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Representing CNAB bundles in OCI Registries
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Representing CNAB bundles in OCI Registries

This section describes how the bundle file and the information contained in the bundle MUST be represented when distributing bundles using OCI registries.

A CNAB is made up of a descriptor file (bundle.json), one or more invocation images, and a list of zero or more component images. Each of these objects can be individually packaged into an OCI manifest or OCI index. An OCI index can then be used to combine these all into a single object.

Runtimes and registries MAY choose to relocate the images and invocation images referenced in the bundle to the registry where the bundle is pushed, but in its simplest form, a CNAB bundle MAY be represented in an OCI registry by the canonical bundle.json file descriptor, referenced in an OCI index (or manifest list).

bundle.json manifest

The bundle.json MUST be serialized as canonical JSON and MUST be stored in the registry as a blob. This blob MUST be referenced by its digest in an OCI manifest. The manifest media type SHOULD be application/vnd.cnab.bundle.config.v1+json but MAY be a standard container image config type (application/vnd.oci.image.config.v1+json) if the target registry does not support the CNAB media type.

Example

The following non-canonical bundle.json file will be used as an example:

{
  "schemaVersion": "v1.0.0",
  "name": "helloworld",
  "version": "0.1.1",
  "description": "A short description of your bundle",
  "keywords": ["helloworld", "cnab", "tutorial"],
  "maintainers": [
    {
      "name": "Jane Doe",
      "email": "[email protected]",
      "url": "https://example.com"
    }
  ],
  "invocationImages": [
    {
      "imageType": "docker",
      "image": "cnab/helloworld:0.1.1",
      "size": 942,
      "contentDigest": "sha256:a59a4e74d9cc89e4e75dfb2cc7ea5c108e4236ba6231b53081a9e2506d1197b6",
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json"
    }
  ],
  "images": null,
  "parameters": null,
  "credentials": null
}

The bundle.json is serialized into canonical JSON and is pushed as a blob to the registry. This blob is then referenced by its digest (for example sha256:e91b9dfcbbb3b88bac94726f276b89de46e4460b55f6e6d6f876e666b150ec5b) as the object config.

The bundle is encapsulated in an OCI manifest which is then referenced as part of an OCI index (or manifest list):

{
  "schemaVersion": 2,
  "config": {
    "mediaType": "application/vnd.cnab.bundle.config.v1+json",
    "digest": "sha256:e91b9dfcbbb3b88bac94726f276b89de46e4460b55f6e6d6f876e666b150ec5b",
    "size": 498
  },
  "layers": null
}

The bundle file MUST be stored in the OCI registry, as it is the canonical representation of a CNAB bundle, and is the object that is used to compute the signature for a thin bundle. While implementations MAY choose to surface information from the bundle as top-level annotations in the OCI representations, they MUST store the unmodified bundle file as a blob.

The bundle file and registry location MAY be used by implementations to generate relocation maps at runtime.

Invocation images

Each invocation image SHOULD be packaged as an OCI image but MAY be packaged as a Docker v2 image. If there are multiple invocation images, these SHOULD be referenced by an OCI index but MAY be referenced by a Docker manifest list.

Component images

Each component image SHOULD be packaged as an OCI image but MAY be packaged as a Docker v2 image. In the case that a component has multiple equivalent forms, such as a multiarch container image, each component MUST be packaged and then SHOULD be referenced by an OCI index but MAY be referenced by a Docker manifest list.

Top-level representation

The top-level representation of the CNAB SHOULD be an OCI index but may be a Docker manifest list.

The manifests list MUST be filled in the following order:

  • A reference to packaged bundle.json manifest
  • References to the invocation images
  • References to the component images

Example

{
  "schemaVersion": 2,
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:6ec4fd695cace0e3d4305838fdf9fcd646798d3fea42b3abb28c117f903a6a5f",
      "size": 188,
      "annotations": {
        "io.cnab.manifest.type": "config"
      }
    },
    {
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "digest": "sha256:a59a4e74d9cc89e4e75dfb2cc7ea5c108e4236ba6231b53081a9e2506d1197b6",
      "size": 942,
      "annotations": {
        "io.cnab.manifest.type": "invocation"
      }
    }
  ],
  "annotations": {
    "io.cnab.keywords": "[\"helloworld\",\"cnab\",\"tutorial\"]",
    "io.cnab.runtime_version": "v1.0.0",
    "org.opencontainers.artifactType": "application/vnd.cnab.manifest.v1",
    "org.opencontainers.image.authors": "[{\"name\":\"Jane Doe\",\"email\":\"[email protected]\",\"url\":\"https://example.com\"}]",
    "org.opencontainers.image.description": "A short description of your bundle",
    "org.opencontainers.image.title": "helloworld",
    "org.opencontainers.image.version": "0.1.1"
  }
}