Working with Moncli part1: Creating a plugin

Producing data

In the previous 2 posts we had an introduction about Moncli and we have seen how to consume Moncli data (reports) from the Message broker with Krolyk and process them. Now its time to let Moncli generate some data we can work with. In the next couple of articles we will go through the process of building a new plugin and use it to generate metrics and perform different kind of evaluations.

Creating a simple plugin

One of the goals for Moncli was to create a framework which allows people to easily create their own plugins. Although Moncli already comes with a set of plugins which provide basic system metrics they will not cover all your needs, so we'll create one as an example.

Let's say we want to measure and evaluate the disk space one or more directories consume. For this I create a small Bash/Perl oneliner which looks like this:

du -s -b $1\|perl -n -e '/(\\d\*)\\s\*(.\*)/ && print "$2:$1\\n"'

This script accepts 1 parameter which is the directory of which we want to process the content.

jelle@indigo:~$ ./ '/tmp/test/*'

Now we have to make sure Moncli can execute this new plugin. The name of the plugin is dir_size, so first we will make a new directory in the Moncli's repository directory which contains all plugins. The name of the directory is the name of our script:

jelle@indigo:~$ mkdir /opt/moncli/lib/repository/dir_size

Then we rename our script to the value of its md5sum and move it to the directory we just created:

jelle@indigo:~$ md5sum
jelle@indigo:~$ mv /opt/moncli/lib/repository/dir_size/a1161fc0d2dfcd6b4e9f52651e88a1d0


The reason why we store the script as its md5sum is for both security as practical reasons. Moncli knows which script to execute because it has the name of the directory. The request submitted to Moncli contains besides the name of the plugin also the md5sum of the version to execute. If somebody changes the content of the script, then Moncli will not execute it, because before executing the script Moncli looks whether the filename of the script matches its hash. Somebody could add a new version of the script inside the directory with the correct hash, but then you must define that hash in your request. In other words, you can have multiple versions of the plugin inside the directory.

Our script is now ready to be used.

In part2 we will cover how to generate a request so we can test and use our newly created plugin.