Plugins

By the end of this exercise, you should be able to:

Install, configure, and delete any Docker plugin
Use the vieux/sshfs plugin to create ssh-mountable volumes that can be mounted into any container in your cluster

Installing a Plugin

Plugins can be hosted on Docker Store or any other (private) repository. Let's start with Docker Store. Browse to https://store.docker.com and enter vieux/sshfs in the search box. The result should show you the plugin that we are going to work with.
Install the plugin into our Docker Engine:
```
[centos@node-0 ~]$ docker plugin install vieux/sshfs
```
The system should ask us for permission to use privileges. In the case of the sshfs plugin there are 4 privileges. Answer with y.
Once we have successfully installed some plugins we can use the ls command to see the status of each of the installed plugins. Execute:
```
[centos@node-0 ~]$ docker plugin ls
```

Enabling and Disabling a Plugin

Once a plugin is installed it is enabled by default. We can disable it using this command:
```
[centos@node-0 ~]$ docker plugin disable vieux/sshfs
```
only when a plugin is disabled can certain operations on it be executed.
The plugin can be (re-) enabled by using this command:
```
[centos@node-0 ~]$ docker plugin enable vieux/sshfs
```
Play with the above commands and notice how the status of the plugin changes when displaying it with docker plugin ls.

Inspecting a Plugin

We can also use the inspect command to further inspect all the attributes of a given plugin. Execute the following command:
```
[centos@node-0 ~]$ docker plugin inspect vieux/sshfs
```
and examine the output. Specifically note that there are two sections in the metadata called Env, one is under Config and the other under Settings. This is where the list of environment variables are listed that the author of the plugin has defined. In this specific situation we can see that there is a single variable called DEBUG defined. Its initial value is 0.
We can use the set command to change values of the environment variables. Execute:
```
[centos@node-0 ~]$ docker plugin set vieux/sshfs DEBUG=1

Error response from daemon: cannot set on an active plugin, disable plugin before setting
```
This is one of those times we have to disable the plugin first; do so, then try the set command again:
```
[centos@node-0 ~]$ docker plugin disable vieux/sshfs
[centos@node-0 ~]$ docker plugin set vieux/sshfs DEBUG=1
[centos@node-0 ~]$ docker plugin enable vieux/sshfs
```
and then inspect again the metadata of the plugin. Notice how the value of DEBUG has been adjusted. Only the one under the Settings node changed but the one under the Config node still shows the original (default) value.

Using the Plugin

Make a directory on node-1 that we will mount as a volume across our cluster:
```
[centos@node-1 ~]$ mkdir ~/demo
```

Back on node-0, use the plugin to create a volume that can be mounted via ssh:

[centos@node-0 ~]$ docker volume create -d vieux/sshfs \
    -o sshcmd=centos@<node-1 public IP>:/home/centos/demo \
    -o password=orca \
    sshvolume

Mount that volume in a new container as per usual:

[centos@node-0 ~]$ docker container run --rm -it -v sshvolume:/data alpine sh

Inside the container navigate to the /data folder and create a new file:
```
/ # cd /data
/ # echo 'Hello from client!' > demo.txt
/ # ls -al
```
Head over to node-1, and confirm that demo.txt got written there.

Removing a Plugin

If we don't want or need this plugin anymore we can remove it using the command:
```
[centos@node-0 ~]$ docker volume rm sshvolume
[centos@node-0 ~]$ docker plugin disable vieux/sshfs
[centos@node-0 ~]$ docker plugin rm vieux/sshfs
```
Note how we first have to disable the plugin before we can remove it.

Conclusion

Docker follows a 'batteries included but swappable' mindset in its product design: everything you need to get started is included, but heavy customization is supported and encouraged. Docker plugins are one aspect of that flexibility, allowing users to define their own volume and networking behavior.