cri-o/vendor/github.com/opencontainers/image-spec/image-layout.md

OCI Image Layout Specification

The OCI Image Layout is a slash separated layout of OCI content-addressable blobs and location-addressable references (refs). This layout MAY be used in a variety of different transport mechanisms: archive formats (e.g. tar, zip), shared filesystem environments (e.g. nfs), or networked file fetching (e.g. http, ftp, rsync).

Given an image layout and a ref, a tool can create an OCI Runtime Specification bundle by:

• Following the ref to find a manifest, possibly via a manifest list
• Applying the filesystem layers in the specified order
• Converting the image configuration into an OCI Runtime Specification config.json

Content

The image layout MUST contain two top level directories:

• blobs contains content-addressable blobs. A blob has no schema and should be considered opaque.
• refs contains descriptors. Commonly pointing to an image manifest or an image manifest list.

Both blobs and refs MAY be empty.

The image layout MUST also contain an oci-layout file:

• It MUST be a JSON object
• It MUST contain an imageLayoutVersion field
  • The imageLayoutVersion value will align with the OCI Image Specification version at the time changes to the layout are made, and will pin a given version until changes to the layout are required
• It MAY include additional fields

Example Layout

This is an example image layout:

$ cd example.com/app/
$ find .
.
./blobs
./blobs/sha256/e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f
./blobs/sha256/afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51
./blobs/sha256/9b97579de92b1c195b85bb42a11011378ee549b02d7fe9c17bf2a6b35d5cb079
./blobs/sha256/5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270
./oci-layout
./refs
./refs/v1.0
./refs/stable-release

Blobs are named by their contents:

$ shasum -a 256 ./blobs/sha256/afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51
afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51 ./blobs/sha256/afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51

Refs

Object names in the refs subdirectories MUST NOT include characters outside of the set of "A" to "Z", "a" to "z", "0" to "9", the hyphen -, the dot ., and the underscore _.

No semantic restriction is given for object names in the refs subdirectory. Each object in the refs subdirectory MUST be of type application/vnd.oci.descriptor.v1+json. In general the mediaType of this descriptor object will be either application/vnd.oci.image.manifest.list.v1+json or application/vnd.oci.image.manifest.v1+json although future versions of the spec MAY use a different mediatype.

Implementor's Note: A common use case of refs is representing "tags" for a container image. For example, an image may have a tag for different versions or builds of the software. In the wild you often see "tags" like "v1.0.0-vendor.0", "2.0.0-debug", etc. Those tags will often be represented in an image-layout repository with matching refs names like "v1.0.0-vendor.0", "2.0.0-debug", etc.

Example Ref

This is an example v1.0 ref with a manifest-list descriptor:

$ cat ./refs/v1.0 | jq
{
  "size": 4096,
  "digest": "sha256:e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f",
  "mediaType": "application/vnd.oci.image.manifest.list.v1+json"
}

Blobs

Object names in the blobs subdirectories are composed of a directory for each hash algorithm, the children of which will contain the actual content. A blob, referenced with digest <alg>:<hex> (per descriptor), MUST have its content stored in a file under blobs/<alg>/<hex>. The character set of the entry name for <hex> and <alg> MUST match the respective grammar elements described in descriptor. For example sha256:5b will map to the layout blobs/sha256/5b.

The blobs directory MAY contain blobs which are not referenced by any of the refs.

The blobs directory MAY be missing referenced blobs, in which case the missing blobs SHOULD be fulfilled by an external blob store.

Example Blobs

$ cat ./blobs/sha256/9b97579de92b1c195b85bb42a11011378ee549b02d7fe9c17bf2a6b35d5cb079 | jq
{
  "schemaVersion": 2,
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "size": 7143,
      "digest": "sha256:afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51",
      "platform": {
        "architecture": "ppc64le",
        "os": "linux"
      }
    },
...

$ cat ./blobs/sha256/afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51 | jq
{
  "schemaVersion": 2,
  "config": {
    "mediaType": "application/vnd.oci.image.config.v1+json",
    "size": 7023,
    "digest": "sha256:5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270"
  },
  "layers": [
    {
      "mediaType": "application/vnd.oci.image.layer.v1.tar+gzip",
      "size": 32654,
      "digest": "sha256:e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f"
    },
...

$ cat ./blobs/sha256/5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270 | jq
{
  "architecture": "amd64",
  "author": "Alyssa P. Hacker <alyspdev@example.com>",
  "config": {
    "Hostname": "8dfe43d80430",
    "Domainname": "",
    "User": "",
    "AttachStdin": false,
    "AttachStdout": false,
    "AttachStderr": false,
    "Tty": false,
    "OpenStdin": false,
    "StdinOnce": false,
    "Env": [
      "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
    ],
    "Cmd": null,
    "Image": "sha256:6986ae504bbf843512d680cc959484452034965db15f75ee8bdd1b107f61500b",
...

$ cat ./blobs/sha256/e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f
[tar stream]