# OCI Image Layout Specification

- The OCI Image Layout is the directory structure for OCI content-addressable blobs and [location-addressable](https://en.wikipedia.org/wiki/Content-addressable_storage#Content-addressed_vs._location-addressed) 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](https://github.com/opencontainers/runtime-spec/blob/v1.0.0/bundle.md) by:

- Following the ref to find a [manifest](manifest.md#image-manifest), possibly via an [image index](image-index.md)
- [Applying the filesystem layers](layer.md#applying) in the specified order
- Converting the [image configuration](config.md) into an [OCI Runtime Specification `config.json`](https://github.com/opencontainers/runtime-spec/blob/v1.0.0/config.md)

## Content

The image layout is as follows:

- `blobs` directory
  - Contains content-addressable blobs
  - A blob has no schema and SHOULD be considered opaque
  - Directory MUST exist and MAY be empty
  - See [blobs](#blobs) section
- `oci-layout` file
  - It MUST exist
  - It MUST be a JSON object
  - It MUST contain an `imageLayoutVersion` field
  - See [oci-layout file](#oci-layout-file) section
  - It MAY include additional fields
- `index.json` file
  - It MUST exist
  - It MUST be an [image index](image-index.md) JSON object.
  - See [index.json](#indexjson-file) section

## Example Layout

This is an example image layout:

```shell
$ cd example.com/app/
$ find . -type f
./index.json
./oci-layout
./blobs/sha256/3588d02542238316759cbf24502f4344ffcc8a60c803870022f335d1390c13b4
./blobs/sha256/4b0bc1c4050b03c95ef2a8e36e25feac42fd31283e8c30b3ee5df6b043155d3c
./blobs/sha256/7968321274dc6b6171697c33df7815310468e694ac5be0ec03ff053bb135e768
```

Blobs are named by their contents:

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

## 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.
- The content of `blobs/<alg>/<encoded>` MUST match the digest `<alg>:<encoded>` (referenced per [descriptor](descriptor.md#digests)). For example, the content of `blobs/sha256/da39a3ee5e6b4b0d3255bfef95601890afd80709` MUST match the digest `sha256:da39a3ee5e6b4b0d3255bfef95601890afd80709`.
- The character set of the entry name for `<alg>` and `<encoded>` MUST match the respective grammar elements described in [descriptor](descriptor.md#digests).
- The blobs directory MAY contain blobs which are not referenced by any of the [refs](#indexjson-file).
- The blobs directory MAY be missing referenced blobs, in which case the missing blobs SHOULD be fulfilled by an external blob store.

### Example Blobs

```shell
$ 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"
      }
    },
...
```

```shell
$ 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": "sha256:9834876dcfb05cb167a5c24953eba58c4ac89b1adf57f28f2f9d09af107ee8f0"
    },
...
```

```shell
$ 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",
...
```

```shell
$ cat ./blobs/sha256/9834876dcfb05cb167a5c24953eba58c4ac89b1adf57f28f2f9d09af107ee8f0
[gzipped tar stream]
```

## oci-layout file

This JSON object serves as a marker for the base of an Open Container Image Layout and to provide the version of the image-layout in use.
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 image layout are required.
This section defines the `application/vnd.oci.layout.header.v1+json` [media type](media-types.md).

### oci-layout Example

```json,title=OCI%20Layout&mediatype=application/vnd.oci.layout.header.v1%2Bjson
{
    "imageLayoutVersion": "1.0.0"
}
```

## index.json file

This REQUIRED file is the entry point for references and descriptors of the image-layout.
The [image index](image-index.md) is a multi-descriptor entry point.

This index provides an established path (`/index.json`) to have an entry point for an image-layout and to discover auxiliary descriptors.

- No semantic restriction is given for the "org.opencontainers.image.ref.name" annotation of descriptors.
- In general the `mediaType` of each [descriptor][descriptors] object in the `manifests` field will be either `application/vnd.oci.image.index.v1+json` or `application/vnd.oci.image.manifest.v1+json`.
- Future versions of the spec MAY use a different mediatype (i.e. a new versioned format).
- An encountered `mediaType` that is unknown MUST NOT generate an error.

**Implementor's Note:**
A common use case of descriptors with a "org.opencontainers.image.ref.name" annotation is representing a "tag" 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 "org.opencontainers.image.ref.name" annotations like "v1.0.0-vendor.0", "2.0.0-debug", etc.

### Index Example

```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.index.v1+json",
      "size": 7143,
      "digest": "sha256:0228f90e926ba6b96e4f39cf294b2586d38fbb5a1e385c05cd1ee40ea54fe7fd",
      "annotations": {
        "org.opencontainers.image.ref.name": "stable-release"
      }
    },
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "size": 7143,
      "digest": "sha256:e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f",
      "platform": {
        "architecture": "ppc64le",
        "os": "linux"
      },
      "annotations": {
        "org.opencontainers.image.ref.name": "v1.0"
      }
    },
    {
      "mediaType": "application/xml",
      "size": 7143,
      "digest": "sha256:b3d63d132d21c3ff4c35a061adf23cf43da8ae054247e32faa95494d904a007e",
      "annotations": {
        "org.freedesktop.specifications.metainfo.version": "1.0",
        "org.freedesktop.specifications.metainfo.type": "AppStream"
      }
    }
  ],
  "annotations": {
    "com.example.index.revision": "r124356"
  }
}
```

This illustrates an index that provides two named references and an auxiliary mediatype for this image layout.

The first named reference (`stable-release`) points to another index that might contain multiple references with distinct platforms and annotations.
Note that the [`org.opencontainers.image.ref.name` annotation](annotations.md) SHOULD only be considered valid when on descriptors on `index.json`.

The second named reference (`v1.0`) points to a manifest that is specific to the linux/ppc64le platform.

[descriptors]: ./descriptor.md
