Python daemon and systemd service

Question

I have a simple Python script working as a daemon. I am trying to create systemd script to be able to start this script during startup.

Current systemd script:

[Unit] Description=Text After=syslog.target  [Service] Type=forking User=node Group=node WorkingDirectory=/home/node/Node/ PIDFile=/var/run/zebra.pid ExecStart=/home/node/Node/node.py  [Install] WantedBy=multi-user.target 

node.py:

if __name__ == '__main__':     with daemon.DaemonContext():         check = Node()         check.run() 

run contains while True loop.

I try to run this service with systemctl start zebra-node.service. Unfortunately service never finished stating sequence - I have to press Ctrl+C. Script is running, but status is activating and after a while it change to deactivating. Now I am using python-daemon (but before I tried without it and the symptoms were similar).

Should I implement some additional features to my script or is systemd file incorrect?

Answer 1

The reason, it does not complete the startup sequence is, that for Type forking your startup process is expected to fork and exit (see $ man systemd.service - search for forking).

Simply use only the main process, do not daemonize

One option is to do less. With systemd, there is often no need to create daemons and you may directly run the code without daemonizing.

#!/usr/bin/python -u from somewhere import Node check = Node() check.run() 

This allows using simpler Type of service called simple, so your unit file would look like.

[Unit] Description=Simplified simple zebra service After=syslog.target  [Service] Type=simple User=node Group=node WorkingDirectory=/home/node/Node/ ExecStart=/home/node/Node/node.py StandardOutput=syslog StandardError=syslog  [Install] WantedBy=multi-user.target 

Note, that the -u in python shebang is not necessary, but in case you print something out to the stdout or stderr, the -u makes sure, there is no output buffering in place and printed lines will be immediately caught by systemd and recorded in journal. Without it, it would appear with some delay.

For this purpose I added into unit file the lines StandardOutput=syslog and StandardError=syslog. If you do not care about printed output in your journal, do not care about these lines (they do not have to be present).

systemd makes daemonization obsolete

While the title of your question explicitly asks about daemonizing, I guess, the core of the question is "how to make my service running" and while using main process seems much simpler (you do not have to care about daemons at all), it could be considered answer to your question.

I think, that many people use daemonizing just because "everybody does it". With systemd the reasons for daemonizing are often obsolete. There might be some reasons to use daemonization, but it will be rare case now.

EDIT: fixed python -p to proper python -u. thanks kmftzg

Answer 2

It is possible to daemonize like Schnouki and Amit describe. But with systemd this is not necessary. There are two nicer ways to initialize the daemon: socket-activation and explicit notification with sd_notify().

Socket activation works for daemons which want to listen on a network port or UNIX socket or similar. Systemd would open the socket, listen on it, and then spawn the daemon when a connection comes in. This is the preferred approch because it gives the most flexibility to the administrator. [1] and [2] give a nice introduction, [3] describes the C API, while [4] describes the Python API.

[1] http://0pointer.de/blog/projects/socket-activation.html
[2] http://0pointer.de/blog/projects/socket-activation2.html
[3] http://www.freedesktop.org/software/systemd/man/sd_listen_fds.html
[4] http://www.freedesktop.org/software/systemd/python-systemd/daemon.html#systemd.daemon.listen_fds

Explicit notification means that the daemon opens the sockets itself and/or does any other initialization, and then notifies init that it is ready and can serve requests. This can be implemented with the "forking protocol", but actually it is nicer to just send a notification to systemd with sd_notify(). Python wrapper is called systemd.daemon.notify and will be one line to use [5].

[5] http://www.freedesktop.org/software/systemd/python-systemd/daemon.html#systemd.daemon.notify

In this case the unit file would have Type=notify, and call systemd.daemon.notify("READY=1") after it has established the sockets. No forking or daemonization is necessary.