---
description: |
    Packer Post-processors are the components of Packer that transform one artifact
    into another, for example by compressing files, or uploading them.
layout: docs
page_title: 'Custom Post-Processors - Extending'
sidebar_current: 'docs-extending-custom-post-processors'
---

# Custom Post-Processors

Packer 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/extending/plugins.html).

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

\~&gt; **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.

## The Interface

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

``` go
type PostProcessor interface {
  ConfigSpec() hcldec.ObjectSpec
  Configure(interface{}) error
  PostProcess(context.Context, Ui, Artifact) (a Artifact, keep, mustKeep bool, err error)
}
```

### The "ConfigSpec" Method

This method returns a hcldec.ObjectSpec, which is a spec necessary for using
HCL2 templates with Packer. For information on how to use and implement this
function, check our
[object spec docs](https://www.packer.io/guides/hcl/component-object-spec)

### 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.
A `PostProcess` call can be cancelled at any moment. Cancellation is triggered
when the done chan of the context struct (`<-ctx.Done()`) unblocks .

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, bool, error)`. Each
return value is explained below:

-   `Artifact` - The newly created artifact if no errors occurred.
-   `bool` - If keep 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.
-   `bool` - If forceOverride is true, then any user input for
    keep_input_artifact is ignored and the artifact is either kept or discarded
    according to the value set in `keep`.
-   `error` - Non-nil if there was an error in any way. If this is the case,
    the other two return values are ignored.
