Application Gateway health monitoring overview

Azure Application Gateway by default monitors the health of all resources in its back-end pool and automatically removes any resource considered unhealthy from the pool. Application Gateway continues to monitor the unhealthy instances and adds them back to the healthy back-end pool once they become available and respond to health probes. By default, Application gateway sends the health probes with the same port that is defined in the back-end HTTP settings. A custom probe port can be configured using a custom health probe.

The source IP address Application Gateway uses for health probes depends on the backend pool:

  • If the server address in the backend pool is a public endpoint, then the source address is the application gateway's frontend public IP address.
  • If the server address in the backend pool is a private endpoint, then the source IP address is from the application gateway subnet's private IP address space.

application gateway probe example

In addition to using default health probe monitoring, you can also customize the health probe to suit your application's requirements. In this article, both default and custom health probes are covered.

Note

This article has been updated to use the new Azure PowerShell Az module. You can still use the AzureRM module, which will continue to receive bug fixes until at least December 2020. To learn more about the new Az module and AzureRM compatibility, see Introducing the new Azure PowerShell Az module. For Az module installation instructions, see Install Azure PowerShell.

Default health probe

An application gateway automatically configures a default health probe when you don't set up any custom probe configuration. The monitoring behavior works by making an HTTP GET request to the IP addresses or FQDN configured in the back-end pool. For default probes if the backend http settings are configured for HTTPS, the probe uses HTTPS to test health of the backend servers.

For example: You configure your application gateway to use back-end servers A, B, and C to receive HTTP network traffic on port 80. The default health monitoring tests the three servers every 30 seconds for a healthy HTTP response with a 30 second timeout for each request. A healthy HTTP response has a status code between 200 and 399. In this case, the HTTP GET request for the health probe will look like http://127.0.0.1/.

If the default probe check fails for server A, the application gateway stops forwarding requests to this server. The default probe still continues to check for server A every 30 seconds. When server A responds successfully to one request from a default health probe, application gateway starts forwarding the requests to the server again.

Default health probe settings

Probe property Value Description
Probe URL <protocol>://127.0.0.1:<port>/ The protocol and port are inherited from the backend HTTP settings to which the probe is associated
Interval 30 The amount of time in seconds to wait before the next health probe is sent.
Time-out 30 The amount of time in seconds the application gateway waits for a probe response before marking the probe as unhealthy. If a probe returns as healthy, the corresponding backend is immediately marked as healthy.
Unhealthy threshold 3 Governs how many probes to send in case there's a failure of the regular health probe. In v1 SKU, these additional health probes are sent in quick succession to determine the health of the backend quickly and don't wait for the probe interval. In the case of v2 SKU, the health probes wait the interval. The back-end server is marked down after the consecutive probe failure count reaches the unhealthy threshold.

The default probe looks only at <protocol>://127.0.0.1:<port> to determine health status. If you need to configure the health probe to go to a custom URL or modify any other settings, you must use custom probes. For more information about HTTPS probes, see Overview of TLS termination and end to end TLS with Application Gateway.

Probe intervals

All instances of Application Gateway probe the backend independent of each other. The same probe configuration applies to each Application Gateway instance. For example, if the probe configuration is to send health probes every 30 seconds and the application gateway has two instances, then both instances send the health probe every 30 seconds.

Also if there are multiple listeners, then each listener probes the backend independent of each other. For example, if there are two listeners pointing to the same backend pool on two different ports (configured by two backend http settings) then each listener probes the same backend independently. In this case, there are two probes from each application gateway instance for the two listeners. If there are two instances of the application gateway in this scenario, the backend virtual machine would see four probes per the configured probe interval.

Custom health probe

Custom probes allow you to have more granular control over the health monitoring. When using custom probes, you can configure a custom hostname, URL path, probe interval, and how many failed responses to accept before marking the back-end pool instance as unhealthy, etc.

Custom health probe settings

The following table provides definitions for the properties of a custom health probe.

Probe property Description
Name Name of the probe. This name is used to identify and refer to the probe in back-end HTTP settings.
Protocol Protocol used to send the probe. This has to match with the protocol defined in the back-end HTTP settings it is associated to
Host Host name to send the probe with. In v1 SKU, this value will be used only for the host header of the probe request. In v2 SKU, it will be used both as host header as well as SNI
Path Relative path of the probe. A valid path starts with '/'
Port If defined, this is used as the destination port. Otherwise, it uses the same port as the HTTP settings that it is associated to. This property is only available in the v2 SKU
Interval Probe interval in seconds. This value is the time interval between two consecutive probes
Time-out Probe time-out in seconds. If a valid response isn't received within this time-out period, the probe is marked as failed
Unhealthy threshold Probe retry count. The back-end server is marked down after the consecutive probe failure count reaches the unhealthy threshold

Probe matching

By default, an HTTP(S) response with status code between 200 and 399 is considered healthy. Custom health probes additionally support two matching criteria. Matching criteria can be used to optionally modify the default interpretation of what makes a healthy response.

The following are matching criteria:

  • HTTP response status code match - Probe matching criterion for accepting user specified http response code or response code ranges. Individual comma-separated response status codes or a range of status code is supported.
  • HTTP response body match - Probe matching criterion that looks at HTTP response body and matches with a user specified string. The match only looks for presence of user specified string in response body and isn't a full regular expression match.

Match criteria can be specified using the New-AzApplicationGatewayProbeHealthResponseMatch cmdlet.

For example:

$match = New-AzApplicationGatewayProbeHealthResponseMatch -StatusCode 200-399
$match = New-AzApplicationGatewayProbeHealthResponseMatch -Body "Healthy"

Once the match criteria is specified, it can be attached to probe configuration using a -Match parameter in PowerShell.

NSG considerations

You must allow incoming Internet traffic on TCP ports 65503-65534 for the Application Gateway v1 SKU, and TCP ports 65200-65535 for the v2 SKU with the destination subnet as Any and source as GatewayManager service tag. This port range is required for Azure infrastructure communication.

Additionally, outbound Internet connectivity can't be blocked, and inbound traffic coming from the AzureLoadBalancer tag must be allowed.

For more information, see Application Gateway configuration overview.

Next steps

After learning about Application Gateway health monitoring, you can configure a custom health probe in the Azure portal or a custom health probe using PowerShell and the Azure Resource Manager deployment model.