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.
This specification uses the following terms:
<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>
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 <alyspdev@example.com>",
"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",
}
}
<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:
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:
</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>
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.
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, 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.
There is also a format for a single archive which contains complete information about an image, including:
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 filejson - The JSON metadata for an image layerlayer.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.
Unpacking a bundle of image layer JSON files and their corresponding filesystem changesets can be done using a series of steps:
Follow the parent IDs of image layers to find the root ancestor (an image with no parent ID specified).
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.
.wh. and the corresponding file or directory named without this prefix.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.