mirror of
https://github.com/genuinetools/reg.git
synced 2024-07-04 12:34:17 -04:00
206 lines
7.8 KiB
Markdown
206 lines
7.8 KiB
Markdown
## OCI Image Layout Specification
|
|
|
|
* The OCI Image Layout is 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:
|
|
|
|
```
|
|
$ 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:
|
|
|
|
```
|
|
$ 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
|
|
|
|
```
|
|
$ 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": "sha256:9834876dcfb05cb167a5c24953eba58c4ac89b1adf57f28f2f9d09af107ee8f0"
|
|
},
|
|
...
|
|
```
|
|
|
|
```
|
|
$ 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/sha256/9834876dcfb05cb167a5c24953eba58c4ac89b1adf57f28f2f9d09af107ee8f0
|
|
[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 SHOULD be safely ignored.
|
|
|
|
|
|
**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,
|
|
"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 manifest references and an auxiliary mediatype for this image layout.
|
|
|
|
|
|
[descriptors]: ./descriptor.md
|