Temperature Logging: Redux

Previously as I was experimenting with logging the temperature using a Raspberry Pi (to monitor the temperatures experienced by fermenting cider), I noted that the Pi was something of a terrible hack, and it should be possible to do more efficiently with some slightly less common hardware.

I decided that improved version would be interesting to build for use at home, since it’s both kind of fun to collect data like that, and actually knowing the temperature in the house is useful at times. The end result of this project is that I can generate graphs like the one below of conditions around the house:

Software requirements

My primary requirement for home monitoring of this sort is that it not depend on a proprietary hub (especially not one that depends on an external service that might go away without warning), and I’d also like something that can be integrated with my existing (but minimal) home automation setup that’s based around Home Assistant running on my home server.

Given my main software is open source it should be possible to integrate an arbitrary solution with it, with varying amount of reverse engineering and implementation necessary. Because reverse-engineering services like that is not my idea of fun, it’s much preferable to find something that’s already supported and take advantage of others’ work. While I don’t mind debugging, I don’t want to build an integration from scratch if I don’t need to.

Hardware selection

As observed last time, the “hub” model for connecting “internet of things” devices to a network seems to be the best choice from a security standpoint- the only externally-visible network device is the hub, which can apply arbitrary security policies to communications between devices and to public networks (in the simplest case, forbidding all communications with public networks). Indeed, recent scholarly work (PDF) suggests systems that work on this model but apply more sophisticated policies to communications passing through the hub.

With that in mind, I decided a Zigbee network for my sensors would be appropriate- the sensors themselves have no ability to talk to public networks because they don’t even run an Internet Protocol stack, and it’s already a fairly common standard for communication. Plus, I was able to get several of the previously-mentioned Xiaomi temperature, humidity and barometric pressure sensors for about $10 each; a quite reasonable cost, given they’re battery powered with very long life and good wireless range. Home assistant already has some support for Zigbee devices; most relevant here seems to be its implementation of the Zigbee Home Automation application standard. Though the documentation isn’t very clear, it supports (or, should support) any radio that communicates with a host processor over a UART interface and speaks either the XBee or EZSP serial protocol. Since the documentation for Home Assistant specifically notes that the Elelabs Zigbee USB adapter is compatible, I bought one of those. Its documentation includes a description of how to configure Home Assistant with it and specifically mentions Xiaomi Aqara devices (which includes the sensors I had selected), so I was confident that radio would meet my needs, though unsure of exactly what protocol was actually used to communicate with the radio over USB at the time I ordered it. Experimenting Once I received the Zigbee radio-on-a-usb-stick, I immediately tried to manually drive it using whatever libraries I could use to set up a network and get one of my sensors connected to it. This ended up not working, but I did learn a lot about how the radio adapter is meant to work. For working with it in Python, the Elelabs documentation points to bellows, a library providing EZSP protocol support for the zigpy Zigbee stack. It also includes a command-line interface exposing some basic commands, perfect for the sort of experimentation I wanted to do. Getting connected was easy; I plugged the USB stick into my Linux workstation and it appeared right away as a PL2303 USB-to-serial converter. Between this and noting that bellows implements the EZSP protocol, I inferred that the Elelabs stick is a Silicon Labs EM35x microcontroller running the EmberZNet stack in a network coordinator mode, with a PL2303 exposing a UART over USB so the host can communicate with the microcontroller (and the rest of the network) by speaking EZSP. Having worked that out and made sense of it, I printed out a label for the stick that says what it is (“Elelabs Zigbee USB adapter”) and how to communicate with it (EZSP at 57600 baud) since the stick is completely unmarked otherwise and being able to tell what it does just by looking at it is very helpful. Trying to use the bellows CLI, the status output seemed okay and the NCP was running. In order to connect one of my sensors, I then needed to figure out how to make the sensor join the network after using bellows permit to let new devices join the network. The sensors each came with a little instruction booklet, but it was all in Chinese. With the help of Google Translate, I was able to take photos of it and find the important bit- holding the button on the sensor for about 5 seconds until the LED blinks three times will reset it, at which point it will attempt to join an open network. On trying to run bellows permit prior to resetting a sensor to get it on the network, I encountered an annoying bug- it didn’t seem to do anything, and Python emitted a warning: RuntimeWarning: coroutine 'permit' was never awaited. I dug into that a little more and found the libraries make heavy use of PEP 492 coroutines, and the warning was fairly clear that a function was declared async when it shouldn’t have been (or its coroutine wasn’t then given to an event loop) so the function actually implementing permit never ran. I eventually tracked down the problem, patched it locally and filed a bug which has since been fixed. Having fixed that bug, I continued to try to get a sensor on my toy network but was ultimately (apparently) unsuccessful. I could permit joins and reset the sensor and see debug output indicating something was happening on the network, but never any conclusive messages saying a new device had joined and rather a lot of messages along the lines of “unrecognized message.” I couldn’t tell if it was working or not, so moved on to hooking up Home Assistant. Setting up Home Assistant Getting set up with Home Assistant was mostly just a matter of following the guide provided with the USB stick, but using my own knowledge of how to set up the software (not using hassio). Configuring the zha component and pointing it at the right USB path is pretty easy. I did discover that specifying the database_path for the zha component alone is not enough to make it work; if the file doesn’t already exist setup just fails. Simply creating an empty file at the configured path is enough- apparently that file is an sqlite database that zigpy uses to track known devices. Still following the Elelabs document, I spent a bit of time invoking zha.permit and trying to get a sensor online to no apparent success. After a little more searching, I found discussion on the Home Assistant forums and in particular one user suggesting that these particular sensors are somewhat finicky when joining a network. They suggested (and my findings agree) that holding the button on the sensor to reset it, then tapping the button approximately every second for a little while (another 5-10 seconds) will keep it awake long enough to successfully join the network. The keep-awake tapping approach did eventually work, though I also found that Home Assistant sometimes didn’t show a new sensor (or parts of a new sensor, like it might show the temperature but not humidity or pressure) until I restarted it. This might be a bug or a misconfiguration on my part, but it’s minor enough not to worry about. At this point I’ve verified that my software and hardware can all work, so it’s time to set up the permanent configuration. Permanent configuration As mentioned above, I run Home Assistant on my Linux home server. Since I was already experimenting on a Linux system, that configuration should be trivial to transfer over, but for one additional desire I had: I want more freedom in where I place the Zigbee radio, in particular not just plugged directly into a free USB port on the server. Putting it in a reasonably central location with other radios (say, near the WiFi router) would be nice. A simple solution might be a USB extension cable, but I didn’t have any of those handy and strewing more wires about the place feels inelegant. My Internet router (a TP-Link Archer C7 running OpenWrt) does have an available USB port though, so I suspected it would be possible to connect the Zigbee radio to the router and make it appear as a serial port on the server. This turned out to be true! Serial over network To find the solution for running a serial port over the network, I first searched for existing protocols; it turns out there’s a standard one that’s somewhat commonly used in fancy networking equipment, specified by RFC 2217. RFC 2217 specifies a set of extensions to Telnet allowing serial port configuration (bit rate, data bits, parity, etc) and flow control over Telnet. Having identified a protocol that does what I want, it’s then a matter of finding software that works as a client (assuming I’ll be able to find or write a suitable server). Suitable clients are somewhat tricky however, since from an applicaton perspective UART use on Linux involves making specialized ioctls to the device to configure it, then reading and writing bytes as usual. Making an RFC2217 network serial device appear like a local device would seem to involve writing a kernel driver that exports a new class of RFC2217 device nodes supporting the relevant ioctls- none exists.1 An alternate approach (not using RFC 2217) might be USB/IP, which is supported in mainline Linux and allows a server to bind USB devices physically connected to it to a virtual USB controller that can then be remotely attached to a different physical machine over a network. This seems like a more complex and potentially fragile solution though, so I put that aside after learning of it. Since Linux doesn’t have any kernel-level support for remote serial ports, I needed to search for support at the application level. It turns out bellows uses pyserial to communicate with radios, and pySerial is a quite featureful library- while most users will only ever provide device names like COM1 or /dev/ttyUSB0, it supports a range of more exotic URLs specifying connections, including RFC 2217. So given a suitable server running on a remote machine, I should be able to configure Home Assistant to use a URL like rfc2217://zigbee.local:25 to reach the Zigbee radio. Serial server The next step in setting up the Zigbee radio plugged into the router is finding an application that can expose a PL2303 over the network with the RFC 2217 protocol. That turned out to be a short search, where I quickly discovered ser2net which does the job and is already packaged for OpenWRT. Installing it on the router was trivial, though I also needed to be sure the kernel module(s) required to expose the USB-serial port were available: # opkg install kmod-usb-serial-pl2303 ser2net Having installed ser2net, I still had to figure out how to configure it. While the documentation describes its configuration format, I know from experience that configuring servers on OpenWRT is usually done differently (as something of a concession to targeting embedded systems without much storage). I quickly found that the package had installed a sample configuration file at /etc/config/ser2net: config ser2net global option enabled 1 config controlport option enabled 0 option host localhost option port 2000 config default option speed 115200 option databits 8 option parity 'none' option stopbits 1 option rtscts false option local false option remctl true config proxy option enabled 0 option port 5000 option protocol telnet option timeout 0 option device '/dev/ttyAPP0' option baudrate 115200 option databits 8 option parity 'none' option stopbits 1 # option led_tx 'tx' # option led_rx 'rx' option rtscts false option local false option xonxoff false Unfortunately, this configuration doesn’t include any comments so the reader is force to guess the meaning of each option. They mostly correspond to words that appear in the ser2net manual, but I didn’t trust guesses so went digging in the OpenWRT packages source code and found the script responsible for converting /etc/config/ser2net into an actual configuration file when starting ser2net. My initial guess at the configuration I wanted looked something like this: config proxy option enabled 1 option port 5000 option protocol telnet option timeout 0 option device '/dev/ttyUSB0' option baudrate 57600 option remctl true The protocol is specified as telnet because RFC 2217 is a layer on top of telnet (my first guess was that I actually wanted raw until actually reading the RFC and seeing it was a set of telnet extensions), and the device is the device name that I found the Zigbee stick appeared as when plugged into the router.2 Unfortunately, this configuration didn’t work and pyserial gave gack a somewhat perplexing error message: serial.serialutil.SerialException: Remote does not seem to support RFC2217 or BINARY mode [we-BINARY:False(INACTIVE), we-RFC2217:False(REQUESTED)]. Without much visibility into what the serial driver was trying to do, I opted to examine the network traffic with Wireshark. I first attempted to use the text-mode interface (tshark -d tcp.port==5000,telnet -f 'port 5000'), but quickly gave up and switched to the GUI instead. I captured the traffic passing between the server and router, but there was almost nothing! The client (pyserial) was sending some Telnet negotiation messages (DO ECHO, WILL suppress go ahead and COM port control), then nothing happened for a few seconds and the connection closed. Since restarting Home Assistant for every one of these serial tests was quite cumbersome, at this point I checked if pyserial includes any programs suitable for testing connectivity. It happily does, provided in my distribution’s package as miniterm.py. Running miniterm.py rfc2217://c7:5000 failed in the same way, so I had a quicker debugging tool. At this point the problem seems like it’s at the server side, so I stopped the ser2net server on the router and started one in the foreground, with a custom configuration specified on the command line: $ /etc/init.d/ser2net stop
$ser2net -n -d -C '5000:telnet:0:/dev/ttyUSB0:57600 remctl' ser2net[14914]: Unable to create network socket(s) on line 0 While ser2net didn’t outright fail, it did print a concerning error message. Does it work if I change the port it’s listening on? $ ser2net -n -d -C '1234:telnet:0:/dev/ttyUSB0:57600 remctl'

And then running miniterm.py succeeds, leaving me with a terminal I could type into (but didn’t, since I don’t know how to speak EZSP with my keyboard).

$miniterm.py rfc2217://c7:1234 57600 --- Miniterm on rfc2217://c7:1234 57600,8,N,1 --- --- Quit: Ctrl+] | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H --- --- exit --- I discovered after a little digging (netstat -lnp) that miniupnpd was already listening on port 5000 of the router, so changing the port fixes the confusing problem. A different sample port in the ser2net configuration would have prevented such an issue, as would ser2net giving up when it fails to bind to a requested port instead of printing a message and pretending nothing happened. But at least I didn’t have to patch anything to make it work. With ser2net listening on port 2525 instead, Home Assistant can connect to it (hooray!). But it immediately throws a different error: NotImplementedError: write_timeout is currently not supported. I’ve found another bug in a rarely-exercised corner of this software stack, have I? Well, kind of. Finding that error message in the pyserial source, something is trying to set the write timeout to zero and it’s simply not implemented in pyserial for RFC2217 connections. This is ultimately because Home Assistant (as alluded to earlier with bellows and zigpy) is all coroutine-based so it uses pyserial-asyncio to adapt the blocking APIs provided by pyserial to something that works nicely with coroutines running on an event loop. When pyserial-asyncio tries to set non-blocking mode by making the timeout zero, we find it’s not supported. def _reconfigure_port(self): """Set communication parameters on opened port.""" if self._socket is None: raise SerialException("Can only operate on open ports") # if self._timeout != 0 and self._interCharTimeout is not None: # XXX if self._write_timeout is not None: raise NotImplementedError('write_timeout is currently not supported') # XXX While I could probably implement non-blocking support for RFC 2217 in pyserial, that seemed rather difficult and not my idea of fun. So instead I looked for a workaround- if RFC 2217 won’t work, does pyserial support a protocol that will? The answer is of course yes: I can use socket:// for a raw socket connection to the ser2net server. This sacrifices the ability to change UART parameters (format, baud rate, etc) on the fly, but since the USB stick doesn’t support changing parameters on the fly anyway (as far as I can tell), this is no problem. Final configuration The ser2net configuration that I’m now using looks like this: config proxy option enabled 1 option port 2525 option protocol raw option timeout 0 option device '/dev/ttyUSB0' option baudrate 57600 option remctl 0 And the relevant stanza in Home Assistant configuration: (The baud rate needs to be specified, but pyserial ignores it for socket:// connections.) zha: usb_path: 'socket://c7:2525' database_path: /srv/homeassistant/.homeassistant/zigbee.db baudrate: 57600 After ensuring the zigbee.db file exists and restarting Home Assistant to reload the configuration, I was able to pair all three sensors by following the procedure defined above: call the permit service in Home Assistant, then reset the sensor by holding the button until its LED blinks three times, then tap the button every second or so for a bit. I did observe some strange behavior on pairing the sensors that made me think they weren’t pairing correctly, like error messages in the log (ERROR (MainThread) [homeassistant.components.sensor] Setup of platform zha is taking longer than 60 seconds. Startup will proceed without waiting any longer.) and some parts of each sensor not appearing (the temperature might be shown, but not humidity or pressure). Restarting Home Assistant after pairing the sensors made everything appear as expected though, so there may be a bug somewhere in there but I can’t be bothered to debug it since there was a very easy workaround. Complaining about async I/O It’s rather interesting to me that the major bugs I encountered in trying to set up this system in a slightly unusual configuration were related to asynchronous I/O running in event loops- this is an issue that’s become something of my pet problem, such that I will argue to just about anybody who will listen that asynchronous I/O is usually unnecessary and more difficult to program. That I discovered two separate bugs in the tools that make this work relating to running asynchronous I/O in event loops seems to support that conclusion. If Home Assistant simply spawned threads for components I believe it would simplify individual parts (perhaps at the cost of some slightly more complex low-level communication primitives) and make the system easier to debug. Instead, it runs all of its dependencies in a way they are not well-exercised in, presumably in search of “maximum performance” that seems entirely irrelevant when considering the program’s main function is acting as a hub for a variety of external devices. I have (slowly) been working on distilling all these complaints into a series of essays on the topic, but for now this is a fine opportunity to wave a finger at something that I think is strictly worse because it’s evented. Conclusion I’m pretty happy with the sensors and software configuration I have now- the sensors are tiny and unobtrusive, while the software does a fine job of logging data and presenting live readings for my edification. I’d like to also configure a “real” database like InfluxDB to store my sensor readings over arbitrarily long time periods (since Home Assistant doesn’t remember data forever, reasonably so), which shouldn’t be too difficult (it’s supported as a module) but is somewhat unrelated to setting up Zigbee sensors in the first place. Until then, I’m pretty happy with these results despite the fact that I think the developers have made a terrible choice with evented I/O. 1. I did find somebody asking for input on the implementation of exactly that, but it looks like nothing ever came of it. A reply suggesting an application at the master end of a pty (pseudoterminal) suggests an interesting alternate option, but it doesn’t appear to be possible to receive parameter change requests from a pty (though flow control is exposed when running in “packet mode”). [return] 2. I was concerned at the outset that the router might be completely unable to see the Zigbee stick, since apparently the Archer C7 doesn’t include a USB 1.1 OHCI or UHCI controller, so it’s incapable of communicating at all with low-speed devices like keyboards! I’ve heard (but not verified myself) that connecting a USB 2.0 hub will allow the router to communicate with low-speed devices downstream of the hub as a workaround. [return] Building a terrible 'IoT' temperature logger I had approximately the following exchange with a co-worker a few days ago: Them: “Hey, do you have a spare Raspberry Pi lying around?” Me: [thinks] “..yes, actually.” T: “Do you want to build a temperature logger with Prometheus and a DS18B20+? M: “Uh, okay?” It later turned out that that co-worker had been enlisted by yet another individual to provide a temperature logger for their project of brewing cider, to monitor the temperature during fermentation. Since I had all the hardware at hand (to wit, a Raspberry Pi 2 that I wasn’t using for anything and temperature sensors provided by the above co-worker), I threw something together. It also turned out that the deadline was quite short (brewing began just two days after this initial exchange), but I made it work in time. Interfacing the thermometer As noted above, the core of this temperature logger is a DS18B20 temperature sensor. Per the manufacturer: The DS18B20 digital thermometer provides 9-bit to 12-bit Celsius temperature measurements … communicates over a 1-Wire bus that by definition requires only one data line (and ground) for communication with a central microprocessor. … Each DS18B20 has a unique 64-bit serial code, which allows multiple DS18B20s to function on the same 1-Wire bus. Thus, it is simple to use one microprocessor to control many DS18B20s distributed over a large area. Indeed, this is a very easy device to interface with. But even given the svelte hardware needs (power, data and ground signals), writing some code that speaks 1-Wire is not necessarily something I’m interested in. Fortunately, these sensors are very commonly used with the Raspberry Pi, as illustrated by an Adafruit tutorial published in 2013. The Linux kernel provided for the Pi in its default Raspbian (Debian-derived) distribution supports bit-banging 1-Wire over its GPIOs by default, requiring only a device tree overlay to activate it. This is as simple as adding a line to /boot/config.txt to make the machine’s boot loader instruct the kernel to apply a change to the hardware configuration at boot time: dtoverlay=w1-gpio With that configuration, one simply needs to wire the sensor up. The w1-gpio device tree configuration by default uses GPIO 4 on the Pi as the data line, then power and grounds need to be connected and a pull-up resistor added to the data line (since 1-Wire is an open-drain bus). The w1-therm kernel module already understands how to interface with these sensors- meaning I don’t need to write any code to talk to the temperature sensor: Linux can do it all for me! For instance, reading the temperature out in an interactive shell to test, after booting with the 1-Wire overlay enabled: $ modprobe w1-gpio w1-therm
$cd /sys/bus/w1/devices$ ls
28-000004b926f1  w1_bus_master1

Temperature exporter

Having connected the thermometer to the Pi and set up Prometheus, we now need to glue them together such that Prometheus can read the temperature. The usual way is for Prometheus to make HTTP requests to its known data sources, where the response is formatted such that Prometheus can make sense of the metrics. There is some support for having metrics sources push their values to Prometheus through a bridge (that basically just remembers the values it’s given until they’re scraped), but that seems inelegant given it would require running another program (the bridge) and goes against the how Prometheus is designed to work.

I’ve published the source for the metrics exporter I ended up writing, and will give it a quick description in the remnants of this section.

The easiest solution to providing a service over HTTP is using the http.server module, so that’s what I chose to use. When the program starts up it scans for temperature sensors and stores them. This has a downside of never returning data if a sensor is accidentally disconnected at startup, but detection is fairly slow and only doing it at startup makes it clearer if sensors are accidentally disconnected during operation, since reading them will fail at that point.

#!/usr/bin/env python3

import socketserver
from http.server import HTTPServer, BaseHTTPRequestHandler
from w1thermsensor import W1ThermSensor

SENSORS = W1ThermSensor.get_available_sensors()

The request handler has a method that builds the whole response at once, which is just plain text based on a simple template.

class Exporter(BaseHTTPRequestHandler):
METRIC_HEADER = ('# HELP w1therm_temperature Temperature in Kelvin of the sensor.\n'
'# TYPE w1therm_temperature gauge\n')

def build_exposition(self, sensor_states):
for sensor, temperature in sensor_states.items():
out += 'w1therm_temperature{{id="{}"}} {}\n'.format(sensor, temperature)
return out

do_GET is called by BaseHTTPRequestHandler for all HTTP GET requests to the server. Since this server doesn’t really care what you want (it only exports one thing- metrics), it completely ignores the request and sends back metrics.

    def do_GET(self):
response = self.build_exposition(self.get_sensor_states())
response = response.encode('utf-8')

# We're careful to send a content-length, so keepalive is allowed.
self.protocol_version = 'HTTP/1.1'
self.close_connection = False

self.send_response(200)
self.wfile.write(response)

The http.server API is somewhat cumbersome in that it doesn’t try to handle setting Content-Length on responses to allow clients to keep connections open between requests, but at least in this case it’s very easy to set the Content-Length on the response and correctly implement HTTP 1.1. The Content-Type used here is the one specified by the Prometheus documentation for exposition formats.

The rest of the program is just glue, for the most part. The console_entry_point function is the entry point for the w1therm_prometheus_exporter script specified in setup.py. The network address and port to listen on are taken from the command line, then an HTTP server is started and allowed to run forever.

As a server

As a Python program with a few non-standard dependencies, installation of this server is not particularly easy. While I could sudo pip install everything and call it sufficient, that’s liable to break unexpectedly if other parts of the system are automatically updated- in particular the Python interpreter itself (though Debian as a matter of policy doesn’t update Python to a different release except as a major update, so it shouldn’t happen without warning). What I’d really like is the ability to build a single standalone program that contains everything in a convenient single-file package, and that’s exactly what PyInstaller can do.

A little bit of wrestling with pyinstaller configuration later (included as the .spec file in the repository), I had successfully built a pretty heavy (5MB) executable containing everything the server needs to run. I placed a copy in /usr/local/bin, for easy accessibility in running it.

I then wrote a simple systemd unit for the temperature server to make it start automatically, installed as /etc/systemd/system/w1therm-prometheus-exporter.service:

[Unit]
Description=Exports 1-wire temperature sensor readings to Prometheus
Documentation=https://bitbucket.org/tari/w1therm-prometheus

[Service]
ExecStart=/usr/local/bin/w1therm-prometheus-exporter localhost 9000
Restart=always

StandardOutput=journal
StandardError=journal

# Standalone binary doesn't need any access beyond its own binary image and
# a tmpfs to unpack itself in.
DynamicUser=true
ProtectSystem=strict
ProtectHome=true
PrivateTmp=true

[Install]
WantedBy=multi-user.target

Enable the service, and it will start automatically when the system boots:

systemctl enable w1therm-prometheus-exporter.service

This unit includes rather more protection than is probably very useful, given the machine is single-purpose, but it seems like good practice to isolate the server from the rest of the system as much as possible.

• DynamicUser will make it run as a system user with ID semi-randomly assigned each time it starts so it doesn’t look like anything else on the system for purposes of resource (file) ownership.
• ProtectSystem makes it impossible to write to most of the filesystem, protecting against accidental or malicious changes to system files.
• ProtectHome makes it impossible to read any user’s home directory, preventing information leak from other users.
• PrivateTmp give the server its own private /tmp directory, so it can’t interfere with temporary files created by other things, nor can its be interfered with- preventing possible races which could be exploited.

Pi connectivity

Having built the HTTP server, I needed a way to get data from it to Prometheus. As discussed earlier, the Raspberry Pi with the sensor is on a WiFi network that doesn’t permit any incoming connections, so how can Prometheus scrape metrics if it can’t connect to the Pi?

One option is to push metrics to Prometheus, using the push gateway. However, I don’t like that option because the push gateway is intended mostly for jobs that run unpredictably, in particular where they can exit without warning. This isn’t true of my sensor server. PushProx provides a rather better solution, wherein clients connect to a proxy which forwards fetches from Prometheus to the relevant client, though I think my ultimate solution is just as effective and simpler.

What I ended up doing is using autossh to open an SSH tunnel at the Prometheus server which connects to the Raspberry Pi’s metrics server. Autossh is responsible for keeping the connection alive, managed by systemd. Code is going to be much more instructive here than a long-form description, so here’s the unit file:

[Unit]
Description=SSH reverse tunnel from %I for Prometheus
After=network-online.target
Wants=network-online.target

[Service]
User=autossh
ExecStart=/usr/bin/autossh -N -p 22 -l autossh -R 9000:localhost:9000 -i /home/autossh/id_rsa %i
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

Installed as /etc/systemd/system/autossh-tunnel@.service, this unit file tells systemd that we want to start autossh when the network is online and try to ensure it always stays online. I’ve increased RestartSec from the default 100 milliseconds because I found that even with the dependency on network-online.target, ssh could fail when the system was booting up with DNS lookup failures, then systemd would give up. Increasing the restart time means it takes much longer for systemd to give up, and in the meantime the network actually comes up.

The autossh process itself runs as a system user I created just to run the tunnels (useradd --system -m autossh), and opens a reverse tunnel from port 9000 on the remote host to the same port on the Pi. Authentication is with an SSH key I created on the Pi and added to the Prometheus machine in Google Cloud, so it can log in to the server without any human intervention. Teaching systemd that this should run automatically is a simple enable command away1:

systemctl enable autossh-tunnel@pitemp.example.com

Then it’s just a matter of configuring Prometheus to scrape the sensor exporter. The entire Prometheus config looks like this:

global:
scrape_interval:     15s # By default, scrape targets every 15 seconds.
evaluation_interval: 15s

scrape_configs:
- job_name: 'w1therm'
static_configs:
- targets: ['localhost:9000']

That’s pretty self-explanatory; Prometheus will fetch metrics from port 9000 on the same machine (which is actually an SSH tunnel to the Raspberry Pi), and do so every 15 seconds. When the Pi gets the request for metrics, it reads the temperature sensors and returns their values.

Data retention

I included InfluxDB in the setup to get arbitrary retention of temperature data- Prometheus is designed primarily for real-time monitoring of computer systems, to alert human operators when things appear to be going wrong. Consequently, in the default configuration Prometheus only retains captured data for a few weeks, and doesn’t provide a convenient way to export data for archival or analysis. While the default retention is probably sufficient for this project’s needs, I wanted better control over how long that data was kept and the ability to save it as long as I liked. So while Prometheus doesn’t offer that control itself, it does support reading and writing data to and from various other databases, including InfluxDB (which I chose only because a package for it is available in Debian without any additional work).

Unfortunately, the version of Prometheus available in Debian right now is fairly old- 1.5.2, where the latest release is 2.2. More problematic, while Prometheus now supports a generic remote read/write API, this was added in version 2.0 and is not yet available in the Debian package. Combined with the lack of documentation (as far as I could find) for the old remote write feature, I was a little bit stuck.

Things ended up working out nicely though- I happened to see flags relating to InluxDB in the Prometheus web UI, which mostly have no default values:

• storage.remote.influxdb-url
• storage.remote.influxdb.database = prometheus
• storage.remote.influxdb.retention-policy
• storage.remote.influxdb.username

These can be specified to Prometheus by editing /etc/defaults/prometheus, which is part of the Debian package for providing the command line arguments to the server without requiring users to directly edit the file that tells the system how to run Prometheus. I ended up with these options there:

ARGS="--storage.local.retention=720h \
--storage.remote.influxdb-url=http://localhost:8086/ \
--storage.remote.influxdb.retention-policy=autogen"

The first option just makes Prometheus keep its data longer than the default, whereas the others tell it how to write data to InfluxDB. I determined where InfluxDB listens for connections by looking at its configuration file /etc/influxdb/influxdb.conf and making a few guesses: a comment in the http section there noted that “these (HTTP endpoints) are the primary mechanism for getting data into and out of InfluxDB” and included the settings bind-address=":8086" and auth-enabled=false, so I guessed (correctly) that telling Prometheus to find InfluxDB at http://localhost:8086/ should be sufficient.

Or, it was almost enough: setting the influxdb-url and restarting Prometheus, it was logging warnings periodically about getting errors back from InfluxDB. Given the influxdb.database settings defaults to prometheus, I (correctly) assumed I needed to create a database. A little browsing of the Influx documentation and a few guesses later, I had done that:

$apt-get install influxdb-client$ influx
Visit https://enterprise.influxdata.com to register for updates, InfluxDB server management, and monitoring.
Connected to http://localhost:8086 version 1.0.2
InfluxDB shell version: 1.0.2
> CREATE DATABASE prometheus;

Examining the Prometheus logs again, now it was failing and complaining that the specified retention policy didn’t exist. Noting that the Influx documentation for the CREATE DATABASE command mentioned that the autogen retention policy will be used if no other is specified, setting the retention-policy flag to autogen and restarting Prometheus made data start appearing, which I verified by waiting a little while and making a query (guessing a little bit about how I would query a particular metric):

> USE prometheus;
> SELECT * FROM w1therm_temperature LIMIT 10;
name: w1therm_temperature
-------------------------
time                    id              instance        job     value
1532423583303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423598303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423613303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423628303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423643303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423658303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423673303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423688303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423703303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423718303000000     000004b926f1    localhost:9000  w1therm 297.9

Results

A sample graph of the temperature over two days:

The fermentation temperature is quite stable, with daily variation of less than one degree in either direction from the baseline.

Refinements

I later improved the temperature server to handle SIGHUP as a trigger to scan for sensors again, which is a slight improvement over restarting it, but not very important because the server is already so simple (and fast to restart).

On reflection, using Prometheus and scraping temperatures is a very strange way to go about solving the problem of logging the temperature (though it has the advantage of using only tools I was already familiar with so it was easy to do quickly). Pushing temperature measurements from the Pi via MQTT would be a much more sensible solution, since that’s a protocol designed specifically for small sensors to report their states. Indeed, there is no shortage of published projects that do exactly that more efficiently than my Raspberry Pi, most of them using ESP8266 microcontrollers which are much lower-power and can still connect to Wi-Fi networks.