Serve maintenance page if file exists

This should be easy enough in BASH cronjob: curl get some status URI and enable or disable the iRule on the virtual servers, via tmsh as needed?

I would not recommend doing this from whith the iRule itself, would be adding too much latency for each and every client request.

You could use an HTTP monitor and configure the 'Send String' to do an HTTP get on a file, /up.txt for example. Then configure 'Receive Disable String' with whatever you put in the up.txt file to indicate that the server should be marked down.

For your maintenance page, you can specify a 'Fallback Host' in the HTTP profile you've associated with the virtual server. This is the page that users will be presented with in the event that no pool members attached to the virtual server are available.

Here's what Cory's description might look like:

ltm monitor http http_file_exists_test {
    defaults-from http
    destination *:*
    interval 1
    recv 404\\sNot\\sFound
    recv-disable 200\\sOK
    send "GET /foo.php HTTP/1.1\\r\\nHost: server\\r\\n\\r\\n"
    time-until-up 0
    timeout 16
}

It's essentially the reverse of a normal monitor. Depending on how the web server responds to a file not found error (ie. 404 status code), the receive string would be the 404 response (doesn't exist so monitor is up), and receive disable string is the 200 OK (exists so monitor is down).

This definitely got us going in the right direction.

We ended up creating an additional pool with a monitor that tests for the existence of the file. We then deployed an iRule that checked the status of that new pool to determine whether or not to display a maintenance page. This is probably less costly than doing a get with each request. We'll let the monitor take care of that.