# 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//` MUST match the digest `:` (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 `` and `` 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 ", "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