Writing systemd gems

docs/gemstones/systemd_service_for_python_script.md

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+---
+title: `systemd` Service - Python Script
+author: Antoine Le Morvan
+contributors: Steven Spencer
+date: 2022-10-14
+tags:
+  - python
+  - systemd
+  - cron
+---
+
+# `systemd` Service for a Python script
+
+If, like many sysadmins, you are an aficionado of cron scripts launched with `* * * * * /I/launch/my/script.sh`, this article should make you think of another way to do it using all the power and ease offered by `systemd`.
+
+We will write a python script that will provide a continuous loop to execute the actions you define.
+
+We will see how to run this script as a `systemd` service, view the logs in journalctl, and see what happens if the script crashes.
+
+## Prerequisites
+
+Let's start by installing some python dependencies needed for the script to use journalctl:
+
+```
+shell > sudo dnf install python36-devel systemd-devel
+shell > sudo pip3 install systemd
+```
+
+## Writing the Script
+
+Let's consider the following script `my_service.py`:
+
+```
+"""
+Sample script to run as script
+"""
+import time
+import logging
+import sys
+from systemd.journal import JournaldLogHandler
+
+# Get an instance of the logger
+LOGGER = logging.getLogger(__name__)
+
+# Instantiate the JournaldLogHandler to hook into systemd
+JOURNALD_HANDLER = JournaldLogHandler()
+JOURNALD_HANDLER.setFormatter(logging.Formatter(
+    '[%(levelname)s] %(message)s'
+))
+
+# Add the journald handler to the current logger
+LOGGER.addHandler(JOURNALD_HANDLER)
+LOGGER.setLevel(logging.INFO)
+
+class Service(): # pylint: disable=too-few-public-methods
+    """
+    Launch an infinite loop
+    """
+    def __init__(self):
+
+        duration = 0
+
+        while True:
+            time.sleep(60)
+            duration += 60
+            LOGGER.info("Total duration: %s", str(duration))
+            # will failed after 4 minutes
+            if duration > 240:
+                sys.exit(1)
+
+if __name__ == '__main__':
+
+    LOGGER.info("Starting the service")
+    Service()
+```
+
+We start by instantiating the necessary variables to send logs in journald. The script then launches an infinite loop and pauses for 60 seconds (which is the minimum of a cron execution, so we can go below this limitation).
+
+!!! Note
+
+    I personally use this script in a more advanced form, which continuously queries a database and executes jobs based on the information retrieved via the rundeck API
+
+## Systemd Integration
+
+Now that we have a script that can serve as a basis for your imagination, we can install it as a systemd service.
+
+Let's create this file `my_service.service` and copy it to `/etc/systemd/system/`.
+
+```
+[Unit]
+Description=My Service
+After=multi-user.target
+
+[Service]
+Type=simple
+Restart=always
+ExecStart=/usr/bin/python3 my_service.py
+WorkingDirectory=/opt/my_service/
+
+StandardOutput=syslog
+StandardError=syslog
+SyslogIdentifier=my_service
+
+[Install]
+WantedBy=multi-user.target
+```
+
+As you can see, the script is launched from `/opt/my_service/`. Remember to adapt the path to your script and the syslog identifier.
+
+Launch and enable the new service:
+
+```
+shell > sudo systemctl daemon-reload
+shell > sudo systemctl enable my_service.service
+shell > sudo systemctl start my_service.service
+```
+
+## Tests
+
+We can now view the logs via journalctl:
+
+```
+shell > journalctl -f -u my_service
+oct. 14 11:07:48 rocky8 systemd[1]: Started My Service.
+oct. 14 11:07:49 rocky8 __main__[270267]: [INFO] Starting the service
+oct. 14 11:08:49 rocky8 __main__[270267]: [INFO] Total duration: 60
+oct. 14 11:09:49 rocky8 __main__[270267]: [INFO] Total duration: 120
+```
+
+Now let's see what happens if the script crashes:
+
+```
+shell > ps -elf | grep my_service
+4 S root      270267       1  0  80   0 - 82385 -      11:07 ?        00:00:00 /usr/bin/python3 my_service.py
+shell > sudo kill -9 270267
+```
+
+```
+shell > journalctl -f -u my_service
+oct. 14 11:10:49 rocky8 __main__[270267]: [INFO] Total duration: 180
+oct. 14 11:11:49 rocky8 __main__[270267]: [INFO] Total duration: 240
+oct. 14 11:12:19 rocky8 systemd[1]: my_service.service: Main process exited, code=killed, status=9/KILL
+oct. 14 11:12:19 rocky8 systemd[1]: my_service.service: Failed with result 'signal'.
+oct. 14 11:12:19 rocky8 systemd[1]: my_service.service: Service RestartSec=100ms expired, scheduling restart.
+oct. 14 11:12:19 rocky8 systemd[1]: my_service.service: Scheduled restart job, restart counter is at 1.
+oct. 14 11:12:19 rocky8 systemd[1]: Stopped My Service.
+oct. 14 11:12:19 rocky8 systemd[1]: Started My Service.
+oct. 14 11:12:19 rocky8 __main__[270863]: [INFO] Starting the service
+```
+
+We can also wait 5 minutes for the script to crash by itself: (remove this for your production)
+
+```
+oct. 14 11:16:02 rocky8 systemd[1]: Started My Service.
+oct. 14 11:16:03 rocky8 __main__[271507]: [INFO] Starting the service
+oct. 14 11:17:03 rocky8 __main__[271507]: [INFO] Total duration: 60
+oct. 14 11:18:03 rocky8 __main__[271507]: [INFO] Total duration: 120
+oct. 14 11:19:03 rocky8 __main__[271507]: [INFO] Total duration: 180
+oct. 14 11:20:03 rocky8 __main__[271507]: [INFO] Total duration: 240
+oct. 14 11:21:03 rocky8 __main__[271507]: [INFO] Total duration: 300
+oct. 14 11:21:03 rocky8 systemd[1]: my_service.service: Main process exited, code=exited, status=1/FAILURE
+oct. 14 11:21:03 rocky8 systemd[1]: my_service.service: Failed with result 'exit-code'.
+oct. 14 11:21:03 rocky8 systemd[1]: my_service.service: Service RestartSec=100ms expired, scheduling restart.
+oct. 14 11:21:03 rocky8 systemd[1]: my_service.service: Scheduled restart job, restart counter is at 1.
+oct. 14 11:21:03 rocky8 systemd[1]: Stopped My Service.
+oct. 14 11:21:03 rocky8 systemd[1]: Started My Service.
+oct. 14 11:21:03 rocky8 __main__[271993]: [INFO] Starting the service
+```
+
+As you can see, the restart feature of systemd is very useful.
+
+## Conclusion
+
+`systemd` and `journald` provide us the tools to make robust and powerful scripts easily enough to replace our old reliable crontab scripts.
+
+I hope this solution will be useful for you.