups will resume autohealing as soon as the resources become available; this, however, may take a long time.

Preemptible instances must be terminated within 24 hours of their launch. In this case, there is a risk that the entire instance group will restart at the same time and stop handling the load of running applications. To avoid this, Instance Groups stops preemptible instances after a random interval of 22 to 24 hours, rather than exactly after 24 hours.

See alsoSee also

• Configuring application health check on the VM.