Extending Juju with plugins

This is a quick introduction on extending Juju with plugins written in Go. What we'll cover:

  • Setting up your Go environment
  • Getting the Juju source code
  • Writing a basic plugin named juju-lyaplugin short for (juju-learnyouaplugin)
  • End result will be a plugin that closely resembles what juju run would do.
  • Running on Ubuntu 14.04 or above
  • Go 1.2.1 or above (Article written using Go 1.2.1)
  • A basic understanding of the Go language, package imports, etc.

This is all a matter of preference but for the sake of this article we'll do it my way :)

On Trusty and above:

sudo apt-get install golang
$ cd /tmp && git clone https://github.com/pote/gvp.git && cd gvp && sudo ./configure && sudo make install
$ cd /tmp && git clone https://github.com/pote/gpm.git && cd gpm && sudo ./configure && sudo make install

Feel free to check out their project pages for additional uses.

$ mkdir ~/Projects/juju-learnyouaplugin
$ cd ~/Projects/juju-learnyouaplugin
$ source gvp in

This will setup your $GOPATH and $GOBIN variables for use when resolving imports, compiling, etc.

$ echo $GOPATH
$ echo $GOBIN

From this point on all package dependencies will be stored in the project's .godeps directory.

From your project's directory run:

$ go get -d -v github.com/juju/juju/...

Now that all the preparatory tasks are complete we can begin the fun stuff. Using your favorite editor open up a new file main.go. Within this file we need to define a few package imports that are necessary for the plugin.

import (
	_ "github.com/juju/juju/provider/all"

Let's go through the imports and list why they are required.

  • github.com/juju/cmd - This import gives us access to the run context of a command DefaultContext
  • github.com/juju/juju/cmd/envcmd - Provides EnvCommandBase for creating new commands and giving us access to the API Client for making queries against the Juju state server.
  • github.com/juju/juju/apiserver/params - Provides access to 2 types [RunParams](https://godoc.org/github.com/juju/juju/apiserver/params#RunParams) and RunResults for executing the api call to Run and return the executed results.
  • github.com/juju/juju/juju - Provides access to InitJujuHome for initializing the necessary bits like charm cache and environment. Required before running any juju cli command.
  • github.com/juju/loggo - Provides access to juju's logging api
  • github.com/juju/names - This package provides some convenience functions in particular we'll use IsValidMachine
  • launchpad.net/gnuflag - Provides the interface for our command definition like setting arguments, usage information, and execution.
  • github.com/juju/juju/provider/all - Registers all known providers (amazon, maas, local, etc)

Spec out the plugin type

This will hold our embedded command base and cli arguments.

type LYAPluginCommand struct {
	out      cmd.Output
	timeout  time.Duration
	machines []string
	services []string
	units    []string
	commands string
	envName string
	description bool

Once defined we can spec out our cli command and its functions.

The info function

First part of the command is the Info() function which returns information about the particular subcommand, in our case that is lyaplugin

var doc = `Run a command on target machine(s)
This example plugin mimics what "juju run" does.
juju lyaplugin -m 1 -e local "touch /tmp/testfile"
func (c *LYAPluginCommand) Info() *cmd.Info {
	return &cmd.Info{
		Name:    "lyaplugin",
		Args:    "<commands>",
		Purpose: "Run a command on remote target",
		Doc:     doc,

SetFlags function

Next we'll define what arguments are available to this new subcommand (lyaplugin).

func (c *LYAPluginCommand) SetFlags(f *gnuflag.FlagSet) {
	f.BoolVar(&c.description, "description", false, "Plugin Description")
	f.Var(cmd.NewStringsValue(nil, &c.machines), "machine", "one or more machine ids")
	f.Var(cmd.NewStringsValue(nil, &c.machines), "m", "")
	f.StringVar(&c.envName, "e", "local", "Juju environment")
	f.StringVar(&c.envName, "environment", "local", "")

Here we are providing a *–description* argument to satisfy a Juju plugin requirement. In addition a target argument -m/–machine MACHINEID and the ability to define which juju environment to execute this in -e/–environment defaults to local environment.

Init function

Here we'll parse the cli arguments, do some basic sanity checking to make sure the passed arguments validate to our liking.

func (c *LYAPluginCommand) Init(args []string) error {
	if c.description {
	if len(args) == 0 {
		return fmt.Errorf("no commands specified")
	if c.envName == "" {
		return fmt.Errorf("Juju environment must be specified.")
	c.commands, args = args[0], args[1:]
	if len(c.machines) == 0 {
		return fmt.Errorf("You must specify a target with --machine, -m")
	for _, machineId := range c.machines {
		if !names.IsValidMachine(machineId) {
			return fmt.Errorf("(%s) not a valid machine id.", machineId)
	return cmd.CheckEmpty(args)

Notice the names.IsValidMachine(machineId) which was imported above as this is the only place where we make use of that particular package.

Run function

To the heart of the command where the execution based on the cli arguments take place. I'll describe inline what is happening:

func (c *LYAPluginCommand) Run(ctx *cmd.Context) error {

Set the environment name pulled from our arguments list so we known which environment to run our command against.

	client, err := c.NewAPIClient()
	if err != nil {
		return fmt.Errorf("Failed to load api client: %s", err)
	defer client.Close()

Grab the api client for the current environment.

	var runResults []params.RunResult
	logger.Infof("Running cmd: %s on machine: %s", c.commands, c.machines[0])
	params := params.RunParams{
		Commands: c.commands,
		Timeout:  c.timeout,
		Machines: c.machines,
		Services: c.services,
		Units:    c.units,

Prepare the RunParams for passing to the api's Run function.

	runResults, err = client.Run(params)
	if err != nil {
		fmt.Errorf("An error occurred: %s", err)
	if len(runResults) == 1 {
		result := runResults[0]
		logger.Infof("Result: out(%s), err(%s), code(%d)", result.Stdout, result.Stderr, result.Code)
	return nil

Execute the api Run function and return the results from the executed command on the machine.


The last bit of code is our main function which ties everything together.

func main() {
	err := juju.InitJujuHome()

Initialize the Juju environment based on the default paths or if $JUJU_HOME is defined.

	if err != nil {
	ctx, err := cmd.DefaultContext()
	if err != nil {

Set the proper command context

	c := &LYAPluginCommand{}
	cmd.Main(c, ctx, os.Args[1:])

Pass our plugin type/command into the supplied command Context and off you go.

With the code written, build and run the command.

$ go build -o juju-lyaplugin -v main.go

Place the executable somewhere in your $PATH

$ mv juju-lyaplugin ~/bin

See if Juju picks it up

$ juju help lyaplugin
usage: lyaplugin [options] <commands>
purpose: Run a command on remote target
--description  (= false)
    Plugin Description
-e, --environment (= "local")
    Juju environment
-m, --machine  (= )
Run a command on target machine(s)
This example plugin mimics what "juju run" does.
juju lyaplugin -m 1 -e local "touch /tmp/testfile"

See it in your list of plugins, requires juju-plugins to be installed:

$ juju help plugins
Juju Plugins
Plugins are implemented as stand-alone executable files somewhere in the user's PATH.
The executable command must be of the format juju-<plugin name>.
git-charm        Clone and keep up-to-date a git repository containing a Juju charm for easy source managing.
kill             Destroy a juju object and reap the environment.
lyaplugin        Run a command on target machine(s)

This should hopefully give you a better idea where to start when you decide to dive into writing a juju plugin :)

Full source code for juju-learnyouaplugin