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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.

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

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 easy reference. The actual interface in the source code contains some basic documentation as well explaining what each method should do.

type PostProcessor interface {
	Configure(interface{}) error
	PostProcess(Ui, Artifact) (a Artifact, keep bool, err error)
}

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 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.