Writing Munin Plugins pt1: Overview

Writing your own Munin Plugins

Around February this year, we at innoQ had the need for setting up a Mac OS based CI for a Project. Besides building of integrating some standard Java Software, we also had to setup an Test Environment with Solaris/Weblogic, Mac OS for doing a CI for an iOS Application and a Linux System that contains the Jenkins CI itself.
Additionally the whole Setup should be reachable via VPN (also the iOS Application itself should be able to reach the Ressources via VPN).

To have the least possible obsticles in Setting up the iOS CI and the iOS (iPad) VPN Access, we decide to use Mac OS Server as the Basic Host OS. As the Need for Resources are somehow limited for the other Systems (Solaris/Weblogic, Linux/Jenkins), we also decide to do a basic VM Setup with VMWare Fusion.

Since we have a decent Munin Monitoring Setup in our Company for all our Systems, we need some Monitoring for all Services used in our Setup:

Beside the Standard Plugins (like Network/CPU/RAM/Disk) that was basically

  • Jenkins CI
  • VMware Fusion
  • VPN

After searching through the Munin Plugin Repository we couldn’t find any plugins providing the necessary monitoring. So the only choice was to write your own set of plugins. Since all three Plugins use different Approaches for collecting the Data, i plan two writer three different posts here. One for each Plugin. The Sources are availible online here and might be added to the main Munin Repo as soon as the Pull Requests are accepted.

How Munin works

But first a brief overview of Munin. Munin is a TCP based Service that has normally one Master and one Node for each System that needs to be monitored. The Master Node ask all Nodes periodicly for Monitoring Updates.
The Node Service, delivering the Updated Data runs on Port 4949 per default. To add some level of security, you normal add a IP to a whitelist, that is allowed to query the Nodes Data.

You can use normal telnet for accessing the Nodes Data:

telnet localhost 4949
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
# munin node at amun

Every Node delivers Information about specific Services provided by Plugins. To get an overview about the configured plugins you do a:

# munin node at amun
list
df df_inode fusion_mem fusion_pcpu fusion_pmem if_en0 if_err_en0 load lpstat netstat ntp_offset processes users

A Plugin always provides a Configuration Output and a Data Output. By Default if you query a Plugin, you will always get the Data Output:

# munin node at amun
df
_dev_disk1s2.value 34
_dev_disk0s1.value 48
_dev_disk3s2.value 62
_dev_disk2s1.value 6
_dev_disk2s2.value 32

To trigger the Config Output you need to add a config to your command:

# munin node at amun
df config
graph_title Filesystem usage (in %)
graph_args --upper-limit 100 -l 0
graph_vlabel %
graph_scale no
_dev_disk0s1.label /Volumes/Untitled
_dev_disk1s2.label /
_dev_disk2s1.label /Volumes/System-reserviert
_dev_disk2s2.label /Volumes/Windows 7
_dev_disk3s2.label /Volumes/Data

You can also use the tool munin-run for doing a basic test (it will be installed when installing your munin-node Binary)

 munin-run df
_dev_disk1s2.value 34
_dev_disk0s1.value 48
_dev_disk3s2.value 62
_dev_disk2s1.value 6
_dev_disk2s2.value 32
munin-run df config
graph_title Filesystem usage (in %)
graph_args --upper-limit 100 -l 0
graph_vlabel %
graph_scale no
_dev_disk0s1.label /Volumes/Untitled
_dev_disk1s2.label /
_dev_disk2s1.label /Volumes/System-reserviert
_dev_disk2s2.label /Volumes/Windows 7
_dev_disk3s2.label /Volumes/Data

Summary

So a Plugin needs to provide an Output both modes:

  • Configuration Output when run with the config argument
  • The (normal) Data Output when called withouth additional arguments

Plugins are Shell Scripts that can be written in every Programming language that is supported by the Nodes Shell (e.g. Bash, Perl, Python, etc.)

Since it is one of the easier Plugins we will have a look at the Plugin, monitoring the VPN Connections at our Mac OS Server in the next Post.

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