Configuring application health check on the VM

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To increase application availability and make sure that an application is responding, you can configure application health checks on a virtual machine. Instance Groups will perform a check and automatically recover a virtual machine if it fails. This is not to be confused with a network load balancer health check. Learn more about automatic recovery and types of checks.

This section describes how to set up application health check for an existing group.

Management console

CLI

Open the folder page in the management console.
Select Compute Cloud.
On the left-hand panel, select Instance groups.
Select a group and click Edit.
Under Health checks, enable the Activate option.
Select the protocol for the health checks: HTTP or TCP.
Set up the health checks:
- Path (for HTTP): The URL path for the HTTP check requests sent from Instance Groups.
- The port in the range 1-32767 to receive the check requests from Instance Groups.
  
  Alert
  
  Only HTTP/1.1 and lower are supported.
- The response timeout in seconds.
  
  If you connected your group to a network load balancer, we recommend setting a higher value here than in the load balancer.
- Check interval in seconds: This is the interval between the health checks done by Instance Groups.
- Performance threshold: The number of successful health checks required for the managed instance to be considered healthy.
  
  If you connected your group to a network load balancer, we recommend setting a lower value here than in the load balancer.
- Failure threshold: The number of failed health checks for the managed instance to be considered unhealthy.
  
  If you connected your group to a network load balancer, we recommend setting a higher value here than in the load balancer.
Click Save.

If you don't have the Yandex Cloud command line interface yet, install and initialize it.

The folder specified in the CLI profile is used by default. You can specify a different folder using the --folder-name or --folder-id parameter.

View a description of the update group command in the CLI:
```
$ yc compute instance-group update --help
```

Get a list of instance groups in the default folder:

yc compute instance-group list
+----------------------+-----------------------+------+
|          ID          |         NAME          | SIZE |
+----------------------+-----------------------+------+
| amc65sbgfqeqf00m02sc | first-instance-group  |    2 |
+----------------------+-----------------------+------+

Select the group ID or NAME (for example, first-instance-group).
Get information about the instance group.
Create a YAML file with any name (for example, group.yaml) and, based on the information received, describe:
- An instance template.
- Policies.
- The service account ID.
- A network load balancer specification (if required).

Add a health check specification to the file (for example, for HTTP checks):

...
health_checks_spec:
  health_check_specs:
    - http_options:
        port: 80
        path: /
      interval: 30s
      timeout: 10s
      unhealthy_threshold: 5
      healthy_threshold: 3
...

Where:

Key	Value
`health_check_specs`	Health check specs.
`http_options`	Settings for HTTP health checks. Only HTTP/1.1 and lower are supported. If you want to use TCP, then in the `tcp_options` property specify the port number only.
`port`	The port in the range 1-32767 to receive the check requests from Instance Groups.
`path`	The URL path for the HTTP health check requests sent from Instance Groups.
`interval`	Check interval in seconds: This is the interval between the health checks done by Instance Groups.
`timeout`	Response timeout in seconds. If you connected your group to a load balancer, we recommend setting a higher value here than in the load balancer.
`unhealthy_threshold`	Unhealthy threshold is the number of failed health checks after which the instance is considered non-functional. If you connected your group to a network load balancer, we recommend setting a higher value here than in the load balancer.
`healthy_threshold`	Healthy threshold is the number of successful health checks after which an instance is considered functional. If you connected your group to a load balancer, we recommend setting a lower value here than in the load balancer.

Update the instance group in the default folder:
```
$ yc compute instance-group update --name first-group --file group.yaml
```
Instance Groups will launch the instance group update process.

Configuring application health check on the VM

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