Python and Systemd

Systemd

Recently I have been tweaking a little bit of my small programs that run as daemons started/managed via systemd.

Systemd is an initialization and process and service management system for Linux kernel-based operating systems. It was introduced as an alternative to the traditional System V-based initialization system (sysvinit) and offers a number of advanced features. Systemd has many tasks and functions, including system initialization, service management, process monitoring or event log.

For me, the most important thing was the logging part, or so-called journal. Systemd Journal is the journaling (logging) system used by systemd on Linux systems. It replaces the traditional text log files (e.g. /var/log/messages) used in older initialization systems.

Journal provides features such as binary format, indexing, encryption, metadata. And most importantly for me, there is a simple way to manipulate journal data using python.

Python and systemd-journal

To use systemd journal using Python, you can use the systemd.journal module, which provides an interface for reading and writing the systemd journal. The systemd.journal module is part of the systemd-python package. Github: https://github.com/systemd/python-systemd

from systemd import journal

# Zapisz wpis do dziennika
journal.send('Wiadomość logu', SYSLOG_IDENTIFIER='my_app', PRIORITY=journal.LOG_INFO)

# Zapisz wpis z dodatkowymi metadanymi
journal.send('Inna wiadomość logu', SYSLOG_IDENTIFIER='my_app', PRIORITY=journal.LOG_DEBUG, MY_CUSTOM_FIELD='Custom value')

We can also use various methods, such as seek_realtime(), seek_monotonic() and add_match(), to filter log entries based on various criteria, such as time interval, logging level, process ID, etc.

To write a log entry, we use the journal.send() function and pass the log message along with optional metadata such as SYSLOG_IDENTIFIER (application identifier), PRIORITY (logging priority), etc.

Using the systemd.journal module, you can flexibly operate on the systemd log and customize it to your needs in an application written in Python.