An example inetd-like socket-activated service. #systemd #inetd #systemd.socket

README-setup-socket-activated-systemd-service.md

Linux sockets and system d

Define a socket unit

The key point here is to specify Accept=yes, which will make the socket accept connections (behaving like inetd) and pass only the resulting connection socket to the service handler.

Create /etc/systemd/system/baz.socket:

[Unit]
Description=Baz Socket

[Socket]
ListenStream=/tmp/baz.socket
Accept=yes

[Install]
WantedBy=sockets.target

Define a service template unit

We now create a service template from which the actual service will be instantiated on-demand. The lifetime of this service is usually very short (thus it may not appear at systemctl).

Create /etc/systemd/system/baz@.service:

[Unit]
Description=Baz Service
Requires=baz.socket

[Service]
Type=simple
ExecStart=/usr/bin/python /opt/baz-service/serve.py %i
StandardInput=socket
StandardError=journal
TimeoutStopSec=5

[Install]
WantedBy=multi-user.target

An example service handler would be (located at /opt/baz-service/serve.py as specified at the service unit file):

#!/usr/bin/python
import sys
import logging
logging.basicConfig(level=logging.INFO)

instance = sys.argv[1]

# The connected socket is duplicated to stdin/stdout
data = sys.stdin.readline().strip()
logging.info('baz-service: at instance %s, got request: %s', instance, data)
sys.stdout.write(data.upper() + '\r\n')

Test

Open the journal in following mode:

sudo journalctl -f

Startthe socket:

sudo systemctl start baz.socket

Verify it via:

sudo systemctl status baz.socket

Make a request to the service:

echo "Hello World" | nc -U /tmp/baz.sock

You should see in the jnournalctl the python logger with the "Hello World".