Plugin Support

Table of Contents

Our requirements for airshipctl contain two very conflicting concepts. One, we’d like to assert that airshipctl is a statically linked executable, such that it can be easily distributed. Two, we’d like to have plugin support. These requirements can’t coincide within the same project under the standard definition of a plugin. Our solution is to provide a more refined definition of what a plugin actually is.

Compile-In Plugins

In order to support plugins to an independent binary file, we use the concept of “compile-in plugins”. A “compile-in plugin” is an add-on that is built into the main application at compile time, as opposed to runtime. This means that while airshipctl is a standalone application, it also acts as a sort of library. In fact, taking a deeper look at airshipctl reveals that the base application is incredibly simple. At its core, airshipctl provides exactly 2 commands: version and help. Take a look at the following snippet to see what this looks like:

package main

import (
	"os"
	"opendev.org/airship/airshipctl/cmd"
)

func main() {
	rootCmd, _ := cmd.NewRootCommand(os.Stdout)
	rootCmd.Execute()
}

Compiling and running the above gives the following output:

$ ./airshipctl
A unified entrypoint to various airship components

Every other command is treated as a plugin. Changing main to the following adds the default commands, or “plugins”, to the airshipctl tool:

func main() {
	rootCmd, settings := cmd.NewRootCommand(os.Stdout)
	cmd.AddDefaultAirshipCTLCommands(rootCmd, cfg.CreateFactory(&settings.AirshipConfigPath))
	rootCmd.Execute()
}

Compiling and running now provides the following output:

$ ./airshipctl
A unified entrypoint to various airship components

Usage:
  airshipctl [command]

Available Commands:
  baremetal   Perform actions on baremetal hosts
  cluster     Manage Kubernetes clusters
  completion  Generate completion script for the specified shell (bash or zsh)
  config      Manage the airshipctl config file
  document    Manage deployment documents
  help        Help about any command
  image       Manage ISO image creation
  phase       Manage phases
  secret      Manage secrets
  version     Show the version number of airshipctl

Flags:
      --airshipconf string   Path to file for airshipctl configuration. (default "$HOME/.airship/config")
      --debug                enable verbose output
  -h, --help                 help for airshipctl

Use "airshipctl [command] --help" for more information about a command.

Downloading and building the main airshipctl project will default to providing the builtin commands (such as phase), much like the above. A plugin author wishing to use airshipctl can then use the rootCmd as the first of a series of building blocks. The following demonstrates the addition of a new command, hello:

package main

import (
	"fmt"
	"os"
	"opendev.org/airship/airshipctl/cmd"
	"github.com/spf13/cobra"
	cfg "opendev.org/airship/airshipctl/pkg/config"
)

func main() {
	rootCmd, settings := cmd.NewRootCommand(os.Stdout)
	cmd.AddDefaultAirshipCTLCommands(rootCmd, cfg.CreateFactory(&settings.AirshipConfigPath))

	helloCmd := &cobra.Command{
		Use: "hello",
		Short: "Prints a friendly message to the screen",
		Run: func(cmd *cobra.Command, args []string) {
			fmt.Println("Hello World!")
		},
	}
	rootCmd.AddCommand(helloCmd)

	rootCmd.Execute()
}

Fine Tuning a Build

There are a couple of ways in which a plugin author can fine tune their version of airshipctl. These manifest as an ability to pick and choose various plugins (including the defaults), and capabilities for accessing the same settings as other airshipctl commands.

Command Selection

In the previous section, we introduced the AddDefaultAirshipCTLCommands function. That command will simply dump all of the builtin commands onto the root. But a plugin author might not need all of the builtins. To deal with this, the author can pick and choose specific commands to add to their airshipctl, much like the following:

package main

import (
	"os"
	"opendev.org/airship/airshipctl/cmd"
	"opendev.org/airship/airshipctl/cmd/phase"
	cfg "opendev.org/airship/airshipctl/pkg/config"
)

func main() {
	rootCmd, settings := cmd.NewRootCommand(os.Stdout)
	rootCmd.AddCommand(phase.NewPhaseCommand(cfg.CreateFactory(&settings.AirshipConfigPath)))
	rootCmd.Execute()
}

This variant of airshipctl will have the phase command, but will not have any other builtins.

This can be particularly useful if a plugin author desires to “override” a specific functionality provided by a builtin command. For example, you might write your own phase command and use it in place of the builtin.

Accessing airshipctl options

A plugin author can define plugin options and/or use root command options. The following snippet demonstrates how to use the debug flag, provided by root command options, as well as a custom alt-message flag, provided by the plugin.

package main

import (
	"fmt"
	"github.com/spf13/cobra"
	"opendev.org/airship/airshipctl/cmd"
	"os"
)

type Options struct {
	*cmd.RootOptions
	AltMessage bool
}

func main() {
	rootCmd, rootOptions := cmd.NewRootCommand(os.Stdout)
	options := &Options{RootOptions: rootOptions}
	helloCmd := &cobra.Command{
		Use:   "hello",
		Short: "Prints a friendly message to the screen",
		Run: func(cmd *cobra.Command, args []string) {
			if options.Debug {
				fmt.Println("Debug message")
			}
			if !options.AltMessage {
				fmt.Println("Hello World!")
			} else {
				fmt.Println("Goodbye World!")
			}
		},
	}

	helloCmd.PersistentFlags().BoolVar(&options.AltMessage, "alt-message", false, "display an alternate message")
	rootCmd.AddCommand(helloCmd)
	rootCmd.Execute()
}

Container Plugins

airshipctl is mostly focused on managing Kubernetes cluster lifecycle using yaml documents. airshipctl uses kustomize capabilities to deal with bundles of yaml documents. In turn, kustomize provides a way to generate/transform yaml documents using plugins (functions). We can define a yaml document with the annotation as follows

apiVersion: airshipit.org/v1alpha1
kind: Templater
metadata:
  annotations:
    config.kubernetes.io/function: |
      container:
        image: quay.io/airshipit/templater:latest
values:
  hosts:
  - macAddress: 00:aa:bb:cc:dd
    name: node-1
  - macAddress: 00:aa:bb:cc:ee
    name: node-2
template: |
  {{ range .hosts -}}
  ---
  apiVersion: metal3.io/v1alpha1
  kind: BareMetalHost
  metadata:
    name: {{ .name }}
  spec:
    bootMACAddress: {{ .macAddress }}
  {{ end -}}

kustomize looks at the annotation config.kubernetes.io/function and runs the container with the image defined in the annotation. The container usually accepts a bunch of yaml documents on its stdin and outputs a generated/modified bunch of yaml documents on its output. The document in the above example defines the configuration for the template plugin. This particular example generates two BareMetalHost documents.