Almost a year after I raised the issue of problems monitoring Mongrel instances on an F5 BigIP, I still get occasional emails asking whether I got it working and how, etc. So, firing off this blog entry to hopefully provide a more findable solution for folks (the mailman mongrel-users archive seems broken, and clearly people find my initial query on the subject, but not the response with the solution).
So, if you need to monitor Mongrel instances with an F5 BigIP, and are doing so with an HTTP monitor health check, you want to set your Send String to be along the lines of the following:
GET /heartbeat/index HTTP/1.1
The key point here is to have the "\r\n" lines (the "\r" is what I was missing and many folks seem to get tripped up by the proper way to specify the line breaks needed for the HTTP request).
Here's a link to the solution provided by Jason Hoffman of Joyent, on the mongrel-users maiing list.
Note that I haven't worked with F5's since September 2007, so likely can't answer any further questions.