---
layout: "docs"
---

# Developing Plugins

This page will document how you can develop your own Packer plugins.
Prior to reading this, it is assumed that you're comfortable with Packer
and also know the [basics of how Plugins work](/docs/extend/plugins.html),
from a user standpoint.

Packer plugins must be written in [Go](http://golang.org/), so it is also
assumed that you're familiar with the language. This page will not be a
Go language tutorial. Thankfully, if you are familiar with Go, the Go toolchain
makes it extremely easy to develop Packer plugins.

<div class="alert alert-block">
  <strong>Warning!</strong> This is an advanced topic. If you're new to Packer,
  we recommend getting a bit more comfortable before you dive into writing
  plugins.
</div>

## Plugin System Architecture

Packer has a fairly unique plugin architecture. Instead of loading plugins
directly into a running application, Packer runs each plugin as a
_separate application_. Inter-process communication and RPC is then used
to communicate between the many running Packer processes. Packer core
itself is responsible for orchestrating the processes and handles cleanup.

The beauty of this is that your plugin can have any dependencies it wants.
Dependencies don't need to line up with what Packer core or any other plugin
uses, because they're completely isolated into the process space of the
plugin itself.

And, thanks to Go's [interfaces](http://golang.org/doc/effective_go.html#interfaces_and_types),
it doesn't even look like inter-process communication is occuring. You just
use the interfaces like normal, but in fact they're being executed in
a remote process. Pretty cool.

## Plugin Development Basics

Developing a plugin is quite simple. All the various kinds of plugins
have a corresponding interface. The plugin simply needs to implement
this interface and expose it using the Packer plugin package (covered here shortly),
and that's it!

There are two packages that really matter that every plugin must use.
Other than the following two packages, you're encouraged to use whatever
packages you want. Because plugins are their own processes, there is
no danger of colliding dependencies.

* `github.com/mitchellh/packer` - Contains all the interfaces that you
  have to implement for any given plugin.

* `github.com/mitchellh/packer/plugin` - Contains the code to serve the
  plugin. This handles all the inter-process communication stuff.

There are two steps involved in creating a plugin:

1. Implement the desired interface. For example, if you're building a
   builder plugin, implement the `packer.Builder` interface.

2. Serve the interface by calling the appropriate plugin serving method
   in your main method. In the case of a builder, this is `plugin.ServeBuilder`.

A basic example is shown below. In this example, assume the `Builder` struct
implements the `packer.Builder` interface:

<pre class="prettyprint">
import (
  "github.com/mitchellh/packer/plugin"
)

// Assume this implements packer.Builder
type Builder struct{}

func main() {
	plugin.ServeBuilder(new(Builder))
}
</pre>

**That's it!** `plugin.ServeBuilder` handles all the nitty gritty of
communicating with Packer core and serving your builder over RPC. It
can't get much easier than that.

Next, just build your plugin like a normal Go application, using `go build`
or however you please. The resulting binary is the plugin that can be
installed using standard installation procedures.

The specifics of how to implement each type of interface are covered
in the relevant subsections available in the navigation to the left.

<div class="alert alert-warn alert-block">
<strong>Lock your dependencies.</strong> Unfortunately, Go's dependency
management story is fairly sad. There are various unofficial methods out
there for locking dependencies, and using one of them is highly recomended
since the Packer codebase will continue to improve, potentially breaking
APIs along the way until there is a stable release. By locking your dependencies,
your plugins will continue to work with the version of Packer you lock to.
</div>

## Logging and Debugging

Plugins can use the standard Go `log` package to log. Anything logged
using this will be available in the Packer log files automatically.
The Packer log is visible on stderr when the `PACKER_LOG` environmental
is set.

Packer will prefix any logs from plugins with the path to that plugin
to make it identifiable where the logs come from. Some example logs are
shown below:

```
2013/06/10 21:44:43 ui: Available commands are:
2013/06/10 21:44:43 Loading command: build
2013/06/10 21:44:43 packer-command-build: 2013/06/10 21:44:43 Plugin minimum port: 10000
2013/06/10 21:44:43 packer-command-build: 2013/06/10 21:44:43 Plugin maximum port: 25000
2013/06/10 21:44:43 packer-command-build: 2013/06/10 21:44:43 Plugin address: :10000
```

As you can see, the log messages from the "build" command plugin are
prefixed with "packer-command-build". Log output is _extremely_ helpful
in debugging issues and you're encouraged to be as verbose as you need to
be in order for the logs to be helpful.

## Plugin Development Tips

Here are some tips for developing plugins, often answering common questions
or concerns.

### Naming Conventions

It is standard practice to name the resulting plugin application
in the format of `packer-TYPE-NAME`. For example, if you're building a
new builder for CustomCloud, it would be standard practice to name the
resulting plugin `packer-builder-custom-cloud`. This naming convention
helps users identify the purpose of a plugin.

### Testing Plugins

While developing plugins, you can configure your Packer configuration
to point directly to the compiled plugin in order to test it. For example,
building the CustomCloud plugin, I may configure packer like so:

<pre class="prettyprint">
{
  "builders": {
    "custom-cloud": "/an/absolute/path/to/packer-builder-custom-cloud"
  }
}
</pre>

This would configure Packer to have the "custom-cloud" plugin, and execute
the binary that I am building during development. This is extremely useful
during development.

### Distributing Plugins

It is recommended you use a tool like [goxc](https://github.com/laher/goxc)
in order to cross-compile your plugin for every platform that Packer supports,
since Go applications are platform-specific. goxc will allow you to build
for every platform from your own computer.
