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layout: "docs"
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# Custom Post-Processor Development

Post-processors are the components of Packer that transform one artifact
into another, for example by compressing files, or uploading them.

In the compression example, the transformation would be taking an artifact
with a set of files, compressing those files, and returning a new
artifact with only a single file (the compressed archive). For the
upload example, the transformation would be taking an artifact with
some set of files, uploading those files, and returning an artifact
with a single ID: the URL of the upload.

Prior to reading this page, it is assumed you have read the page on
[plugin development basics](/docs/extend/developing-plugins.html).

Post-processor plugins implement the `packer.PostProcessor` interface and
are served using the `plugin.ServePostProcessor` function.

<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>


## The Interface

The interface that must be implemented for a post-processor is the
`packer.PostProcessor` interface. It is reproduced below for easy reference.
The actual interface in the source code contains some basic documentation as well explaining
what each method should do.


<pre class="prettyprint">
type PostProcessor interface {
	Configure(interface{}) error
	PostProcess(Ui, Artifact) (a Artifact, keep bool, err error)
}
</pre>

### The "Configure" Method

The `Configure` method for each post-processor is called early in the
build process to configure the post-processor. The configuration is passed
in as a raw `interface{]`. The configure method is responsible for translating
this configuration into an internal structure, validating it, and returning
any errors.

For decoding the `interface{]` into a meaningful structure, the
[mapstructure](https://github.com/mitchellh/mapstructure) library is
recommended. Mapstructure will take an `interface{}` and decode it into an
arbitrarily complex struct. If there are any errors, it generates very
human-friendly errors that can be returned directly from the configure
method.

While it is not actively enforced, **no side effects** should occur from
running the `Configure` method. Specifically, don't create files, don't
create network connections, etc. Configure's purpose is solely to setup
internal state and validate the configuration as much as possible.

`Configure` being run is not an indication that `PostProcess` will ever
run. For example, `packer validate` will run `Configure` to verify the
configuration validates, but will never actually run the build.

### The "PostProcess" Method

The `PostProcess` method is where the real work goes. PostProcess is
responsible for taking one `packer.Artifact` implementation, and transforming
it into another.

When we say "transform," we don't mean actually modifying the existing
`packer.Artifact` value itself. We mean taking the contents of the artifact
and creating a new artifact from that. For example, if we were creating
a "compress" post-processor that is responsible for compressing files,
the transformation would be taking the `Files()` from the original artifact,
compressing them, and creating a new artifact with a single file: the
compressed archive.

The result signature of this method is `(Artifact, bool, error)`. Each
return value is explained below:

* `Artifact` - The newly created artifact if no errors occurred.
* `bool` - If true, the input artifact will forcefully be kept. By default,
  Packer typically deletes all input artifacts, since the user doesn't generally
  want intermediary artifacts. However, some post-processors depend on the
  previous artifact existing. If this is `true`, it forces packer to keep the
  artifact around.
* `error` - Non-nil if there was an error in any way. If this is the case,
  the other two return values are ignored.
