---
description: >
  Data sources fetch data to use in the Packer configuration. Learn how to define custom data sources so that Packer can use external data in builds.
page_title: Custom Data Sources - Extending
---

# Custom Data Sources

Packer data sources let Packer fetch data to use within the configuration, including information defined outside of Packer. For example, the [amazon-ami data source](/packer/plugins/datasources/amazon/ami), outputs the data from an Amazon AMI.

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()`.

~> **Warning:** This is an advanced topic that requires strong knowledge of Packer and Packer plugins.

## Before You Begin

We recommend reviewing the following resources before you begin development:

- [Developing Plugins - Overview](/packer/docs/plugins/creation)
- The [Go](https://go.dev/) language. You must write custom plugins in Go, so this guide assumes you are familiar with the language.

## 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.

```go
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](/packer/guides/hcl/component-object-spec)

### The "OutputSpec" Method

This method returns a [hcldec.ObjectSpec](/packer/guides/hcl/component-object-spec) of the data source output.
The object spec can be generated using the command [`packer-sdc mapstructure-to-hcl2`](https://github.com/hashicorp/packer-plugin-sdk/tree/main/cmd/packer-sdc)
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](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 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](https://github.com/hashicorp/packer-plugin-sdk/blob/v0.0.7/hcl2helper/values.go).

## Scaffolding template

To make your experience easier when developing your new data source plugin, we provide you a
[Packer plugin scaffolding](https://github.com/hashicorp/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.
