After following Dan’s tutorial on installing munin on your servers, you already get the benefits of munin’s default plugins. You have graphs showing your CPU, RAM, I/O, as well as MySQL, Exim, and quite some other stats. But most of the time you run some additional software which you also want to montior.
Install Some Custom Munin Plugins
In our case, we have a full Ruby on Rails stack featuring nginx front end web server, HA Proxy load balancer, a cluster of thin instances for the Rails application, memcached for caching page fragments and a MySQL database backend.
No matter how your stack looks like, you’ll find some good munin plugins at MuninExchange. Some of them are pretty basic (or not working as required) but give you a great head start. Just grab one of them and put it into the
/usr/share/munin/plugins dir on any munin node. Make it executable (
chmod 755 thin_memory) and enable the plugin by symlinking it to
/etc/munin/plugins (you must restart your munin-node after adding new plugins).
Alternatively to manually symlinking your plugin, just call
root@node:/usr/share/munin/plugins# munin-node-configure --shell to do it for you. It will only link plugins compatible with your configuration (see the munin-node config docs for details)
Test Your Custom Munin Plugins
You’ve got basically three ways of making sure your plugin works:
- Run the munin plugin script directly:
This is the most basic way to ensure, that your munin plugin returns reasonable values. Just execute the script directly on your munin node.
Your script should return something like this (depending on how many thin instances you run on which ports):
thin_8000.value 180 thin_8001.value 178 thin_8002.value 175 thin_8003.value 169
munin-runto execute the plugin locally:
The next stage is to test whether the munin plugin still returns reasonable values when executed as a munin-node process:
root@node:/etc/munin/plugins# munin-run thin_memory --debug
This should return something like this:
# Saving mysql*->env->mysqlopts = --defaults-extra-file=/etc/mysql/debian.cnf... ... # file: 'thin_memory' # Checking thin_*... # Checking thin_*... # Checking thin_*... # Checking thin_*... # Want to run as euid/egid 0/106 # Running as uid/gid/euid/egid 0/106 106/0/106 106 # DEBUG: About to exec "/etc/munin/plugins/thin_memory" thin_8000.value 180 thin_8001.value 178 thin_8002.value 175 thin_8003.value 169
munin-updateat your munin server: The final stage is to manually invoke the munin server process to see, whether it is able to collect data from your munin plugin remotely:
root@munin-server:~# /usr/share/munin/munin-update --debug --nofork --stdout --host node.example.com --service thin_memory
Watch out for lines like these:
... Dec 17 18:41:31 -  Fetching service: node.example.com->thin_memory Dec 17 18:41:31 -  Updating /var/lib/munin/reference/node.example.com-thin_memory-thin_8000-g.rrd with 180 Dec 17 18:41:31 -  Updating /var/lib/munin/reference/node.example.com-thin_memory-thin_8001-g.rrd with 179 Dec 17 18:41:31 -  Updating /var/lib/munin/reference/node.example.com-thin_memory-thin_8002-g.rrd with 175 Dec 17 18:41:31 -  Updating /var/lib/munin/reference/node.example.com-thin_memory-thin_8003-g.rrd with 169 Dec 17 18:41:31 -  Fetched service: node.example.com -> thin_memory (0.08 sec) ...
And, last, but not least, there’s good, old telnet which you can use to contact and query any munin-node:
me@anywhere:~$ telnet node 4949
You may use
list to see, which services are monitored at your node, and
fetch thin_memory to get the current values. Just type
help to see what else you can do there.
Customizing your munin installation with additional munin plugins is quite easy and straight forward. You just need some scripts from MuninExchange and enable them for use via
munin-node. The various ways of debugging your plugins really help to get your monitoring up fast.