»Custom Data Sources

Packer Data Sources are the components of Packer that allow data to be fetched for use within the configuration. Use of data sources allows a build to use information defined outside of Packer. An example of data source is the amazon-ami data source, which outputs the data of a fetched Amazon AMI.

Prior to reading this page, it is assumed you have read the page on plugin development basics.

Data Source plugins implement the packersdk.Datasource interface and are registered within a plugin Set with set.RegisterDatasource(...) function and served using the set.Run().

»The Interface

The interface that must be implemented for a datasource is the packersdk.Datasource 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.

type Datasource interface {
    ConfigSpec() hcldec.ObjectSpec
    OutputSpec() hcldec.ObjectSpec
    Configure(...interface{}) error
    Execute() (cty.Value, 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

»The "OutputSpec" Method

This method returns a hcldec.ObjectSpec of the data source output. The object spec can be generated using the command mapstructure-to-hcl2 just like the configuration spec for the ConfigSpec method.

This method is used in packer validate command. Packer will use the spec to assign unknown values to the data source, instead of executing it and fetching real values.

»The "Configure" Method

The Configure method is called prior to any runs with the configuration that was given in the template. This is passed in as an array of interface{} types, but is generally map[string]interface{}. The Configure method is responsible for translating this configuration into an internal structure, validating it, and returning any errors.

For multiple parameters, they should be merged together into the final configuration, with later parameters overwriting any previous configuration. The exact semantics of the merge are left to the builder author.

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 launch virtual machines, etc. Configure's purpose is solely to configure the data source and validate the configuration.

The Configure method is called very early in the build process so that errors may be displayed to the user before anything actually happens.

»The "Execute" Method

This method returns an HCL cty.Value and an error. Packer will run the Execute method on packer build command so that the data source output will be available prior to evaluating build and locals blocks. Is expected that the Execute command will fetch the data and return it as a cty.Value.

To get the equivalent cty.Value from an output config, we suggest using our packer-plugin-sdk hcl2helper functions.

»Scaffolding template

To make your experience easier when developing your new data source plugin, we provide you a Packer plugin scaffolding to use as a template to your plugin repository. The template is structure with the basics and contain the necessary configuration to start creating your own plugin.