Using watchdog and sd-notify functionality for systemd in Python 3

systemd python

Revision history

I published my first package to Python Package Index (PyPI) today after my package has been tested for a while.

Publishing packages to PyPI was pretty simple using the official guide, but I created a Makefile to help me out the next time around.

The package itself is a tiny client library to interface with the systemd watchdog functionality and sd-notify(3). I wrote a similar post for a C integration on the Hackeriet blog some time ago.

A systemd service of Type=notify waits for the executable program to send a notification message to systemd before it is considered activated. Up until the service is active, its state is starting. systemctl start will block until the service is active, or failed.

Similarly, a service which has WatchdogSec set will expect to receive a notification message no less than at every specified time interval. If no message has been received, systemd will kill the process with SIGABRT and place the service in a failed state.

Example

Install the library

$ pip install sd-notify

Create a Python script file with the following code taken from the project readme.

# /home/sshow/tmp/sd-notify-test.py
import time
import sd_notify

notify = sd_notify.Notifier()
if not notify.enabled():
    # Then it's probably not running is systemd with watchdog enabled
    raise Exception("Watchdog not enabled")

# Report a status message
notify.status("Initialising my service...")
time.sleep(3)

# Report that the program init is complete
notify.ready()
notify.status("Waiting for web requests...")
time.sleep(3)

# Report an error to the service manager
notify.notify_error("An irrecoverable error occured!")
# The service manager will probably kill the program here
time.sleep(3)

Then create a systemd.service file to run and watchdog this script. I created this in my systemd user directory.

$ systemctl --user cat sd-notify-test.service
# /home/sshow/.config/systemd/user/sd-notify-test.service
[Unit]
Description=Watchdog test service

[Service]
ExecStart=/usr/bin/python3 /home/sshow/tmp/sd-notify-test.py
Type=notify
WatchdogSec=15
Restart=on-failure
RestartSec=10

[Install]
WantedBy=default.target

Then start the service

$ systemctl --user daemon-reload
$ systemctl --user start sd-notify-test

You can follow the log output of the user service with journalctl

$ journalctl --user-unit sd-notify-test
jan. 26 11:11:04 systemd[620]: Stopped Watchdog test service.
jan. 26 11:11:04 systemd[620]: Starting Watchdog test service...
jan. 26 11:11:07 systemd[620]: Started Watchdog test service.
jan. 26 11:11:10 systemd[620]: sd-notify-test.service: Watchdog request (last status: An irrecoverable error occured!)!
jan. 26 11:11:10 systemd[620]: sd-notify-test.service: Killing process 358538 (python3) with signal SIGABRT.
jan. 26 11:11:10 systemd[620]: sd-notify-test.service: Main process exited, code=dumped, status=6/ABRT
jan. 26 11:11:10 systemd[620]: sd-notify-test.service: Failed with result 'watchdog'.
jan. 26 11:11:10 systemd-coredump[358542]: Process 358538 (python3) of user 1000 dumped core.

                                           Stack trace of thread 358538:...

Another way to look at a service status message is by running systemctl

$ systemctl --user status sd-notify-test.service
● sd-notify-test.service - Watchdog test service
     Loaded: loaded (/home/sshow/.config/systemd/user/sd-notify-test.service; disabled; vendor preset: enabled)
     Active: active (running) since Thu 2020-01-26 11:15:46 CET; 1s ago
   Main PID: 358886 (python3)
     Status: "Waiting for web requests..."
     CGroup: /user.slice/user-1000.slice/user@1000.service/sd-notify-test.service
             └─358886 /usr/bin/python3 /home/sshow/tmp/sd-notify-test.py

jan. 26 11:15:43 <hostname> systemd[620]: Stopped Watchdog test service.
jan. 26 11:15:43 <hostname> systemd[620]: Starting Watchdog test service...
jan. 26 11:15:46 <hostname> systemd[620]: Started Watchdog test service.

If you have any issues, please post to the issue tracker of the source repository.

If you have any comments or feedback, please send me an e-mail. (stig at stigok dotcom)