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spec/manifest: clarify relationship between urls and foreign layers

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Previously, the specificiation incorrectly bound the fates of `urls` and
foreign layers. These are complementary but unrelated features, in that
the `urls` field may be populated for layers that aren't foreign. The
type of the layer only dictates the push behavior of the layer, rather
than involving where it came from.

For example, one may pull a foreign layer from a registry, but they may
not push it back to another registry. Conversely, a layer that has no
restrictions on push/pull behavior may be fetched via `urls` entries.

Signed-off-by: Stephen J Day <stephen.day@docker.com>
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Stephen J Day 2016-08-29 17:12:01 -07:00
parent b1b100cf01
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docs/spec/manifest-v2-2.md
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@ -31,8 +31,9 @@ the resources they reference:
- `application/vnd.docker.distribution.manifest.v1+json`: schema1 (existing manifest format)
- `application/vnd.docker.distribution.manifest.v2+json`: New image manifest format (schemaVersion = 2)
- `application/vnd.docker.distribution.manifest.list.v2+json`: Manifest list, aka "fat manifest"
- `application/vnd.docker.image.rootfs.diff.tar.gzip`: "Layer", as a gzipped tar
- `application/vnd.docker.container.image.v1+json`: Container config JSON
- `application/vnd.docker.image.rootfs.diff.tar.gzip`: "Layer", as a gzipped tar
- `application/vnd.docker.image.rootfs.foreign.diff.tar.gzip`: "Layer", as a gzipped tar that should never be pushed
## Manifest List
@ -203,6 +204,9 @@ image. It's the direct replacement for the schema-1 manifest.
The MIME type of the referenced object. This should
generally be `application/vnd.docker.image.rootfs.diff.tar.gzip`.
Layers of type
`application/vnd.docker.image.rootfs.foreign.diff.tar.gzip` may be
pulled from a remote location but they should never be pushed.
- **`size`** *int*
@ -218,11 +222,9 @@ image. It's the direct replacement for the schema-1 manifest.
- **`urls`** *array*
For an ordinary layer, this is empty, and the layer contents can be
retrieved directly from the registry. For a layer with *`mediatype`* of
`application/vnd.docker.image.rootfs.foreign.diff.tar.gzip`, this
contains a non-empty list of URLs from which this object can be
downloaded.
Provides a list of URLs from which the content may be fetched. Content
should be verified against the `digest` and `size`. This field is
optional and uncommon.
## Example Image Manifest