github.com/resin-io/docker@v1.13.1/image/spec/v1.md

# Docker Image Specification v1.0.0

An *Image* is an ordered collection of root filesystem changes and the
corresponding execution parameters for use within a container runtime. This
specification outlines the format of these filesystem changes and corresponding
parameters and describes how to create and use them for use with a container
runtime and execution tool.

## Terminology

This specification uses the following terms:

<dl>
    <dt>
        Layer
    </dt>
    <dd>
        Images are composed of <i>layers</i>. <i>Image layer</i> is a general
        term which may be used to refer to one or both of the following:

        <ol>
            <li>The metadata for the layer, described in the JSON format.</li>
            <li>The filesystem changes described by a layer.</li>
        </ol>

        To refer to the former you may use the term <i>Layer JSON</i> or
        <i>Layer Metadata</i>. To refer to the latter you may use the term
        <i>Image Filesystem Changeset</i> or <i>Image Diff</i>.
    </dd>
    <dt>
        Image JSON
    </dt>
    <dd>
        Each layer has an associated JSON structure which describes some
        basic information about the image such as date created, author, and the
        ID of its parent image as well as execution/runtime configuration like
        its entry point, default arguments, CPU/memory shares, networking, and
        volumes.
    </dd>
    <dt>
        Image Filesystem Changeset
    </dt>
    <dd>
        Each layer has an archive of the files which have been added, changed,
        or deleted relative to its parent 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.
    </dd>
    <dt>
        Image ID <a name="id_desc"></a>
    </dt>
    <dd>
        Each layer is given an ID upon its creation. It is 
        represented as a hexadecimal encoding of 256 bits, e.g.,
        <code>a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9</code>.
        Image IDs should be sufficiently random so as to be globally unique.
        32 bytes read from <code>/dev/urandom</code> is sufficient for all
        practical purposes. Alternatively, an image ID may be derived as a
        cryptographic hash of image contents as the result is considered
        indistinguishable from random. The choice is left up to implementors.
    </dd>
    <dt>
        Image Parent
    </dt>
    <dd>
        Most layer metadata structs contain a <code>parent</code> field which
        refers to the Image from which another directly descends. An image
        contains a separate JSON metadata file and set of changes relative to
        the filesystem of its parent image. <i>Image Ancestor</i> and
        <i>Image Descendant</i> are also common terms.
    </dd>
    <dt>
        Image Checksum
    </dt>
    <dd>
        Layer metadata structs contain a cryptographic hash of the contents of
        the layer's filesystem changeset. Though the set of changes exists as a
        simple Tar archive, two archives with identical filenames and content
        will have different SHA digests if the last-access or last-modified
        times of any entries differ. For this reason, image checksums are
        generated using the TarSum algorithm which produces a cryptographic
        hash of file contents and selected headers only. Details of this
        algorithm are described in the separate <a href="https://github.com/docker/docker/blob/master/pkg/tarsum/tarsum_spec.md">TarSum specification</a>.
    </dd>
    <dt>
        Tag
    </dt>
    <dd>
        A tag serves to map a descriptive, user-given name to any single image
        ID. An image name suffix (the name component after <code>:</code>) is
        often referred to as a tag as well, though it strictly refers to the
        full name of an image. Acceptable values for a tag suffix are
        implementation specific, but they SHOULD be limited to the set of
        alphanumeric characters <code>[a-zA-z0-9]</code>, punctuation
        characters <code>[._-]</code>, and MUST NOT contain a <code>:</code>
        character.
    </dd>
    <dt>
        Repository
    </dt>
    <dd>
        A collection of tags grouped under a common prefix (the name component
        before <code>:</code>). For example, in an image tagged with the name
        <code>my-app:3.1.4</code>, <code>my-app</code> is the <i>Repository</i>
        component of the name. Acceptable values for repository name are
        implementation specific, but they SHOULD be limited to the set of
        alphanumeric characters <code>[a-zA-z0-9]</code>, and punctuation
        characters <code>[._-]</code>, however it MAY contain additional
        <code>/</code> and <code>:</code> characters for organizational
        purposes, with the last <code>:</code> character being interpreted
        dividing the repository component of the name from the tag suffix
        component.
    </dd>
</dl>

## Image JSON Description

Here is an example image JSON file:

```
{  
    "id": "a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9",
    "parent": "c6e3cedcda2e3982a1a6760e178355e8e65f7b80e4e5248743fa3549d284e024",
    "checksum": "tarsum.v1+sha256:e58fcf7418d2390dec8e8fb69d88c06ec07039d651fedc3aa72af9972e7d046b",
    "created": "2014-10-13T21:19:18.674353812Z",
    "author": "Alyssa P. Hacker &ltalyspdev@example.com&gt",
    "architecture": "amd64",
    "os": "linux",
    "Size": 271828,
    "config": {
        "User": "alice",
        "Memory": 2048,
        "MemorySwap": 4096,
        "CpuShares": 8,
        "ExposedPorts": {  
            "8080/tcp": {}
        },
        "Env": [  
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
            "FOO=docker_is_a_really",
            "BAR=great_tool_you_know"
        ],
        "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",
    }
}
```

### Image JSON Field Descriptions

<dl>
    <dt>
        id <code>string</code>
    </dt>
    <dd>
        Randomly generated, 256-bit, hexadecimal encoded. Uniquely identifies
        the image.
    </dd>
    <dt>
        parent <code>string</code>
    </dt>
    <dd>
        ID of the parent image. If there is no parent image then this field
        should be omitted. A collection of images may share many of the same
        ancestor layers. This organizational structure is strictly a tree with
        any one layer having either no parent or a single parent and zero or
        more descendant layers. Cycles are not allowed and implementations
        should be careful to avoid creating them or iterating through a cycle
        indefinitely.
    </dd>
    <dt>
        created <code>string</code>
    </dt>
    <dd>
        ISO-8601 formatted combined date and time at which the image was
        created.
    </dd>
    <dt>
        author <code>string</code>
    </dt>
    <dd>
        Gives the name and/or email address of the person or entity which
        created and is responsible for maintaining the image.
    </dd>
    <dt>
        architecture <code>string</code>
    </dt>
    <dd>
        The CPU architecture which the binaries in this image are built to run
        on. Possible values include:
        <ul>
            <li>386</li>
            <li>amd64</li>
            <li>arm</li>
        </ul>
        More values may be supported in the future and any of these may or may
        not be supported by a given container runtime implementation.
    </dd>
    <dt>
        os <code>string</code>
    </dt>
    <dd>
        The name of the operating system which the image is built to run on.
        Possible values include:
        <ul>
            <li>darwin</li>
            <li>freebsd</li>
            <li>linux</li>
        </ul>
        More values may be supported in the future and any of these may or may
        not be supported by a given container runtime implementation.
    </dd>
    <dt>
        checksum <code>string</code>
    </dt>
    <dd>
        Image Checksum of the filesystem changeset associated with the image
        layer.
    </dd>
    <dt>
        Size <code>integer</code>
    </dt>
    <dd>
        The size in bytes of the filesystem changeset associated with the image
        layer.
    </dd>
    <dt>
        config <code>struct</code>
    </dt>
    <dd>
        The execution parameters which should be used as a base when running a
        container using the image. This field can be <code>null</code>, in
        which case any execution parameters should be specified at creation of
        the container.

        <h4>Container RunConfig Field Descriptions</h4>

        <dl>
            <dt>
                User <code>string</code>
            </dt>
            <dd>
                <p>The username or UID which the process in the container should
                run as. This acts as a default value to use when the value is
                not specified when creating a container.</p>

                <p>All of the following are valid:</p>

                <ul>
                    <li><code>user</code></li>
                    <li><code>uid</code></li>
                    <li><code>user:group</code></li>
                    <li><code>uid:gid</code></li>
                    <li><code>uid:group</code></li>
                    <li><code>user:gid</code></li>
                </ul>

                <p>If <code>group</code>/<code>gid</code> is not specified, the
                default group and supplementary groups of the given
                <code>user</code>/<code>uid</code> in <code>/etc/passwd</code>
                from the container are applied.</p>
            </dd>
            <dt>
                Memory <code>integer</code>
            </dt>
            <dd>
                Memory limit (in bytes). This acts as a default value to use
                when the value is not specified when creating a container.
            </dd>
            <dt>
                MemorySwap <code>integer</code>
            </dt>
            <dd>
                Total memory usage (memory + swap); set to <code>-1</code> to
                disable swap. This acts as a default value to use when the
                value is not specified when creating a container.
            </dd>
            <dt>
                CpuShares <code>integer</code>
            </dt>
            <dd>
                CPU shares (relative weight vs. other containers). This acts as
                a default value to use when the value is not specified when
                creating a container.
            </dd>
            <dt>
                ExposedPorts <code>struct</code>
            </dt>
            <dd>
                A set of ports to expose from a container running this image.
                This JSON structure value is unusual because it is a direct
                JSON serialization of the Go type
                <code>map[string]struct{}</code> and is represented in JSON as
                an object mapping its keys to an empty object. Here is an
                example:

<pre>{
    "8080": {},
    "53/udp": {},
    "2356/tcp": {}
}</pre>

                Its keys can be in the format of:
                <ul>
                    <li>
                        <code>"port/tcp"</code>
                    </li>
                    <li>
                        <code>"port/udp"</code>
                    </li>
                    <li>
                        <code>"port"</code>
                    </li>
                </ul>
                with the default protocol being <code>"tcp"</code> if not
                specified.

                These values act as defaults and are merged with any specified
                when creating a container.
            </dd>
            <dt>
                Env <code>array of strings</code>
            </dt>
            <dd>
                Entries are in the format of <code>VARNAME="var value"</code>.
                These values act as defaults and are merged with any specified
                when creating a container.
            </dd>
            <dt>
                Entrypoint <code>array of strings</code>
            </dt>
            <dd>
                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.
            </dd>
            <dt>
                Cmd <code>array of strings</code>
            </dt>
            <dd>
                Default arguments to the entry point of the container. These
                values act as defaults and are replaced with any specified when
                creating a container. If an <code>Entrypoint</code> value is
                not specified, then the first entry of the <code>Cmd</code>
                array should be interpreted as the executable to run.
            </dd>
            <dt>
                Volumes <code>struct</code>
            </dt>
            <dd>
                A set of directories which should be created as data volumes in
                a container running this image. This JSON structure value is
                unusual because it is a direct JSON serialization of the Go
                type <code>map[string]struct{}</code> and is represented in
                JSON as an object mapping its keys to an empty object. Here is
                an example:
<pre>{
    "/var/my-app-data/": {},
    "/etc/some-config.d/": {},
}</pre>
            </dd>
            <dt>
                WorkingDir <code>string</code>
            </dt>
            <dd>
                Sets the current working directory of the entry point process
                in the container. This value acts as a default and is replaced
                by a working directory specified when creating a container.
            </dd>
        </dl>
    </dd>
</dl>

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.

## Creating an Image Filesystem Changeset

An example of creating an Image Filesystem Changeset follows.

An image root filesystem is first created as an empty directory named with the
ID of the image being created. Here is the initial empty directory structure
for the changeset for an image with ID `c3167915dc9d` ([real IDs are much
longer](#id_desc), but this example use a truncated one here for brevity.
Implementations need not name the rootfs directory in this way but it may be
convenient for keeping record of a large number of image layers.):

```
c3167915dc9d/
```

Files and directories are then created:

```
c3167915dc9d/
    etc/
        my-app-config
    bin/
        my-app-binary
        my-app-tools
```

The `c3167915dc9d` directory is then committed as a plain Tar archive with
entries for the following files:

```
etc/my-app-config
bin/my-app-binary
bin/my-app-tools
```

The TarSum checksum for the archive file is then computed and placed in the
JSON metadata along with the execution parameters.

To make changes to the filesystem of this container image, create a new
directory named with a new ID, such as `f60c56784b83`, and initialize it with
a snapshot of the parent image's root filesystem, so that the directory is
identical to that of `c3167915dc9d`. NOTE: a copy-on-write or union filesystem
can make this very efficient:

```
f60c56784b83/
    etc/
        my-app-config
    bin/
        my-app-binary
        my-app-tools
```

This example change is going add a configuration directory at `/etc/my-app.d`
which contains a default config file. There's also a change to the
`my-app-tools` binary to handle the config layout change. The `f60c56784b83`
directory then looks like this:

```
f60c56784b83/
    etc/
        my-app.d/
            default.cfg
    bin/
        my-app-binary
        my-app-tools
```

This reflects the removal of `/etc/my-app-config` and creation of a file and
directory at `/etc/my-app.d/default.cfg`. `/bin/my-app-tools` has also been
replaced with an updated version. Before committing this directory to a
changeset, because it has a parent image, it is first compared with the
directory tree of the parent snapshot, `f60c56784b83`, looking for files and
directories that have been added, modified, or removed. The following changeset
is found:

```
Added:      /etc/my-app.d/default.cfg
Modified:   /bin/my-app-tools
Deleted:    /etc/my-app-config
```

A Tar Archive is then created which contains *only* this changeset: The added
and modified files and directories in their entirety, and for each deleted item
an entry for an empty file at the same location but with the basename of the
deleted file or directory prefixed with `.wh.`. The filenames prefixed with
`.wh.` are known as "whiteout" files. NOTE: For this reason, it is not possible
to create an image root filesystem which contains a file or directory with a
name beginning with `.wh.`. The resulting Tar archive for `f60c56784b83` has
the following entries:

```
/etc/my-app.d/default.cfg
/bin/my-app-tools
/etc/.wh.my-app-config
```

Any given image is likely to be composed of several of these Image Filesystem
Changeset tar archives.

## Combined Image JSON + Filesystem Changeset Format

There is also a format for a single archive which contains complete information
about an image, including:

 - repository names/tags
 - all image layer JSON files
 - all tar archives of each layer filesystem changesets

For example, here's what the full archive of `library/busybox` is (displayed in
`tree` format):

```
.
├── 5785b62b697b99a5af6cd5d0aabc804d5748abbb6d3d07da5d1d3795f2dcc83e
│   ├── VERSION
│   ├── json
│   └── layer.tar
├── a7b8b41220991bfc754d7ad445ad27b7f272ab8b4a2c175b9512b97471d02a8a
│   ├── VERSION
│   ├── json
│   └── layer.tar
├── a936027c5ca8bf8f517923169a233e391cbb38469a75de8383b5228dc2d26ceb
│   ├── VERSION
│   ├── json
│   └── layer.tar
├── f60c56784b832dd990022afc120b8136ab3da9528094752ae13fe63a2d28dc8c
│   ├── VERSION
│   ├── json
│   └── layer.tar
└── repositories
```

There are one or more directories named with the ID for each layer in a full
image. Each of these directories contains 3 files:

 * `VERSION` - The schema version of the `json` file
 * `json` - The JSON metadata for an image layer
 * `layer.tar` - The Tar archive of the filesystem changeset for an image
   layer.

The content of the `VERSION` files is simply the semantic version of the JSON
metadata schema:

```
1.0
```

And the `repositories` file is another JSON file which describes names/tags:

```
{  
    "busybox":{  
        "latest":"5785b62b697b99a5af6cd5d0aabc804d5748abbb6d3d07da5d1d3795f2dcc83e"
    }
}
```

Every key in this object is the name of a repository, and maps to a collection
of tag suffixes. Each tag maps to the ID of the image represented by that tag.

## Loading an Image Filesystem Changeset

Unpacking a bundle of image layer JSON files and their corresponding filesystem
changesets can be done using a series of steps:

1. Follow the parent IDs of image layers to find the root ancestor (an image
with no parent ID specified).
2. For every image layer, in order from root ancestor and descending down,
extract the contents of that layer's filesystem changeset archive into a
directory which will be used as the root of a container filesystem.

    - Extract all contents of each archive.
    - Walk the directory tree once more, removing any files with the prefix
    `.wh.` and the corresponding file or directory named without this prefix.


## Implementations

This specification is an admittedly imperfect description of an
imperfectly-understood problem. The Docker project is, in turn, an attempt to
implement this specification. Our goal and our execution toward it will evolve
over time, but our primary concern in this specification and in our
implementation is compatibility and interoperability.