A Sample Socket-Based Activation

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This is an example of a socket-activated per-connection service (which is usually referred to as inetd-like service).A thorough explanation can be found athttp://0pointer.de/blog/projects/inetd.html.

Define a socket unit

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

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

[Unit]Description=Baz Socket[Socket]ListenStream=127.0.0.1:9999Accept=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 isusually very short (thus it may not appear atsystemctl).

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

[Unit]Description=Baz ServiceRequires=baz.socket[Service]Type=simpleExecStart=/usr/bin/python /opt/baz-service/serve.py %iStandardInput=socketStandardError=journalTimeoutStopSec=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/pythonimport sysimport logginglogging.basicConfig(level=logging.INFO)instance = sys.argv[1]# The connected socket is duplicated to stdin/stdoutdata = sys.stdin.readline().strip()logging.info('baz-service: at instance %s, got request: %s', instance, data)sys.stdout.write(data.upper() + '\r\n')

Test

Start (or enable it to start at boot time) the socket (verify it via systemctl status baz.socket), and make a request to the service:

echo Hello| nc localhost 9999

You 'll notice that the socket's status has changed (i.e. Accepted: 1). Also, any log message generated by the on-demandservice should be present at the journal (e.g.journalctl --all| grep -e baz).

 systemctl  status baz.socket● baz.socket - Baz Socket   Loaded: loaded (/etc/systemd/system/baz.socket; enabled; vendor preset: enabled)   Active: active (listening) since Mon 2017-04-10 00:08:22 CST; 1h 13min ago   Listen: 127.0.0.1:9999 (Stream) Accepted: 3; Connected: 0Apr 10 00:08:22 china systemd[1]: Listening on Baz Socket.

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To enable socket, run command:

systemctl enable baz.socket

systemctl start baz.socket

https://gist.github.com/drmalex07/28de61c95b8ba7e5017c

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