Every man and his dog has written a Nagios Apache monitor that scrapes the mod_status "/server-status/" URL to tell you how many workers are in use or idle. However all of these suffer the same problem; because you're using a TCP connection when Apache runs out of connections, (because your application is bricked waiting for database connections, etc.) your monitoring breaks/times out, etc. rather than say all of your workers are busy.

An alternative idea is to monitor Apache "out-of-band" using the scoreboard directly.

All you need is something like the following in your httpd.conf:

    ScoreboardFile /var/run/httpd/httpd.scoreboard

You don't need to have mod_status enabled for this to work. Then run the monitor like so:

    # ./check_apache -w 100 -c 200 /var/run/httpd/httpd.scoreboard
    WARNING: 150 workers in use

-w and -c refer to the warning and critical thresholds for the number of workers in use.