Using God Gem to Monitor mySQL
I recently ran into a problem on a client server where the msyqld process died, and neither of us noticed it for almost 24 hours. That meant that the site was completely useless that entire time. Needless to say, the client wasn’t thrilled about this.
I told him that I’d look into setting up a monitoring service to, at the very least, notify me if msyqld ever crashed. During my research, I stumbled upon the god gem (which I’d used in the past to monitor mongrel processes back in the days before Passenger). I didn’t realize it could be used to monitor other processes as well, such as msyqld.
God is a rubygem, so it’s easy to install on any *nix operating system. It’s config files are also written in Ruby, so that makes it even better.
What you’ll need to get the script running:
Install the god gem (as root).
sudo gem install god
Find the location of your msyqld pid file.
mysqladmin -u root -p variables | grep pid_file
Determine the command to start/stop/restart the msyqld service. Usually /etc/init.d/mysqld
Here is the config file I wrote to watch my mysqld process:
A few things to note about my script:
In my start and restart commands, I also restart Apache. Running my site with Passenger, I would get a Rack error when mysqld restarted, but apache hadn’t. YMMV.
I’m using an SMTP server for my email notification. You can also use sendmail, or you can exclude the email notifications altogether.
When running the script for the first time, it’s a good idea to add the -D flag. This runs god in non-daemonized mode so all output is piped to STDOUT. That way, you can watch what it’s doing to ensure everything is working correctly.