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docs/spec: Proposal for new manifest format
This is a follow-on to PR #62, and it borrows much of the format from #993, but uses specific formats for the image manifest and manifest list (fat manifest) instead of a combined generic format. The intent of this proposed manifest format is to allow multi-arch, and allow for full content-addressability of images in the Docker engine. Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
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docs/spec/manifest-v2-2.md
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docs/spec/manifest-v2-2.md
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# Image Manifest Version 2, Schema 2
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This document outlines the format of of the V2 image manifest, schema version 2.
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The original (and provisional) image manifest for V2 (schema 1), was introduced
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in the Docker daemon in the [v1.3.0
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release](https://github.com/docker/docker/commit/9f482a66ab37ec396ac61ed0c00d59122ac07453)
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and is specified in the [schema 1 manifest definition](./manifest-v2-1.md)
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This second schema version has two primary goals. The first is to allow
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multi-architecture images, through a "fat manifest" which references image
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manifests for platform-specific versions of an image. The second is to
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move the Docker engine towards content-addressable images, by supporting
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an image model where the image's configuration can be hashed to generate
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an ID for the image.
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# Media Types
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The following media types are used by the manifest formats described here, and
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the resources they reference:
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- `application/vnd.docker.distribution.manifest.v1+json`: schema1 (existing manifest format)
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- `application/vnd.docker.distribution.manifest.v2+json`: New image manifest format (schemaVersion = 2)
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- `application/vnd.docker.distribution.manifest.list.v2+json`: Manifest list, aka "fat manifest"
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- `application/vnd.docker.image.rootfs.diff.tar.gzip`: "Layer", as a gzipped tar
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- `application/vnd.docker.container.image.v1+json`: Container config JSON
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## Manifest List
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The manifest list is the "fat manifest" which points to specific image manifests
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for one or more platforms. Its use is optional, and relatively few images will
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use one of these manifests. A client will distinguish a manifest list from an
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image manifest based on the Content-Type returned in the HTTP response.
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## *Manifest List* Field Descriptions
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- **`schemaVersion`** *int*
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This field specifies the image manifest schema version as an integer. This
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schema uses the version `2`.
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- **`mediaType`** *string*
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The MIME type of the manifest list. This should be set to
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`application/vnd.docker.distribution.manifest.list.v2+json`.
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- **`manifests`** *array*
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The manifests field contains a list of manifests for specific platforms.
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Fields of a object in the manifests list are:
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- **`mediaType`** *string*
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The MIME type of the referenced object. This will generally be
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`application/vnd.docker.image.manifest.v2+json`, but it could also
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be `application/vnd.docker.image.manifest.v1+json` if the manifest
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list references a legacy schema-1 manifest.
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- **`size`** *int*
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The size in bytes of the object. This field exists so that a client
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will have an expected size for the content before validating. If the
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length of the retrieved content does not match the specified length,
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the content should not be trusted.
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- **`digest`** *string*
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The digest of the content, as defined by the
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[Registry V2 HTTP API Specificiation](https://docs.docker.com/registry/spec/api/#digest-parameter).
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- **`platform`** *object*
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The platform object describes the platform which the image in the
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manifest runs on.
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- **`architecture`** *string*
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The architecture field specifies the CPU architecture, for example
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`amd64` or `ppc64`.
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- **`os`** *string*
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The architecture field specifies the operating system, for example
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`linux` or `windows`.
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- **`variant`** *string*
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The optional variant field specifies a variant of the CPU, for
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example `ppc64le` to specify a little-endian version of a PowerPC
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CPU.
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- **`features`** *array*
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The optional features field specifies an array of strings, each
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listing a required CPU feature (for example `sse4` or `aes`).
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## Example Manifest List
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*Example showing a simple manifest list pointing to image manifests for two platforms:*
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```json
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{
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"schemaVersion": 2,
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"mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
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"manifests": [
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{
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"mediaType": "application/vnd.docker.image.manifest.v2+json",
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"size": 7143,
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"digest": "sha256:e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f",
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"platform": {
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"architecture": "ppc64",
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"os": "linux",
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"variant": "ppc64le",
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}
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},
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{
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"mediaType": "application/vnd.docker.image.manifest.v2+json",
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"size": 7682,
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"digest": "sha256:5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270",
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"platform": {
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"architecture": "x86-64",
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"os": "linux",
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"features": [
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"sse4"
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]
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}
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}
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]
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}
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```
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# Image Manifest
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The image manifest provides a configuration and a set of layers for a container
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image. It's the direct replacement for the schema-1 manifest.
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## *Image Manifest* Field Descriptions
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- **`schemaVersion`** *int*
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This field specifies the image manifest schema version as an integer. This
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schema uses version `2`.
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- **`mediaType`** *string*
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The MIME type of the manifest. This should be set to
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`application/vnd.docker.distribution.manifest.v2+json`.
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- **`config`** *object*
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The config field references a configuration object for a container, by
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digest. This configuration item is a JSON blob that the runtime uses
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to set up the container. This new schema uses a tweaked version
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of this configuration to allow image content-addressability on the
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daemon side.
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Fields of a config object are:
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- **`mediaType`** *string*
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The MIME type of the referenced object. This should generally be
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`application/vnd.docker.container.image.v1+json`.
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- **`size`** *int*
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The size in bytes of the object. This field exists so that a client
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will have an expected size for the content before validating. If the
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length of the retrieved content does not match the specified length,
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the content should not be trusted.
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- **`digest`** *string*
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The digest of the content, as defined by the
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[Registry V2 HTTP API Specificiation](https://docs.docker.com/registry/spec/api/#digest-parameter).
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- **`layers`** *array*
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The layer list is ordered starting from the base image (opposite order of schema1).
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Fields of an item in the layers list are:
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- **`mediaType`** *string*
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The MIME type of the referenced object. This should
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generally be `application/vnd.docker.image.rootfs.diff.tar.gzip`.
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- **`size`** *int*
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The size in bytes of the object. This field exists so that a client
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will have an expected size for the content before validating. If the
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length of the retrieved content does not match the specified length,
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the content should not be trusted.
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- **`digest`** *string*
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The digest of the content, as defined by the
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[Registry V2 HTTP API Specificiation](https://docs.docker.com/registry/spec/api/#digest-parameter).
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## Example Image Manifest
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*Example showing an image manifest:*
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```json
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{
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"schemaVersion": 2,
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"mediaType": "application/vnd.docker.distribution.manifest.v2+json",
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"config": {
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"mediaType": "application/vnd.docker.container.image.v1+json",
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"size": 7023,
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"digest": "sha256:b5b2b2c507a0944348e0303114d8d93aaaa081732b86451d9bce1f432a537bc7"
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},
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"layers": [
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{
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"mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
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"size": 32654,
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"digest": "sha256:e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f"
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},
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{
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"mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
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"size": 16724,
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"digest": "sha256:3c3a4604a545cdc127456d94e421cd355bca5b528f4a9c1905b15da2eb4a4c6b"
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},
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{
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"mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
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"size": 73109,
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"digest": "sha256:ec4b8955958665577945c89419d1af06b5f7636b4ac3da7f12184802ad867736"
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}
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],
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}
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```
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# Backward compatibility
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The registry will continue to accept uploads of manifests in both the old and
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new formats.
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When pushing images, clients which support the new manifest format should first
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construct a manifest in the new format. If uploading this manifest fails,
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presumably because the registry only supports the old format, the client may
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fall back to uploading a manifest in the old format.
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When pulling images, clients indicate support for this new version of the
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manifest format by sending the
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`application/vnd.docker.distribution.manifest.v2+json` and
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`application/vnd.docker.distribution.manifest.list.v2+json` media types in an
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`Accept` header when making a request to the `manifests` endpoint. Updated
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clients should check the `Content-Type` header to see whether the manifest
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returned from the endpoint is in the old format, or is an image manifest or
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manifest list in the new format.
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If the manifest being requested uses the new format, and the appropriate media
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type is not present in an `Accept` header, the registry will assume that the
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client cannot handle the manifest as-is, and rewrite it on the fly into the old
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format. If the object that would otherwise be returned is a manifest list, the
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registry will look up the appropriate manifest for the x86-64 platform and
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linux OS, rewrite that manifest into the old format if necessary, and return
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the result to the client. If no suitable manifest is found in the manifest
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list, the registry will return a 404 error.
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One of the challenges in rewriting manifests to the old format is that the old
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format involves an image configuration for each layer in the manifest, but the
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new format only provides one image configuration. To work around this, the
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registry will create synthetic image configurations for all layers except the
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top layer. These image configurations will not result in runnable images on
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their own, but only serve to fill in the parent chain in a compatible way.
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The IDs in these synthetic configurations will be derived from hashes of their
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respective blobs. The registry will create these configurations and their IDs
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using the same scheme as Docker 1.10 when it creates a legacy manifest to push
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to a registry which doesn't support the new format.
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