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OCI Image Configuration

An OCI Image is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime. This specification outlines the JSON format describing images for use with a container runtime and execution tool and its relationship to filesystem changesets, described in Layers.

This section defines the application/vnd.oci.image.config.v1+json media type.

Terminology

This specification uses the following terms:

Layer

Image filesystems are composed of layers. Each layer represents a set of filesystem changes in a tar-based layer format, recording files to be added, changed, or deleted relative to its parent layer. Layers do not have configuration metadata such as environment variables or default arguments - these are properties of the image as a whole rather than any particular layer. Using a layer-based or union filesystem such as AUFS, or by computing the diff from filesystem snapshots, the filesystem changeset can be used to present a series of image layers as if they were one cohesive filesystem.

Image JSON

Each image has an associated JSON structure which describes some basic information about the image such as date created, author, as well as execution/runtime configuration like its entrypoint, default arguments, networking, and volumes. The JSON structure also references a cryptographic hash of each layer used by the image, and provides history information for those layers. This JSON is considered to be immutable, because changing it would change the computed ImageID. Changing it means creating a new derived image, instead of changing the existing image.

Layer DiffID

A layer DiffID is a SHA256 digest over the layer's uncompressed tar archive and serialized in the descriptor digest format, e.g., sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9. Layers must be packed and unpacked reproducibly to avoid changing the layer DiffID, for example by using tar-split to save the tar headers.

NOTE: the DiffID is different than the layer digest because it is taken over the uncompressed gzipped layer for application/vnd.oci.image.layer.v1.tar+gzip types.

Layer ChainID

For convenience, it is sometimes useful to refer to a stack of layers with a single identifier. This is called a ChainID. For a single layer (or the layer at the bottom of a stack), the ChainID is equal to the layer's DiffID. Otherwise the ChainID is given by the formula: ChainID(layerN) = SHA256hex(ChainID(layerN-1) + " " + DiffID(layerN)).

ImageID

Each image's ID is given by the SHA256 hash of its configuration JSON. It is represented as a hexadecimal encoding of 256 bits, e.g., sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9. Since the configuration JSON that gets hashed references hashes of each layer in the image, this formulation of the ImageID makes images content-addresable.

Properties

Note: Any OPTIONAL field MAY also be set to null, which is equivalent to being absent.

  • created string, OPTIONAL

    An ISO-8601 formatted combined date and time at which the image was created.

  • author string, OPTIONAL

    Gives the name and/or email address of the person or entity which created and is responsible for maintaining the image.

  • architecture string, REQUIRED

    The CPU architecture which the binaries in this image are built to run on. Configurations SHOULD use, and implementations SHOULD understand, values supported by runtime-spec's platform.arch.

  • os string, REQUIRED

    The name of the operating system which the image is built to run on. Configurations SHOULD use, and implementations SHOULD understand, values supported by runtime-spec's platform.os.

  • config object, OPTIONAL

    The execution parameters which should be used as a base when running a container using the image. This field can be null, in which case any execution parameters should be specified at creation of the container.

    • User string, OPTIONAL

      The username or UID which is a platform-specific structure that allows specific control over which user the process run as. This acts as a default value to use when the value is not specified when creating a container. For Linux based systems, all of the following are valid: user, uid, user:group, uid:gid, uid:group, user:gid. If group/gid is not specified, the default group and supplementary groups of the given user/uid in /etc/passwd from the container are applied.

    • ExposedPorts object, OPTIONAL

      A set of ports to expose from a container running this image. Its keys can be in the format of: port/tcp, port/udp, port with the default protocol being tcp if not specified. These values act as defaults and are merged with any specified when creating a container. NOTE: This JSON structure value is unusual because it is a direct JSON serialization of the Go type map[string]struct{} and is represented in JSON as an object mapping its keys to an empty object.

    • Env array of strings, OPTIONAL

      Entries are in the format of VARNAME="var value". These values act as defaults and are merged with any specified when creating a container.

    • Entrypoint array of strings, OPTIONAL

      A list of arguments to use as the command to execute when the container starts. This value acts as a default and is replaced by an entrypoint specified when creating a container.

    • Cmd array of strings, OPTIONAL

      Default arguments to the entrypoint of the container. These values act as defaults and are replaced with any specified when creating a container. If an Entrypoint value is not specified, then the first entry of the Cmd array should be interpreted as the executable to run.

    • Volumes object, OPTIONAL

      A set of directories which should be created as data volumes in a container running this image. If a file or folder exists within the image with the same path as a data volume, that file or folder is replaced with the data volume and is never merged. NOTE: This JSON structure value is unusual because it is a direct JSON serialization of the Go type map[string]struct{} and is represented in JSON as an object mapping its keys to an empty object.

    • WorkingDir string, OPTIONAL

      Sets the current working directory of the entrypoint process in the container. This value acts as a default and is replaced by a working directory specified when creating a container.

    • Labels object, OPTIONAL

      The field contains arbitrary metadata for the container. Labels MUST be a key-value map where both the key and value MUST be strings. Keys MUST be unique within this map, and best practice is to namespace the keys. Keys SHOULD be named using a reverse domain notation - e.g. com.example.myKey. Keys using the org.opencontainers namespace are reserved and MUST NOT be used by subsequent specifications. If there are no labels then this property MAY either be absent or an empty map. Implementations that are reading/processing this configuration file MUST NOT generate an error if they encounter an unknown labels key.

  • rootfs object, REQUIRED

    The rootfs key references the layer content addresses used by the image. This makes the image config hash depend on the filesystem hash.

    • type string, REQUIRED

      MUST be set to layers. Implementations MUST generate an error if they encounter a unknown value while verifying or unpacking an image.

    • diff_ids array of strings, REQUIRED

      An array of layer content hashes (DiffIDs), in order from first to last.

  • history array of objects, OPTIONAL

    Describes the history of each layer. The array is ordered from first to last. The object has the following fields:

    • created string, OPTIONAL

      Creation time, expressed as an ISO-8601 formatted combined date and time.

    • author string, OPTIONAL

      The author of the build point.

    • created_by string, OPTIONAL

      The command which created the layer.

    • comment string, OPTIONAL

      A custom message set when creating the layer.

    • empty_layer boolean, OPTIONAL

      This field is used to mark if the history item created a filesystem diff. It is set to true if this history item doesn't correspond to an actual layer in the rootfs section (for example, Dockerfile's ENV command results in no change to the filesystem).

Any extra fields in the Image JSON struct are considered implementation specific and should be ignored by any implementations which are unable to interpret them.

Whitespace is OPTIONAL and implementations MAY have compact JSON with no whitespace.

Example

Here is an example image configuration JSON document:

{
    "created": "2015-10-31T22:22:56.015925234Z",
    "author": "Alyssa P. Hacker <alyspdev@example.com>",
    "architecture": "amd64",
    "os": "linux",
    "config": {
        "User": "alice",
        "ExposedPorts": {
            "8080/tcp": {}
        },
        "Env": [
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
            "FOO=oci_is_a",
            "BAR=well_written_spec"
        ],
        "Entrypoint": [
            "/bin/my-app-binary"
        ],
        "Cmd": [
            "--foreground",
            "--config",
            "/etc/my-app.d/default.cfg"
        ],
        "Volumes": {
            "/var/job-result-data": {},
            "/var/log/my-app-logs": {}
        },
        "WorkingDir": "/home/alice",
        "Labels": {
            "com.example.project.git.url": "https://example.com/project.git",
            "com.example.project.git.commit": "45a939b2999782a3f005621a8d0f29aa387e1d6b"
        }
    },
    "rootfs": {
      "diff_ids": [
        "sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1",
        "sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef"
      ],
      "type": "layers"
    },
    "history": [
      {
        "created": "2015-10-31T22:22:54.690851953Z",
        "created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /"
      },
      {
        "created": "2015-10-31T22:22:55.613815829Z",
        "created_by": "/bin/sh -c #(nop) CMD [\"sh\"]",
        "empty_layer": true
      }
    ]
}