diff --git a/image/spec/v1.2.md b/image/spec/v1.2.md new file mode 100644 index 0000000000..c014bf9a09 --- /dev/null +++ b/image/spec/v1.2.md @@ -0,0 +1,696 @@ +# Docker Image Specification v1.2.0 + +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. + +This version of the image specification was adopted starting in Docker 1.12. + +## Terminology + +This specification uses the following terms: + +
+
+ Layer +
+
+ Images are composed of layers. Each layer is a set of filesystem + changes. 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. +
+
+ Image JSON +
+
+ Each image 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. 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. +
+
+ Image Filesystem Changeset +
+
+ 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. +
+
+ Layer DiffID +
+
+ Layers are referenced by cryptographic hashes of their serialized + representation. This is a SHA256 digest over the tar archive used to + transport the layer, represented as a hexadecimal encoding of 256 bits, e.g., + sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9. + Layers must be packed and unpacked reproducibly to avoid changing the + layer ID, for example by using tar-split to save the tar headers. Note + that the digest used as the layer ID is taken over an uncompressed + version of the tar. +
+
+ 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. +
+
+ Tag +
+
+ A tag serves to map a descriptive, user-given name to any single image + ID. Tag values are limited to the set of characters + [a-zA-Z0-9_.-], except they may not start with a . + or - character. Tags are limited to 127 characters. +
+
+ Repository +
+
+ A collection of tags grouped under a common prefix (the name component + before :). For example, in an image tagged with the name + my-app:3.1.4, my-app is the Repository + component of the name. A repository name is made up of slash-separated + name components, optionally prefixed by a DNS hostname. The hostname + must follow comply with standard DNS rules, but may not contain + _ characters. If a hostname is present, it may optionally + be followed by a port number in the format :8080. + Name components may contain lowercase characters, digits, and + separators. A separator is defined as a period, one or two underscores, + or one or more dashes. A name component may not start or end with + a separator. +
+
+ +## Image JSON Description + +Here is an example image JSON file: + +``` +{ + "created": "2015-10-31T22:22:56.015925234Z", + "author": "Alyssa P. Hacker <alyspdev@example.com>", + "architecture": "amd64", + "os": "linux", + "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", + }, + "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 + } + ] +} +``` + +Note that image JSON files produced by Docker don't contain formatting +whitespace. It has been added to this example for clarity. + +### Image JSON Field Descriptions + +
+
+ created string +
+
+ ISO-8601 formatted combined date and time at which the image was + created. +
+
+ author string +
+
+ Gives the name and/or email address of the person or entity which + created and is responsible for maintaining the image. +
+
+ architecture string +
+
+ The CPU architecture which the binaries in this image are built to run + on. Possible values include: + + More values may be supported in the future and any of these may or may + not be supported by a given container runtime implementation. +
+
+ os string +
+
+ The name of the operating system which the image is built to run on. + Possible values include: + + More values may be supported in the future and any of these may or may + not be supported by a given container runtime implementation. +
+
+ config struct +
+
+ 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. + +

Container RunConfig Field Descriptions

+ +
+
+ User string +
+
+

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.

+ +

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.

+
+
+ Memory integer +
+
+ Memory limit (in bytes). This acts as a default value to use + when the value is not specified when creating a container. +
+
+ MemorySwap integer +
+
+ Total memory usage (memory + swap); set to -1 to + disable swap. This acts as a default value to use when the + value is not specified when creating a container. +
+
+ CpuShares integer +
+
+ 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. +
+
+ ExposedPorts struct +
+
+ 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 + map[string]struct{} and is represented in JSON as + an object mapping its keys to an empty object. Here is an + example: + +
{
+    "8080": {},
+    "53/udp": {},
+    "2356/tcp": {}
+}
+ + 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. +
+
+ Env array of strings +
+
+ 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 +
+
+ 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 +
+
+ 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 Entrypoint value is + not specified, then the first entry of the Cmd + array should be interpreted as the executable to run. +
+
+ Healthcheck struct +
+
+ A test to perform to determine whether the container is healthy. + Here is an example: +
{
+  "Test": [
+      "CMD-SHELL",
+      "/usr/bin/check-health localhost"
+  ],
+  "Interval": 30000000000,
+  "Timeout": 10000000000,
+  "Retries": 3
+}
+ The object has the following fields. +
+
+ Test array of strings +
+
+ The test to perform to check that the container is healthy. + The options are: +
    +
  • [] : inherit healthcheck from base image
    • +
  • ["NONE"] : disable healthcheck
    • +
  • ["CMD", arg1, arg2, ...] : exec arguments directly
    • +
  • ["CMD-SHELL", command] : run command with system's default shell
    • +
+ + The test command should exit with a status of 0 if the container is healthy, + or with 1 if it is unhealthy. +
+
+ Interval integer +
+
+ Number of nanoseconds to wait between probe attempts. +
+
+ Timeout integer +
+
+ Number of nanoseconds to wait before considering the check to have hung. +
+
+ Retries integer +
+
+ The number of consecutive failures needed to consider a container as unhealthy. +
+
+ + In each case, the field can be omitted to indicate that the + value should be inherited from the base layer. + + These values act as defaults and are merged with any specified + when creating a container. +
+
+ Volumes struct +
+
+ 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 map[string]struct{} and is represented in + JSON as an object mapping its keys to an empty object. Here is + an example: +
{
+    "/var/my-app-data/": {},
+    "/etc/some-config.d/": {},
+}
+
+
+ WorkingDir string +
+
+ 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. +
+
+
+
+ rootfs struct +
+
+ The rootfs key references the layer content addresses used by the + image. This makes the image config hash depend on the filesystem hash. + rootfs has two subkeys: + + + + + Here is an example rootfs section: + +
"rootfs": {
+  "diff_ids": [
+    "sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1",
+    "sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef",
+    "sha256:13f53e08df5a220ab6d13c58b2bf83a59cbdc2e04d0a3f041ddf4b0ba4112d49"
+  ],
+  "type": "layers"
+}
+
+
+ history struct +
+
+ history is an array of objects describing the history of + each layer. The array is ordered from bottom-most layer to top-most + layer. The object has the following fields. + + + +Here is an example history section: + +
"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
+  }
+]
+
+
+ +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. Here is the +initial empty directory structure for the a changeset using the +randomly-generated directory name `c3167915dc9d` ([actual layer DiffIDs are +generated based on the content](#id_desc)). + +``` +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 +``` + +To make changes to the filesystem of this container image, create a new +directory, 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 + - image configuration JSON file + - all tar archives of each layer filesystem changesets + +For example, here's what the full archive of `library/busybox` is (displayed in +`tree` format): + +``` +. +├── 47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb.json +├── 5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a +│   ├── VERSION +│   ├── json +│   └── layer.tar +├── a65da33792c5187473faa80fa3e1b975acba06712852d1dea860692ccddf3198 +│   ├── VERSION +│   ├── json +│   └── layer.tar +├── manifest.json +└── repositories +``` + +There is a directory for each layer in the image. Each directory is named with +a 64 character hex name that is deterministically generated from the layer +information. These names are not necessarily layer DiffIDs or ChainIDs. Each of +these directories contains 3 files: + + * `VERSION` - The schema version of the `json` file + * `json` - The legacy JSON metadata for an image layer. In this version of + the image specification, layers don't have JSON metadata, but in + [version 1](v1.md), they did. A file is created for each layer in the + v1 format for backward compatibility. + * `layer.tar` - The Tar archive of the filesystem changeset for an image + layer. + +Note that this directory layout is only important for backward compatibility. +Current implementations use the paths specified in `manifest.json`. + +The content of the `VERSION` files is simply the semantic version of the JSON +metadata schema: + +``` +1.0 +``` + +The `repositories` file is another JSON file which describes names/tags: + +``` +{ + "busybox":{ + "latest":"5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a" + } +} +``` + +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. +This file is only used for backwards compatibility. Current implementations use +the `manifest.json` file instead. + +The `manifest.json` file provides the image JSON for the top-level image, and +optionally for parent images that this image was derived from. It consists of +an array of metadata entries: + +``` +[ + { + "Config": "47bcc53f74dc94b1920f0b34f6036096526296767650f223433fe65c35f149eb.json", + "RepoTags": ["busybox:latest"], + "Layers": [ + "a65da33792c5187473faa80fa3e1b975acba06712852d1dea860692ccddf3198/layer.tar", + "5f29f704785248ddb9d06b90a11b5ea36c534865e9035e4022bb2e71d4ecbb9a/layer.tar" + ] + } +] +``` + +There is an entry in the array for each image. + +The `Config` field references another file in the tar which includes the image +JSON for this image. + +The `RepoTags` field lists references pointing to this image. + +The `Layers` field points to the filesystem changeset tars. + +An optional `Parent` field references the imageID of the parent image. This +parent must be part of the same `manifest.json` file. + +This file shouldn't be confused with the distribution manifest, used to push +and pull images. + +Generally, implementations that support this version of the spec will use +the `manifest.json` file if available, and older implementations will use the +legacy `*/json` files and `repositories`.