It involves adding an import, and the plugin registers itself. Extension/plugin/addon -- call them what you will, I guess. We chose the term "plugin" before Go

It involves adding an import, and the plugin registers itself. Extension/plugin/addon -- call them what you will, I guess. We chose the term "plugin" before Go 1.8 had its experimental plugin package that, on Linux, allows dynamic linking of libraries. (There are a lot of limitations with that, for now.) In Caddy, all plugins are statically linked and compiled in.