K5917: Using Regular Expressions in a Health Monitor Receive String
K5917: Using regular expressions in a health monitor receive string Non-Diagnostic Original Publication Date: Oct 20, 2015 Update Date: Aug 24, 2021 Topic Some BIG-IP application health monitors, such as the HTTP monitor, include a Receive String field. This field specifies a string for comparison with the server response. You can specify a regular expression in that field to provide some flexibility in matching an expected response from a healthy server. Health monitors that support regular expressions include TCP, HTTP, HTTPS, and User Datagram Protocol (UDP). Regular expressions provide the flexibility for identifying strings of text that are of interest, such as certain characters, words, or a pattern of characters. With some exceptions, health monitors on BIG-IP systems support the use of POSIX Extended Regular Expressions (ERE). ERE syntax treats most characters as literals, meaning that they match only themselves, and defines metacharacters, which can be used to represent special characters, multiple characters, or a sequences of characters. To reduce operating overhead for the BIG-IP system, F5 recommends that, whenever possible, you use static strings for monitoring. Using static strings instead of regular expressions for monitoring is simpler and more efficient. However, the performance impact is minimal in comparison to the administrative flexibility offered from using regular expressions. For example, your HTTP health monitor requests the /admin/monitor.html page from the web server, which invokes a simple local script. The script checks whether vital local operating parameters, such as connections, memory, processor, or disk space, are within acceptable limits. If so, the script returns the monitor.html page, which includes a single line of body text that varies for each server, based on the server name: Server1: OK Server2: OK Server3: OK If you do not use a regular expression to match this response, you must create three separate monitors, each with a different receive string, and ensure the correct monitor is applied to each server.
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