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xpkg Specification	305

Overview

Crossplane supports two types of [packages]: Providers and Configurations. These packages are distributed as generic [OCI images], which contain [YAML] content informing the Crossplane package manager how to alter the state of a cluster by installing objects that configure new resource types, and starting controllers to reconcile them. An OCI image that contains valid Crossplane package content is commonly referred to as an xpkg ("ex-package"). This document provides the specification for a valid xpkg, which can be considered a superset of the requirements detailed in the [OCI image specification]. It is divided into two broad sections: requirements related to OCI image format and requirements related to Crossplane package.yaml contents.

OCI Image Format
package.yaml Contents

OCI Image Format

OCI images are comprised of [manifests], [configuration], and [layers]. Additionally, an image reference could refer to an image [index], which may reference multiple image manifests and is frequently used for multi-platform images. A valid Crossplane xpkg imposes various requirements on the components of an OCI, each of which are described in the following sections.

Indexes

The components of an xpkg that Crossplane interacts with do not contain any platform-specific information, so Crossplane is broadly agnostic to the formatting of an image index. Crossplane does impose the following requirements on an image index:

At least one (1) manifest MUST be referenced in the manifest descriptor array for a package to be successfully fetched and processed.

The OCI image specification allows for zero-length manifest descriptor arrays in an index.

The following default behavior when interacting with image indexes is implemented in the Crossplane package manager:

If one manifest is referenced in the image index, the image it points to will be used.
If multiple manifests are referenced in the image index, Crossplane will use the linux/amd64 variant by default.

It is important to note that the platform of the package image that is used by Crossplane does not necessarily mean that the same platform will be used for the controller if the package is a Provider. The decision of selecting a platform for a Provider controller image is deferred to the configured container runtime.

Manifests

A manifest defines the layers and configuration of a specific image. Crossplane is only concerned with the layer descriptors array in an image manifest and does not impose additional requirements on any other portions of the manifest. The following requirements are imposed on the layer descriptors array:

One (1) layer descriptor in the array MAY have an [annotation] with key io.crossplane.xpkg and value base.
Any number of layer descriptors in the array MAY have an annotation with key io.crossplane.xpkg and arbitrary value. Whether multiple layer descriptors may have the same value is left to the specification of the consumer of those layers.

As evidenced by the fact that annotations are provided as a map of string-string, no single descriptor will contain multiple io.crossplane.xpkg annotations.

Crossplane is only concerned with the layer with the base annotation, and any other layers with the io.crossplane.xpkg key are used to signify to third-party consumers that a layer contains content related to the xpkg that may be specific to a given consumer.

If no layer descriptors have an annotation in the form io.crossplane.xpkg: base, the resultant filesystem from [applying changesets] from all layers will be used. It is preferred to use layer descriptor annotations.

Motivation

Crossplane prefers the usage of annotated layer descriptors because it allows for fetching and processing individual layers, rather than all layers in the image. In the event that the image contains a single layer, this overhead is minimal. However, larger images with many layers, whether they contain third-party xpkg content or unrelated data, will result in multiple network calls and more data to process.

Crossplane also prefers the usage of annotated layer descriptors to define additive package content (i.e. third-party xpkg content) as it provides a clean mechanism to build an xpkg through a series of stages. A valid xpkg can be produced and later modified while verifying that the integrity of the existing content is not violated, which ensures that Crossplane's package manager will process the resulting xpkg in the same manner as the it would prior to modification.

While not explicitly forbidden, modifying content from a preceding layer with the io.crossplane.xpkg annotation in any subsequent layers is discouraged, as it may lead to confusion if a third-party is consuming content from the flattened filesystem.

Configuration

Crossplane imposes no additional requirements on image configuration and does not consider its contents when processing a package.

Layers

As described above, Crossplane is only concerned with the single layer referenced by the descriptor containing io.crossplane.xpkg: base if distinguished. Crossplane imposes no additional restrictions on any other layers, including those with a io.crossplane.xpkg annotation but a value other than base, but does require the following of the xpkg base layer:

A single file with name package.yaml MUST exist in the root directory of the xpkg base layer if distinguished, or in the root of the image filesystem after all layer changesets are applied.
The package.yaml file MUST contain a valid [YAML stream].
All other content in either the xpkg base layer, or the full image filesystem is ignored by Crossplane.

The ability to use the image's flattened filesystem is primarily for backwards compatibility and is not encouraged, especially in the event that an image contains more than just xpkg related content, due to the fact that accidentally overwriting or modifying the xpkg layer contents in subsequent layers when constructing an image could cause the package to be invalid.

package.yaml Contents

Depending on the type of package, the YAML stream in the xpkg base layer package.yaml may contain different content. Additionally, the objects in the YAML stream may contain common annotations that are suitable for the given object type.

Configuration Package Requirements

The package.yaml for Configuration packages must adhere to the following requirements:

One (1) and only one Configuration.meta.pkg.crossplane.io object MUST be defined in the YAML stream.
Zero (0) or more CompositeResourceDefinition.apiextensions.crossplane.io objects MAY be defined in the YAML stream.
Zero (0) or more Composition.apiextensions.crossplane.io objects MAY be defined in the YAML stream.
Zero (0) other object types may be defined in the YAML stream.

Provider Package Requirements

The package.yaml for Provider packages must adhere to the following requirements:

One (1) and only one Provider.meta.pkg.crossplane.io object MUST be defined in the YAML stream.
Zero (0) or more CustomResourceDefinition.apiextensions.k8s.io objects MAY be defined in the YAML stream.
Zero (0) or more AdmissionWebhookConfiguration.admissionregistration.k8s.io objects MAY be defined in the YAML stream.
Zero (0) or more MutatingWebhookConfiguration.admissionregistration.k8s.io objects MAY be defined in the YAML stream.
Zero (0) other object types may be defined in the YAML stream.

Object Annotations

Though not used directly by Crossplane, the following object metadata annotations (not to be confused with descriptor annotations in an OCI image manifest) are defined for Configuration.meta.pkg.crossplane.io and Provider.meta.pkg.crossplane.io and should be honored over any competing annotations by third-party consumers of Crossplane packages:

meta.crossplane.io/maintainer: The package's maintainers, as a short opaque text string.
meta.crossplane.io/source: The URL at which the package's source can be found.
meta.crossplane.io/license: The license under which the package's source is released. This should be a valid [SPDX License Identifier].
meta.crossplane.io/description: A one sentence description of the package.
meta.crossplane.io/readme: A longer description, documentation, etc.

Third party consumers may define additional arbitrary annotations with any key and value on any object in a package. All annotations on "meta" types (i.e. Configuration.meta.pkg.crossplane.io and Provider.meta.pkg.crossplane.io) are propagated to the respective package revision (i.e. ConfigurationRevision.pkg.crossplane.io and ProviderRevision.pkg.crossplane.io) on package install. Annotations on all other objects in a package are propagated to their in-cluster representation unmodified.

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