On my home computer, I use Docker from time to time. Since I don't need it every day, I only start the daemon on demand though.
A pattern that occurs frequently is that I turn on my computer, try to start a container I need, and then the following happens:
$ docker-compose up ERROR: Couldn't connect to Docker daemon at http+docker://localunixsocket - is it running? If it's at a non-standard location, specify the URL with the DOCKER_HOST environment variable.
Of course I could then manually start docker, re-run the docker-compose command, and everything would work. But the people behind systemd had some better ideas on how to solve the issue of network services that are only needed infrequently.
Socket Activation
Systemd implements a technique called socket activation. It basically means that instead of starting a service that listens on a socket, a supervisor daemon listens on behalf of those services. When a new connection comes in that wants to talk to that socket, the service is started transparently in the background. The connection is kept alive and handed over to the service as soon as the service is up and running.
One of the downsides of this approach is that the service needs to be aware of socket activation. Luckily support for systemd socket activation was implemented in the Docker daemon in 2014. (Unfortunately it's still missing in PostgreSQL...)
Starting Docker on Demand
Arch Linux ships with appropriate unit files for a socket activated Docker daemon.
The first requirement for socket activation to work, is that the daemon is started appropriately. For Docker, the daemon needs to be started with the -H fd:// argument. This is being done correctly, as you can see in the service file shipped with the docker package:
# /usr/lib/systemd/system/docker.service
[Unit]
Description=Docker Application Container Engine
Documentation=https://docs.docker.com
After=network.target docker.socket
Requires=docker.socket
[Service]
ExecStart=/usr/bin/dockerd -H fd://
# ...
The second component that needs to be present is the socket unit configuration. This is what the file looks like on Arch Linux:
# /usr/lib/systemd/system/docker.socket
[Unit]
Description=Docker Socket for the API
PartOf=docker.service
[Socket]
ListenStream=/var/run/docker.sock
SocketMode=0660
SocketUser=root
SocketGroup=docker
[Install]
WantedBy=sockets.target
To start Docker only on demand, we need to disable the service (to prevent it for starting on boot) and to enable the socket.
$ sudo systemctl disable docker.service $ sudo systemctl stop docker.service $ sudo systemctl enable docker.socket $ sudo systemctl start docker.socket
Now if you run any command that tries to connect to the Docker daemon, the service should be started automatically!
$ docker info Containers: 52 Running: 2 Paused: 0 Stopped: 50 Images: 323 Server Version: 1.12.6 ...
Hooray.