From 4fa0eccd10c5c120fe1f641285db920cf643dbe8 Mon Sep 17 00:00:00 2001 From: Aaron Lehmann Date: Fri, 22 Apr 2016 13:27:07 -0700 Subject: [PATCH] Update image specification for content-addressability changes The image spec in image/spec/v1.md is quite a bit out of date. Not only is it missing the changes that went into 1.10 for content addressability, but it has inaccuracies that date back further, such as mentioning storing tarsum in the image configuration. This commit creates image/spec/v1.1.md which brings the specification up to date. It discusses content addressability, new fields in the image configuration, the repository/tag grammar, and the current mechanism for exporting an image. Signed-off-by: Aaron Lehmann --- image/spec/v1.1.md | 639 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 639 insertions(+) create mode 100644 image/spec/v1.1.md diff --git a/image/spec/v1.1.md b/image/spec/v1.1.md new file mode 100644 index 0000000000..0b7d5395f6 --- /dev/null +++ b/image/spec/v1.1.md @@ -0,0 +1,639 @@ +# Docker Image Specification v1.1.0 + +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 1.10. + +## 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-Z_0-9]. +
+
+ 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: +
    +
  • 386
    • +
  • amd64
    • +
  • arm
    • +
+ 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: +
    +
  • darwin
    • +
  • freebsd
    • +
  • linux
    • +
+ 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. +
+
+ 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: + +
    +
  • + type is usually set to layers. There is + also a Windows-specific value layers+base that allows + a base layer to be specified in a field of rootfs + called base_layer. +
    • +
  • + diff_ids is an array of layer content hashes (DiffIDs), in order from bottom-most to top-most. +
    • +
+ + + 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. + +
    +
  • + created: Creation time, expressed as a ISO-8601 formatted + combined date and time +
    • +
  • + author: The author of the build point +
    • +
  • + created_by: The command which created the layer +
    • +
  • + comment: A custom message set when creating the layer +
    • +
  • + empty_layer: 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, a command like ENV which results in no change to the + filesystem). +
    • +
+ +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 compatiblity. + * `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`.