volmex

Volmex is a docker volume driver that allows to execute arbitrary commands before a volume is mounted (pre-mount hook).

Context

Using a swarm cluster, we found that there is no satisfying solution to setup and use multi-host persistent volumes.

As there are numerous protocols and implementations for multi-host storage solutions, e.g. rsync, syncthing, … we decided to not implement a new protocol but strive for a more abstract solution.

Hence, we created a Docker volume plugin (v1) that simply executes a user defined command whenever a volume is mounted.

General

Besides the ‘pre-mount hook’, volmex volumes are equivalent to named volumes in Docker (With a different storage base directory though - /var/local/volmex)

Usage (Evaluation/Testing)

# get volmex
$ wget https://github.com/volmex/volmex/releases/download/v0.9/volmex-0.9.tar
$ tar xf volmex-0.9.1.tar
# create dir where local volumes are stored
$ sudo mkdir /var/local/volmex
# start volmex driver (later you can move this to a systemd service file)
$ sudo ./volmex-daemon
# create a volume 'foo' using the volmex driver and specify a command or shell script that shall be executed pre-mount
$ docker volume create \
  --driver volmex \
  --opt cmd="/usr/local/sbin/do-something" \
  foo

When the command is executed, the following variables are available in the command’s environment:

• VOLMEX_NAME = foo
• VOLMEX_MOUNTPOINT = /var/local/volmex/foo
• VOLMEX_CMD = /usr/local/sbin/do-something

Install on systemd systems (Persistent)

• Download and extract volmex

# cd /tmp
# wget https://github.com/volmex/volmex/releases/download/v0.9/volmex-0.9.tar
# tar xf volmex-0.9.1.tar

• Install volmex

# mkdir -p /usr/lib/docker
# install -D -m 744 volmex-daemon /usr/lib/docker/volmex-daemon
# install -D -m 644 volmex.service /etc/systemd/system/volmex.service
# mkdir -p /var/local/volmex
# systemctl daemon-reload

• Start/enable volmex

# systemctl enable volmex
# systemctl restart docker

• Volmex should be started before Docker - which is configured in the service file. That’s why it’s sufficient to systemctl restart docker.

Usage

• Create script that shall be executed on volume mount:

➜  ~ sudo vim /usr/local/sbin/do-something
...
#!/usr/bin/env bash
echo $VOLMEX_NAME
echo sleeping for 10 seconds...
sleep 10
echo done
...
➜  ~ sudo chmod +x /usr/local/sbin/do-something

• Create the volume using the volmex driver

➜  ~ docker volume create --driver volmex --opt cmd="/usr/local/sbin/do-something" foo

• Add content

➜  ~ sudo mkdir -p /var/local/volmex/foo
➜  ~ sudo vim /var/local/volmex/foo/volmex-test
...
volmex-test
...

• Create a container which uses the volume

➜  ~ docker run --rm -ti --volume foo:/foo alpine sh
/ # cat /foo/volmex-test
volmex-test
/ # exit

• Check log output of the executed command

➜  ~ journalctl -u volmex | tail
...
volmex-daemon[2115]: received 'Get' request for volume: foo
volmex-daemon[2115]: received 'Mount' request for volume: foo
volmex-daemon[2115]:         executing volmex command: /usr/local/sbin/do-something
volmex-daemon[2115]: foo
volmex-daemon[2115]: sleeping for 10 seconds...
volmex-daemon[2115]: done
volmex-daemon[2115]: received 'Unmount' request for volume: foo
...