Systemd socket units

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These are the notes of a training course on systemd I gave as part of my work with Truelite.

`.socket` units

Socket units tell systemd to listen on a given IPC, network socket, or file system FIFO, and use another unit to service requests to it.

For example, this creates a network service that listens on port 55555:

# /etc/systemd/system/ddate.socket
[Unit]
Description=ddate service on port 55555

[Socket]
ListenStream=55555
Accept=true

[Install]
WantedBy=sockets.target

# /etc/systemd/system/ddate@.service
[Unit]
Description=Run ddate as a network service

[Service]
Type=simple
ExecStart=/bin/sh -ec 'while true; do /usr/bin/ddate; sleep 1m; done'
StandardOutput=socket
StandardError=journal

Note that the .service file is called ddate@ instead of ddate: units whose name ends in '@' are template units which can be activated multiple times, by adding any string after the '@' in the unit name.

If I run nc localhost 55555 a couple of times, and then check the list of running units, I see ddate@… instantiated twice, adding the local and remote socket endpoints to the unit name:

$ systemctl list-units 'ddate@*'
  UNIT                                             LOAD   ACTIVE SUB     DESCRIPTION
  ddate@15-127.0.0.1:55555-127.0.0.1:36936.service loaded active running Run ddate as a network service (127.0.0.1:36936)
  ddate@16-127.0.0.1:55555-127.0.0.1:37002.service loaded active running Run ddate as a network service (127.0.0.1:37002)

This allows me to monitor each running service individually.

systemd also automatically creates a slice unit called system-ddate.slice grouping all services together:

$ systemctl status system-ddate.slice
● system-ddate.slice
   Loaded: loaded
   Active: active since Thu 2017-09-21 14:25:02 CEST; 9min ago
    Tasks: 4
   CGroup: /system.slice/system-ddate.slice
           ├─ddate@15-127.0.0.1:55555-127.0.0.1:36936.service
           │ ├─18214 /bin/sh -ec while true; do /usr/bin/ddate; sleep 1m; done
           │ └─18661 sleep 1m
           └─ddate@16-127.0.0.1:55555-127.0.0.1:37002.service
             ├─18228 /bin/sh -ec while true; do /usr/bin/ddate; sleep 1m; done
             └─18670 sleep 1m

This allows to also work with all running services for this template unit as a whole, sending a signal to all their processes and setting up resource control features for the service as a whole.

See:

Systemd socket units

.socket units

`.socket` units