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# OCI Image Index Specification
The image index is a higher-level manifest which points to specific [image manifests](manifest.md), ideal for one or more platforms.
While the use of an image index is OPTIONAL for image providers, image consumers SHOULD be prepared to process them.
This section defines the `application/vnd.oci.image.index.v1+json` [media type](media-types.md).
For the media type(s) that this document is compatible with, see the [matrix][matrix].
## *Image Index* Property Descriptions
- **`schemaVersion`** *int*
This REQUIRED property specifies the image manifest schema version.
For this version of the specification, this MUST be `2` to ensure backward compatibility with older versions of Docker.
The value of this field will not change.
This field MAY be removed in a future version of the specification.
- **`mediaType`** *string*
This property SHOULD be used and [remain compatible][matrix] with earlier versions of this specification and with other similar external formats.
When used, this field MUST contain the media type `application/vnd.oci.image.index.v1+json`.
This field usage differs from the [descriptor](descriptor.md#properties) use of `mediaType`.
- **`artifactType`** *string*
This OPTIONAL property contains the type of an artifact when the manifest is used for an artifact.
If defined, the value MUST comply with [RFC 6838][rfc6838], including the [naming requirements in its section 4.2][rfc6838-s4.2], and MAY be registered with [IANA][iana].
- **`manifests`** *array of objects*
This REQUIRED property contains a list of [manifests](manifest.md) for specific platforms.
While this property MUST be present, the size of the array MAY be zero.
Each object in `manifests` includes a set of [descriptor properties](descriptor.md#properties) with the following additional properties and restrictions:
- **`mediaType`** *string*
This [descriptor property](descriptor.md#properties) has additional restrictions for `manifests`.
Implementations MUST support at least the following media types:
- [`application/vnd.oci.image.manifest.v1+json`](manifest.md)
Also, implementations SHOULD support the following media types:
- `application/vnd.oci.image.index.v1+json` (nested index)
Image indexes concerned with portability SHOULD use one of the above media types.
Future versions of the spec MAY use a different mediatype (i.e. a new versioned format).
An encountered `mediaType` that is unknown to the implementation MUST NOT generate an error.
- **`platform`** *object*
This OPTIONAL property describes the minimum runtime requirements of the image.
This property SHOULD be present if its target is platform-specific.
- **`architecture`** *string*
This REQUIRED property specifies the CPU architecture.
Image indexes SHOULD use, and implementations SHOULD understand, values listed in the Go Language document for [`GOARCH`][go-environment2].
- **`os`** *string*
This REQUIRED property specifies the operating system.
Image indexes SHOULD use, and implementations SHOULD understand, values listed in the Go Language document for [`GOOS`][go-environment2].
- **`os.version`** *string*
This OPTIONAL property specifies the version of the operating system targeted by the referenced blob.
Implementations MAY refuse to use manifests where `os.version` is not known to work with the host OS version.
Valid values are implementation-defined. e.g. `10.0.14393.1066` on `windows`.
- **`os.features`** *array of strings*
This OPTIONAL property specifies an array of strings, each specifying a mandatory OS feature.
When `os` is `windows`, image indexes SHOULD use, and implementations SHOULD understand the following values:
- `win32k`: image requires `win32k.sys` on the host (Note: `win32k.sys` is missing on Nano Server)
When `os` is not `windows`, values are implementation-defined and SHOULD be submitted to this specification for standardization.
- **`variant`** *string*
This OPTIONAL property specifies the variant of the CPU.
Image indexes SHOULD use, and implementations SHOULD understand, `variant` values listed in the [Platform Variants](#platform-variants) table.
- **`features`** *array of strings*
This property is RESERVED for future versions of the specification.
If multiple manifests match a client or runtime's requirements, the first matching entry SHOULD be used.
- **`subject`** *[descriptor](descriptor.md)*
This OPTIONAL property specifies a [descriptor](descriptor.md) of another manifest.
This value, used by the [`referrers` API](https://github.com/opencontainers/distribution-spec/blob/main/spec.md#listing-referrers), indicates a relationship to the specified manifest.
- **`annotations`** *string-string map*
This OPTIONAL property contains arbitrary metadata for the image index.
This OPTIONAL property MUST use the [annotation rules](annotations.md#rules).
See [Pre-Defined Annotation Keys](annotations.md#pre-defined-annotation-keys).
## Platform Variants
When the variant of the CPU is not listed in the table, values are implementation-defined and SHOULD be submitted to this specification for standardization.
| ISA/ABI | `architecture` | `variant` |
|-----------------|----------------|-------------|
| ARM 32-bit, v6 | `arm` | `v6` |
| ARM 32-bit, v7 | `arm` | `v7` |
| ARM 32-bit, v8 | `arm` | `v8` |
| ARM 64-bit, v8 | `arm64` | `v8` |
## Example Image Index
*Example showing a simple image index pointing to image manifests for two platforms:*
```json,title=Image%20Index&mediatype=application/vnd.oci.image.index.v1%2Bjson
{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.index.v1+json",
"manifests": [
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"size": 7143,
"digest": "sha256:e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f",
"platform": {
"architecture": "ppc64le",
"os": "linux"
}
},
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"size": 7682,
"digest": "sha256:5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270",
"platform": {
"architecture": "amd64",
"os": "linux"
}
}
],
"annotations": {
"com.example.key1": "value1",
"com.example.key2": "value2"
}
}
```
## Example Image Index with multiple media types
*Example showing an image index pointing to manifests with multiple media types:*
```json,title=Image%20Index&mediatype=application/vnd.oci.image.index.v1%2Bjson
{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.index.v1+json",
"manifests": [
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"size": 7143,
"digest": "sha256:e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f",
"platform": {
"architecture": "ppc64le",
"os": "linux"
}
},
{
"mediaType": "application/vnd.oci.image.index.v1+json",
"size": 7682,
"digest": "sha256:601570aaff1b68a61eb9c85b8beca1644e698003e0cdb5bce960f193d265a8b7"
}
],
"annotations": {
"com.example.key1": "value1",
"com.example.key2": "value2"
}
}
```
[go-environment2]: https://golang.org/doc/install/source#environment
[matrix]: media-types.md#compatibility-matrix
|
