December 20, 2008

Day 20 - Ganglia

Ganglia is a monitoring tool designed originally to help scalably monitor computing grids and clusters. How can it help you, even if you don't run traditional computing grids or clusters?

Ganglia is an RRDtool-based (like Cacti) monitoring and graphing system. Ganglia differs from Cacti in that configuration is much more automatic. Ganglia's design centers around two programs: gmond and gmetad. The gmond program listens for metric reports from other gmond programs (or tools that emit the same messages). The gmetad program periodically polls a single gmond for data on an entire cluster. The trick, here, is that every time gmond gets data, it sends that data via multicast to other gmonds, so every gmond has state for the whole cluster. I presume that the actual gmond used by gmetad is chosen at random, and if the chosen gmond host fails, another gmond host is chosen.

In addition to clusters (one gmetad for N gmonds), Ganglia supports a higher level collection they call a grid. A grid is automatically learned when you have one gmetad polling from another gmetad. I am unaware if you can have more than these levels (host, cluster, grid).

Multicast: This means your network gear will need multicast routing enabled if you hope to span broadcast domains with this monitoring. Alternately, gmond can be configured to send updates to a unicast address which can avoid needing multicast routing and other potentially difficult network features.

Both gmond and gmetad have reasonably easy-to-use configuration files and come with very reasonable default values. Simply running gmond and gmetad from the default configurations will result in data you can access easily. The primary Ganglia human interface is through a webserver.

Getting data out of Ganglia is easy. The historical data is stored in RRD files in a known location organized by cluster and hostname, so you can use your favorite rrdtool interface to query data. The current data is stored on any gmond which is queryable by connecting to ganglia's xml port (default 8649). The service listening on that port will dump the metric data by cluster and host in XML. XML might make you groan, but it's use will help you write tools (like nagios checks) to use the current data.

My first question after playing with Ganglia for a few minutes was, "How do I monitor my network gear?" Typical network gear won't allow you to run arbitrary binaries on them. Luckily, Ganglia comes with a tool for broadcasting metric messages, gmetric. With gmetric, you can spoof the source of a piece of data and easily claim that it came from your switch or router. This tool is also the easiest way to extend the metrics ganglia monitors for you. For example:

% gmetric -S "" -n uptime -v 3644 -t uint32 -u seconds
  spoofName: myrouter    spoofIP: 
And the metric reported on the xml port is:
<HOST NAME="myrouter" IP="" REPORTED="1229761211" TN="28" TMAX="20" DMAX="0" LOCATION="unspecified" GMOND_STARTED="0">
<METRIC NAME="uptime" VAL="3644" TYPE="uint32" UNITS="seconds" TN="28" TMAX="60" DMAX="0" SLOPE="both" SOURCE="gmetric"/>
You can also specify the lifetime of the value, which will cause it to automatically be dropped from your gmond processes.

In addition to the gmetric command, there is are C and Python interfaces to gmetric. Additionally, if you want your own software to emit gmetric messages, there's an embedded gmetric library you can use.

My second question was, "What if a host is retired, is renamed, or is reconfigured?" If a host never again reports data about itself, the last state is still kept on every gmond and gmetad. To expire hosts that haven't spoken in a while, you should set the host_dmax value in gmond.conf to some value of seconds after which the host's state will be dropped. However, I am not sure the RRDs for this host will be cleaned up automatically. Host renaming probably requires that you rename the rrd directory holding the host's data if you wish to maintain historical data across the rename. If you reconfigure a host, deleting no-longer-used metric rrds is probably prudent. All of the above changes will likely require walking around in the ganglia rrd storage directory in your gmetad.

My last concern with ganglia was that the monitoring unit was a host. I think most often in services, not hosts. Thankfully, there are workarounds. Since you can spoof hostnames with gmetric, I decided to try something not-hostname-like to identify a service+host combo. I tried to spoof "service/hostname," but Ganglia uses this as the directory name and fails mkdir for not doing it recursively. Choosing another delimiter, comma, works fine:

Note: gmetric -S "foo/bar" will succeed, but gmetad will crash trying to write to that file path (mkdir not so smart). If you try this, you'll have to stop all gmond and gmetad instances then start them all again to clear the knowledge of the host named "foo/bar"

% gmetric -S ",aaa123" -n "apache errors" -v 5 -t uint32 -u "errors"
 spoofName: apache,aaa123    spoofIP: 
However, it appears that ganglia keys on IP as unique, so trying to add another entry of ",aaa123" will appear in the web interface. Further testing revealed that the IP portion of the spoof is not validated. If we use the unique combination of "service,host" as both the IP and hostname, everything is peachy:
% gmetric -S "apache,aaa123:apache,aaa123" ...
% gmetric -S "mysql,aaa123:mysql,aaa123" ...
Pushing only apache-related data to the 'apache' host prefixes will help you organize your monitoring by service. This means you could easily view only apache data for a host, etc.

Ganglia makes me happy because I can worry only about giving it data. Adding a new data source only requires knowing how to get the data and feed it periodically to gmetric. After that, the data is automatically available in Ganglia's web interface. I fed (with gmetric) some fake data about mysql connections to see what happens, and this was the result.-

The wish-list of Ganglia features includes a talk of making it easy to provide custom graphs and views. Adding your own custom graphs requires, currently, hacking on the php code that presents the web interface. Glancing at the PHP powering the web interface doesn't make me cringe (easy to read!), so extending ganglia by adding your own views is probably reasonable.

Considering what I've found so far with Ganglia, the shortcomings mentioned here are easily worked around, and the potential benefits of less-painfully-configured data trending and monitoring seem quite good.

Oh, by the way, gmond works in Windows. The project has a cygwin binary that works by itself and can report data about a windows host. You can also use gmetric from windows hosts to report info about themselves.

Further reading:

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