ロギングにSystemD JournalHandlerを使用するPythonで書かれた単純なサービスがあります（SysLogHandlerも同様に機能する必要があります）。
# myservice.py
import time
import logging
from systemd.journal import JournalHandler
log = logging.getLogger('demo')
log.addHandler(JournalHandler())
log.setLevel(logging.DEBUG)
while True:
log.debug("debug")
log.info("info")
log.warning("warning")
log.error("error")
time.sleep(3)
このサービスを実行する SystemD 構成は次のとおりです。
# myservice.service
[Unit]
Description=Python logging test
[Service]
ExecStart=/usr/bin/python3 .../myservice.py
Type=simple
[Install]
WantedBy=multi-user.target
これで、常にすべてがJournalDに書き込まれます。
# journalctl -f
Mar 06 15:58:13 myhost .../myservice.py[12852]: debug
Mar 06 15:58:13 myhost .../myservice.py[12852]: info
Mar 06 15:58:13 myhost .../myservice.py[12852]: warning
Mar 06 15:58:13 myhost .../myservice.py[12852]: error
実行時にこのSystemDサービスのロギング設定を変更できますか？後ろにすでに起動しています（DEBUGレベル）。サービスを再ロードまたは再起動する必要はありませんか？ Pythonコードでログレベルを完全に設定せずにSystemDにログレベルを処理させたいと思います。
基本的に私はこれを次のように呼びたいと思います。
# Log everything to JournalD
change-systemd-service-loglevel myservice DEBUG
# Ignore all logs < WARNING, these should not show up on journalctl
change-systemd-service-loglevel myservice WARNING
答え1
代わりに、ログに書き込む内容を選択すると、通常はすべての内容を記録し、ログから目的の内容をフィルタリングできます。これにより、問題が発生しても問題を再現することなくデバッグ内容を読み取ることができます。
journalctl --priorityフィルタログレベルを使用できます。man journalctl説明する：
-p, --priority=
Filter output by message priorities or priority ranges. Takes
either a single numeric or textual log level (i.e. between
0/"emerg" and 7/"debug"), or a range of numeric/text log levels
in the form FROM..TO. The log levels are the usual syslog log
levels as documented in syslog(3), i.e. "emerg" (0),
"alert" (1), "crit" (2), "err" (3), "warning" (4), "notice" (5),
"info" (6), "debug" (7). If a single log level is specified, all
messages with this log level or a lower (hence more important)
log level are shown. If a range is specified, all messages within
the range are shown, including both the start and the end value
of the range. This will add "PRIORITY=" matches for the specified
priorities.
デモンストレーションするために、数秒間Pythonスクリプトを実行しました。私の端末には灰色、白、琥珀、赤がdebugあります。infowarningerror
ケース1：フィルタはありません。
$ journalctl --user -u pylog --no-hostname
Mar 07 08:54:03 systemd[1064]: Started Python logging test.
Mar 07 08:54:03 /home/stew/bin/pylog.py[953165]: debug
Mar 07 08:54:03 /home/stew/bin/pylog.py[953165]: info
Mar 07 08:54:03 /home/stew/bin/pylog.py[953165]: warning
Mar 07 08:54:03 /home/stew/bin/pylog.py[953165]: error
Mar 07 08:54:06 /home/stew/bin/pylog.py[953165]: debug
Mar 07 08:54:06 /home/stew/bin/pylog.py[953165]: info
Mar 07 08:54:06 /home/stew/bin/pylog.py[953165]: warning
Mar 07 08:54:06 /home/stew/bin/pylog.py[953165]: error
ケース2：警告フィルタ（詳細構文）
$ journalctl --user -u pylog --no-hostname --priority=warning
Mar 07 08:54:03 /home/stew/bin/pylog.py[953165]: warning
Mar 07 08:54:03 /home/stew/bin/pylog.py[953165]: error
Mar 07 08:54:06 /home/stew/bin/pylog.py[953165]: warning
Mar 07 08:54:06 /home/stew/bin/pylog.py[953165]: error
ケース3：情報と警告範囲フィルタ（簡単な構文）
$ journalctl --user -u pylog --no-hostname -p 4..6
Mar 07 08:54:03 systemd[1064]: Started Python logging test.
Mar 07 08:54:03 /home/stew/bin/pylog.py[953165]: info
Mar 07 08:54:03 /home/stew/bin/pylog.py[953165]: warning
Mar 07 08:54:06 /home/stew/bin/pylog.py[953165]: info
Mar 07 08:54:06 /home/stew/bin/pylog.py[953165]: warning
特定のレベルのログ記録を実際に防止するには、次を参照してくださいman systemd.exec。
LogLevelMax=
Configures filtering by log level of log messages generated by
this unit. Takes a syslog log level, one of emerg (lowest log
level, only highest priority messages), alert, crit, err,
warning, notice, info, debug (highest log level, also lowest
priority messages). See syslog(3) for details. By default no
filtering is applied (i.e. the default maximum log level is
debug). Use this option to configure the logging system to drop
log messages of a specific service above the specified level. For
example, set LogLevelMax=info in order to turn off debug logging
of a particularly chatty unit. Note that the configured level is
applied to any log messages written by any of the processes
belonging to this unit, as well as any log messages written by
the system manager process (PID 1) in reference to this unit,
sent via any supported logging protocol. The filtering is applied
early in the logging pipeline, before any kind of further
processing is done. Moreover, messages which pass through this
filter successfully might still be dropped by filters applied at
a later stage in the logging subsystem. For example,
MaxLevelStore= configured in journald.conf(5) might prohibit
messages of higher log levels to be stored on disk, even though
the per-unit LogLevelMax= permitted it to be processed.
man journal.conf。ここにあります：
MaxLevelStore=
Controls the maximum log level of messages that are stored in the
journal. As argument, takes one of "emerg", "alert", "crit",
"err", "warning", "notice", "info", "debug", or integer values in
the range of 0–7 (corresponding to the same levels). Messages
equal or below the log level specified are stored/forwarded,
messages above are dropped. Defaults to "debug" to ensure that
the all messages are stored in the journal and forwarded to
syslog. These settings may be overridden at boot time with the
kernel command line options "systemd.journald.max_level_store="
ただし、リアルタイムで構成することはできません。リアルタイム構成の場合は、すべてを記録し、フィルターを使用して読みます。