Munin—the Raven Reports

Long-term monitoring with Munin is not restricted to system parameters. Why not monitor data of personal interest or data of interest to your colleagues?
How Individual Parameters Are Displayed

In addition to these variables related to the entire graph, you also can specify details for each parameter the plugin monitors, so in the case of the muc plugin, calling, boarding, departed, planned and cancelled. The <parameter>.label variable sets the legend entry for <parameter>.

<parameter>.draw specifies the type of diagram the <parameter> data is to be presented as. AREA asks the Munin master to draw the relevant curve and fill the entire area between the x-axis and data point with color. Parameter data of the drawing type STACK will be stacked on top of this basic area. This way, we sum up all flights that, in the current five-minute interval, are labeled calling, boarding and departed. The airport timetable won't correct their departure time later; they all count as dispatched at this point of time.

Flights tagged as planned and cancelled behave differently. For delayed flights, the Munich Airport authorities will issue a new departure time later. This way, the plugin will see planned flights twice: as planned within their time slot according to schedule, and as calling, boarding or departed at their actual time interval of departure. That's why we draw delayed flights in a separate line of the type LINE2. The number denotes the thickness of the line in pixels. A LINE1 line is one pixel thick; a LINE2 line is two pixels, and a LINE3 line is three pixels. Cancelled flights won't reappear in the time schedule, but as they will never depart, we also draw them as a separate line of two pixels thick.

All this plugin output is written to the standard output. The final version of our muc script is shown in Listing 1.

To activate the muc plugin, we simply create a symlink in the plugins directory and restart the Munin dæmon:


# ln -s <path/to/>muc /etc/munin/plugins
# /etc/init.d/munin-node restart

By Telnetting from the Munin master machine to port 4949 of the node machine on which we have activated the muc plugin, we can check whether all is well. Let's see if our config method works:

$ telnet localhost 4949
[...]
config muc
graph_title Departures Munich Airport
graph_vlabel Number
[...]
cancelled.label Cancelled
cancelled.draw LINE2

If we can do this by hand, the Munin master should generate some nice little graphs and present them via the Web, as shown in Figure 3.

Figure 3. Munin creates daily, weekly, monthly and annual graphs. If you want to preserve them for later comparison, you can write a cron job that archives the relevant PNGs and/or RRD databases at certain points of time.

Drawbacks

The simple way to write and integrate custom plugins is one of the huge advantages of Munin—even the more complex wild-card plugins are no big deal.

Unfortunately, simple sometimes also means simplistic. Although it is possible to include Munin-generated diagrams in customized Web pages, Munin does not provide any functionality to customize the Web pages generated by the Munin master. Especially on sites with many hosts and plugins to monitor, the simple approach that combines all daily and weekly graphs in one page results in an extremely slow-loading overview page.

Another example of a rigid approach in the Munin architecture is the fixed resolution of data. Not all data changes fast enough that the five-minute interval is appropriate.

Unfortunately, a configuration option for individual plugins remains an item on the wish list. On the other hand, Munin allows quite powerful configuration for plugins by means of environment variables set in the /etc/munin/plugin-conf.d/ directory.

Because the documentation could be more extensive, and because the code isn't well commented, the English and German users' mailing lists remain helpful resources.

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