Merge pull request #9560 from jlhawn/image_spec

Adds Docker Image v1 Spec Documention

image/spec/v1.md

@@ -0,0 +1,573 @@
+# 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 A 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 hexidecimal 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 [TarSum specification](https://github.com/docker/docker/blob/master/pkg/tarsum/tarsum_spec.md).
+    </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 suffic
+        component.
+    </dd>
+</dl>
+
+## Image JSON Schema
+
+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 decendent 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 image.
+
+        <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 creating 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.