205 lines
7.9 KiB
Markdown
205 lines
7.9 KiB
Markdown
## OCI Image Layout Specification
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* The OCI Image Layout is a slash separated layout of OCI content-addressable blobs and [location-addressable](https://en.wikipedia.org/wiki/Content-addressable_storage#Content-addressed_vs._location-addressed) references (refs).
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* 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).
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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-rc3/bundle.md) by:
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* Following the ref to find a [manifest](manifest.md#image-manifest), possibly via an [image index](image-index.md)
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* [Applying the filesystem layers](layer.md#applying) in the specified order
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* Converting the [image configuration](config.md) into an [OCI Runtime Specification `config.json`](https://github.com/opencontainers/runtime-spec/blob/v1.0.0-rc3/config.md)
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# Content
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The image layout is as follows:
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- `blobs` directory
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- Contains content-addressable blobs
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- A blob has no schema and SHOULD be considered opaque
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- Directory MUST exist and MAY be empty
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- See [blobs](#blobs) section
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- `oci-layout` file
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- It MUST exist
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- It MUST be a JSON object
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- It MUST contain an `imageLayoutVersion` field
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- See [oci-layout file](#oci-layout-file) section
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- It MAY include additional fields
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- `index.json` file
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- It MUST exist
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- It MUST be an [image index](image-index.md) JSON object.
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- See [index.json](#indexjson-file) section
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## Example Layout
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This is an example image layout:
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```
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$ cd example.com/app/
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$ find . -type f
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./index.json
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./oci-layout
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./blobs/sha256/3588d02542238316759cbf24502f4344ffcc8a60c803870022f335d1390c13b4
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./blobs/sha256/4b0bc1c4050b03c95ef2a8e36e25feac42fd31283e8c30b3ee5df6b043155d3c
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./blobs/sha256/7968321274dc6b6171697c33df7815310468e694ac5be0ec03ff053bb135e768
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```
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Blobs are named by their contents:
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```
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$ shasum -a 256 ./blobs/sha256/afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51
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afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51 ./blobs/sha256/afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51
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```
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## Blobs
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* Object names in the `blobs` subdirectories are composed of a directory for each hash algorithm, the children of which will contain the actual content.
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* The content of `blobs/<alg>/<encoded>` MUST match the digest `<alg>:<encoded>` (referenced per [descriptor](descriptor.md#digests-and-verification)). For example, the content of `blobs/sha256/da39a3ee5e6b4b0d3255bfef95601890afd80709` MUST match the digest `sha256:da39a3ee5e6b4b0d3255bfef95601890afd80709`.
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* The character set of the entry name for `<alg>` and `<encoded>` MUST match the respective grammar elements described in [descriptor](descriptor.md#digests-and-verification).
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* The blobs directory MAY contain blobs which are not referenced by any of the [refs](#indexjson-file).
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* The blobs directory MAY be missing referenced blobs, in which case the missing blobs SHOULD be fulfilled by an external blob store.
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### Example Blobs
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```
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$ cat ./blobs/sha256/9b97579de92b1c195b85bb42a11011378ee549b02d7fe9c17bf2a6b35d5cb079 | jq
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{
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"schemaVersion": 2,
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"manifests": [
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{
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"mediaType": "application/vnd.oci.image.manifest.v1+json",
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"size": 7143,
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"digest": "sha256:afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51",
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"platform": {
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"architecture": "ppc64le",
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"os": "linux"
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}
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},
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...
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```
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```
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$ cat ./blobs/sha256/afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51 | jq
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{
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"schemaVersion": 2,
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"config": {
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"mediaType": "application/vnd.oci.image.config.v1+json",
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"size": 7023,
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"digest": "sha256:5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270"
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},
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"layers": [
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{
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"mediaType": "application/vnd.oci.image.layer.v1.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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```
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```
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$ cat ./blobs/sha256/5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270 | jq
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{
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"architecture": "amd64",
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"author": "Alyssa P. Hacker <alyspdev@example.com>",
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"config": {
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"Hostname": "8dfe43d80430",
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"Domainname": "",
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"User": "",
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"AttachStdin": false,
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"AttachStdout": false,
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"AttachStderr": false,
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"Tty": false,
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"OpenStdin": false,
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"StdinOnce": false,
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"Env": [
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"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
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],
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"Cmd": null,
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"Image": "sha256:6986ae504bbf843512d680cc959484452034965db15f75ee8bdd1b107f61500b",
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...
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```
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```
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$ cat ./blobs/sha256/e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f
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[tar stream]
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```
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## oci-layout file
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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.
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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.
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This section defines the `application/vnd.oci.layout.header.v1+json` [media type](media-types.md).
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### oci-layout Example
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```json,title=OCI%20Layout&mediatype=application/vnd.oci.layout.header.v1%2Bjson
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{
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"imageLayoutVersion": "1.0.0"
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}
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```
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## index.json file
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This REQUIRED file is the entry point for references and descriptors of the image-layout.
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The [image index](image-index.md) is a multi-descriptor entry point.
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This index provides an established path (`/index.json`) to have an entry point for an image-layout and to discover auxiliary descriptors.
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* No semantic restriction is given for the "org.opencontainers.image.ref.name" annotation of descriptors.
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* 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`.
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* Future versions of the spec MAY use a different mediatype (i.e. a new versioned format).
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* An encountered `mediaType` that is unknown SHOULD be safely ignored.
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**Implementor's Note:**
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A common use case of descriptors with a "org.opencontainers.image.ref.name" annotation is representing a "tag" for a container image.
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For example, an image may have a tag for different versions or builds of the software.
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In the wild you often see "tags" like "v1.0.0-vendor.0", "2.0.0-debug", etc.
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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.
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### Index Example
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```json,title=Image%20Index&mediatype=application/vnd.oci.image.index.v1%2Bjson
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{
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"schemaVersion": 2,
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"manifests": [
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{
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"mediaType": "application/vnd.oci.image.index.v1+json",
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"size": 7143,
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"digest": "sha256:0228f90e926ba6b96e4f39cf294b2586d38fbb5a1e385c05cd1ee40ea54fe7fd",
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"annotations": {
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"org.opencontainers.image.ref.name": "stable-release"
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}
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},
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{
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"mediaType": "application/vnd.oci.image.manifest.v1+json",
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"size": 7143,
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"digest": "sha256:e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f",
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"platform": {
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"architecture": "ppc64le",
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"os": "linux"
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},
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"annotations": {
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"org.opencontainers.image.ref.name": "v1.0"
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}
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},
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{
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"mediaType": "application/xml",
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"size": 7143,
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"digest": "sha256:b3d63d132d21c3ff4c35a061adf23cf43da8ae054247e32faa95494d904a007e",
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"annotations": {
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"org.freedesktop.specifications.metainfo.version": "1.0",
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"org.freedesktop.specifications.metainfo.type": "AppStream"
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}
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}
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],
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"annotations": {
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"com.example.index.revision": "r124356"
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}
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}
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```
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This illustrates an index that provides two named manifest references and an auxiliary mediatype for this image layout.
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[descriptors]: ./descriptor.md
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