This is a bit of a fringe case, but if I don't write down what I just learned, I'll totally forget it. I got a DHT22 temperature and humidity sensor, and unlike the 1-wire DS18B20 temperature sensor, there isn't a convenient kernel module so I can't just read from the /sys filesystem. Thankfully, Adafruit has a nice guide to using the DHT22 with the Raspberry Pi, and they've got a GitHub repository with the code needed to query the sensor.
Of course, due to my love of Graphite, I need to immediately get my DHT22 not only working, but logging to Graphite, because METRICS. (funny how that word used to annoy me) I could simply modify the Adafruit code to output in Graphite plaintext format, but since I use collectd for gathering my host-based metrics anyway, let's have it do the work and submit everything to graphite together.
I could modify the python script to output in the collectd format, and call that with the Exec plugin, but since the python code needs to be run as root, I decided to keep it pretty minimal, and write a shell wrapper around it, because I know shell. ^_^
The biggest problem I ran into was getting my data to log, even though the script was working, and I discovered this limitation: with the exec data format, the identifier - the path of the metric being collected - has to have a very specific format: hostname/exec-instance/type-instance. "hostname" is pretty obvious, and is defined by COLLECTD_HOSTNAME. (as documented in the Exec plugin docs) exec-instance just has to be a unique instance name, and with this being the only exec plugin I'm running, uniqueness is easy. The last entry, type-instance has to have "type" being a valid type as defined in /usr/share/collectd/types.db, and "instance" again is any unique name. Once I changed my metric path identifier to match this standard, my stuff started logging.
Here's my modified Adafruit python script to gather the data from the DHT22:
https://github.com/ChrisHeerschap/lakehouse/blob/master/dht.py
Here's the shell script wrapper called by collectd:
https://github.com/ChrisHeerschap/lakehouse/blob/master/dht-collectd
And, here's what it looks like when the data gets into Graphite, referencing the DHT22 against a DS18B20 1-Wire sensor:
