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Health probes

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Important

Azure Front Door (classic) will be retired on March 31, 2027. To avoid any service disruption, it's important that you migrate your Azure Front Door (classic) profiles to Azure Front Door Standard or Premium tier by March 2027. For more information, see Azure Front Door (classic) retirement.

Note

An origin and an origin group in this article refers to the backend and backend pool of an Azure Front Door (classic) configuration.

To determine the health and proximity of each origin for a given Azure Front Door environment, each Front Door profile periodically sends a synthetic HTTP/HTTPS request to all your configured origins. Front Door then uses responses from the health probe to determine the best origin to route your client requests to.

Warning

Since each Azure Front Door edge location is sending health probes to your origins, the health probe volume for your origins can be high. The number of probes depends on your customer's traffic location and your health probe frequency. If the Azure Front Door edge locations don’t receive real traffic from your end users, the frequency of the health probe from the edge location is decreased from the configured frequency. If there's traffic to all the Azure Front Door edge locations, the health probe volume can be high depending on your health probes frequency.

An example to roughly estimate the health probe volume per minute to an origin when using the default probe frequency of 30 seconds. The probe volume on each of your origin is equal to the number of edge locations times two requests per minute. The probing requests are less if there's no traffic sent to all of the edge locations. For a list of edge locations, see edge locations by region.

Supported protocols

Azure Front Door supports sending probes over either HTTP or HTTPS protocols. These probes are sent over the same TCP ports configured for routing client requests, and can't be overridden. Front Door HTTP/HTTPS probes are sent with User-Agent header set with value: Edge Health Probe.

Supported HTTP methods for health probes

Azure Front Door supports the following HTTP methods for sending the health probes:

  1. GET: The GET method means retrieve whatever information (in the form of an entity) gets identified by the Request-URI.
  2. HEAD: The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response. For new Front Door profiles, by default, the probe method is set as HEAD.

Tip

To lower the load and cost to your origins, Front Door recommends using HEAD requests for health probes.

Health probe responses

Responses Description
Determining health A 200 OK status code indicates the origin is healthy. Any other status code is considered a failure. If for any reason a valid HTTP response isn't received for a probe, the probe is counted as a failure.
Measuring latency Latency is the wall-clock time measured from the moment immediately before the probe request gets sent to the moment when Front Door receives the last byte of the response. Front Door uses a new TCP connection for each request. The measurement isn't biased towards origins with existing warm connections.

How Front Door determines origin health

Azure Front Door uses a three-step process across all algorithms to determine health.

  1. Exclude disabled origins.

  2. Exclude origins that have health probes errors:

    • This selection is done by looking at the last n health probe responses. If at least x are healthy, the origin is considered healthy.

    • n is configured by changing the SampleSize property in load-balancing settings.

    • x is configured by changing the SuccessfulSamplesRequired property in load-balancing settings.

  3. For sets of healthy origins in an origin group, Front Door measures and maintains the latency for each origin.

Note

If a single endpoint is a member of multiple origin groups, Front Door optimizes the number of health probes sent to the origin to reduce the load on the origin. Health probe requests are sent based on the lowest configured sample interval. The responses from same health probes determine the health of the endpoint in all origin groups.

Complete health probe failure

If health probes fail for every origin in an origin group, then Front Door considers all origins unhealthy and routes traffic in a round robin distribution across all of them.

Once an origin returns to a healthy state, Front Door resumes the normal load-balancing algorithm.

Disabling health probes

If you have a single origin in your origin group, you can choose to disable health probes to reduce the load on your application. If you have multiple origins in your origin group and more than one of them is in enabled state, you can't disable health probes.

Note

If there's only a single origin in your origin group, the single origin gets few health probes. This might lead to a dip in origin health metrics but your traffic doesn't get impacted.

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