​​Health probes

In order to determine the health and proximity of each backend from a given Front Door environment, each Front Door environment periodically sends a synthetic HTTP/HTTPS request to each of your configured backends. Front Door then uses responses from these probes to determine the "best" backends to which it should route real client requests.

Warning

Since Front Door has many edge environments globally, health probe requests volume to your backends can be quite high - ranging from 25 requests every minute to as high as 1200 requests per minute, depending on the health probe frequency configured. With the default probe frequency of 30 seconds, the probe volume on your backend should be about 200 requests per minute.

Supported protocols

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 cannot be overridden.

Supported HTTP methods for health probes

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) is 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.

Note

For lower load and cost on your backends, Front Door recommends using HEAD requests for health probes.

Health probe responses

Responses Description
Determining Health ​A 200 OK status code indicates the backend is healthy. Everything else is considered a failure. If for any reason (including network failure) a valid HTTP response is not 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 we send the probe request to the moment when we receive the last byte of the response. We use a new TCP connection for each request, so this measurement is not biased towards backends with existing warm connections.

How Front Door determines backend health

Azure Front Door uses the same three-step process below across all algorithms to determine health.

  1. ​Exclude disabled backends.

  2. Exclude backends 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 backend 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. Out of the set of healthy backends in the backend pool, Front Door additionally measures and maintains the latency (round-trip time) for each backend.

Complete health probe failure

If health probes fail for every backend in a backend pool, then Front Door considers all backends healthy and routes traffic in a round robin distribution across all of them.

Once any backend returns to a healthy state, then Front Door will resume the normal load-balancing algorithm.

Disabling health probes

If you have a single backend in your backend pool, you can choose to disable the health probes reducing the load on your application backend. Even if you have multiple backends in the backend pool but only one of them is in enabled state, you can disable health probes.

Next steps