packer-cn/website/source/docs/extend/developing-plugins.html.md

---
description: |
    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, from a user standpoint.
layout: docs
page_title: Developing Plugins
...

# 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](https://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.

\~> **Warning!** This is an advanced topic. If you're new to Packer, we
recommend getting a bit more comfortable before you dive into writing plugins.

## 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](https://golang.org/doc/effective_go.html#interfaces_and_types), it
doesn't even look like inter-process communication is occurring. 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/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:

``` {.go}
import (
  "github.com/mitchellh/packer/packer/plugin"
)

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

func main() {
  plugin.ServeBuilder(new(Builder))
}
```

**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.

\~> **Lock your dependencies!** 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 recommended 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.

## 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:

``` {.text}
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:

``` {.javascript}
{
  "builders": {
    "custom-cloud": "/an/absolute/path/to/packer-builder-custom-cloud"
  }
}
```

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.