Section – Out-of-band Checks

Out-of-band checks allow constant monitoring of a specific page independent of any end-user traffic. This page is called in a defined interval and the response is analyzed to decide whether a back-end server is in a good or a bad state. As soon as at least one of the failure patterns matches, the request is treated as a failed request. If out-of-band checks are configured, no probe requests from end-users are being sent to bad servers.
you can configure how often the test page should be called for the good and bad states, and how many subsequent failed or successful attempts are necessary to initiate a state change from good to bad or vice versa.

A back-end host variable or a static value can be set to add a specific host header to the out-of-band requests. This may be necessary e.g. for back-ends behind a load balancer.

• Examples:
• %BACKENDHOST%
• back-end.example.com
• 192.0.2.48

If the Host header field is left empty, the original back-end hostname is used as the host header.

This is the address for the test page, e.g. a login page, or a dedicated test page explicitly implemented to return status information on the back-end application.

To analyze the return status of the response, you can define a regular expression to match the response status code. Activate/deactivate the pattern by clicking on the gray/green button to the left of the pattern input field. This regular expression is always case insensitive, but can be inverted if required. If the response status code matches with the pattern, the request is counted as a failed request.

Example:

^503$                   = interpret response status 503 as failed
^5                      = interpret response status 5xx as failed
^(200|3..)$, inverted   = only 200 and 3xx responses are expected, every other status is interpreted as a failure

If this regular expression matches against the response body, then the request is counted as a failed request. The regular expression can be configured to be either case sensitive or case insensitive. Checking the content can be turned on or off by clicking on the grey/green button to the left of the pattern input field.

The "Interval" value defines how many seconds pass between individual calls to the check URL in case of a good back-end. The second parameter on this line defines the number of subsequent failed requests after which the back-end will be marked as "bad".

The "Interval" value defines how many seconds pass between individual calls to the check URL in case of a bad back-end. The second parameter on this line defines the number of subsequent successful requests after which the back-end will be marked as "good".

Since out-of-band checks only call one specific page, their expected timing behavior can differ considerably from that of general pages within an application. This field allows setting an HTTP request time-out which will only apply to Out-of-band calls to the check URL. All other requests use the time-out defined on their corresponding Timeout.