Understanding a plugin

So far, we have established that all the plugins that we have installed have actually nothing to do with Docker directly, so what does a plugin do?

Docker describes a plugin as:

"Docker plugins are out-of-process extensions which add capabilities to the Docker Engine."

This is exactly what we have seen when installing third-party tools, they all run alongside Docker as separate daemons.

Let's assume that we are going to be creating a volume plugin called mobyfs for the remainder of this chapter. The mobyfs plugin is a fictional service which is written in Go and it runs as a daemon.

Discovery

Typically, a plugin will be installed on the same host as the Docker binary. We can register our mobyfs plugin with Docker by creating the following files in either /run/docker/plugins if it's a Unix socket file, or /etc/docker/plugins or /usr/lib/docker/plugins if it is one of the other two files:

• mobyfs.sock
• mobyfs.spec
• mobyfs.json

Plugins that use a Unix socket file...