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Apply custom metadata

You can apply metadata to your images, containers, or daemons via labels. Labels serve a wide range of uses, such as adding notes or licensing information to an image, or to identify a host.

A label is a <key> / <value> pair. Docker stores the label values as strings. You can specify multiple labels but each <key> must be unique or the value will be overwritten. If you specify the same key several times but with different values, newer labels overwrite previous labels. Docker uses the last key=value you supply.

Note: Support for daemon-labels was added in Docker 1.4.1. Labels on containers and images were added in Docker 1.6.0

Label keys (namespaces)

Docker puts no hard restrictions on the key used for a label. However, using simple keys can easily lead to conflicts. For example, you have chosen to categorize your images by CPU architecture using “architecture” labels in your Dockerfiles:

LABEL architecture="amd64"

LABEL architecture="ARMv7"

Another user may apply the same label based on a building’s “architecture”:

LABEL architecture="Art Nouveau"

To prevent naming conflicts, Docker recommends using namespaces to label keys using reverse domain notation. Use the following guidelines to name your keys:

  • All (third-party) tools should prefix their keys with the reverse DNS notation of a domain controlled by the author. For example, com.example.some-label.

  • The com.docker.*, io.docker.* and org.dockerproject.* namespaces are reserved for Docker’s internal use.

  • Keys should only consist of lower-cased alphanumeric characters, dots and dashes (for example, [a-z0-9-.]).

  • Keys should start and end with an alpha numeric character.

  • Keys may not contain consecutive dots or dashes.

  • Keys without namespace (dots) are reserved for CLI use. This allows end- users to add metadata to their containers and images without having to type cumbersome namespaces on the command-line.

These are simply guidelines and Docker does not enforce them. However, for the benefit of the community, you should use namespaces for your label keys.

Store structured data in labels

Label values can contain any data type as long as it can be represented as a string. For example, consider this JSON document:

{
    "Description": "A containerized foobar",
    "Usage": "docker run --rm example/foobar [args]",
    "License": "GPL",
    "Version": "0.0.1-beta",
    "aBoolean": true,
    "aNumber" : 0.01234,
    "aNestedArray": ["a", "b", "c"]
}

You can store this struct in a label by serializing it to a string first:

LABEL com.example.image-specs="{\"Description\":\"A containerized foobar\",\"Usage\":\"docker run --rm example\\/foobar [args]\",\"License\":\"GPL\",\"Version\":\"0.0.1-beta\",\"aBoolean\":true,\"aNumber\":0.01234,\"aNestedArray\":[\"a\",\"b\",\"c\"]}"

While it is possible to store structured data in label values, Docker treats this data as a ‘regular’ string. This means that Docker doesn’t offer ways to query (filter) based on nested properties. If your tool needs to filter on nested properties, the tool itself needs to implement this functionality.

Add labels to images

To add labels to an image, use the LABEL instruction in your Dockerfile:

LABEL [<namespace>.]<key>=<value> ...

The LABEL instruction adds a label to your image. A LABEL consists of a <key> and a <value>. Use an empty string for labels that don’t have a <value>, Use surrounding quotes or backslashes for labels that contain white space characters in the <value>:

LABEL vendor=ACME\ Incorporated
LABEL com.example.version.is-beta=
LABEL com.example.version.is-production=""
LABEL com.example.version="0.0.1-beta"
LABEL com.example.release-date="2015-02-12"

The LABEL instruction also supports setting multiple <key> / <value> pairs in a single instruction:

LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12"

Long lines can be split up by using a backslash (\) as continuation marker:

LABEL vendor=ACME\ Incorporated \
      com.example.is-beta= \
      com.example.is-production="" \
      com.example.version="0.0.1-beta" \
      com.example.release-date="2015-02-12"

Docker recommends you add multiple labels in a single LABEL instruction. Using individual instructions for each label can result in an inefficient image. This is because each LABEL instruction in a Dockerfile produces a new IMAGE layer.

You can view the labels via the docker inspect command:

$ docker inspect 4fa6e0f0c678

...
"Labels": {
    "vendor": "ACME Incorporated",
    "com.example.is-beta": "",
    "com.example.is-production": "",
    "com.example.version": "0.0.1-beta",
    "com.example.release-date": "2015-02-12"
}
...

# Inspect labels on container
$ docker inspect -f "{{json .Config.Labels }}" 4fa6e0f0c678

{"Vendor":"ACME Incorporated","com.example.is-beta":"", "com.example.is-production":"", "com.example.version":"0.0.1-beta","com.example.release-date":"2015-02-12"}

# Inspect labels on images
$ docker inspect -f "{{json .ContainerConfig.Labels }}" myimage

Query labels

Besides storing metadata, you can filter images and containers by label. To list all running containers that have the com.example.is-beta label:

# List all running containers that have a `com.example.is-beta` label
$ docker ps --filter "label=com.example.is-beta"

List all running containers with the label color that have a value blue:

$ docker ps --filter "label=color=blue"

List all images with the label vendor that have the value ACME:

$ docker images --filter "label=vendor=ACME"

Container labels

docker run \
   -d \
   --label com.example.group="webservers" \
   --label com.example.environment="production" \
   busybox \
   top

Please refer to the Query labels section above for information on how to query labels set on a container.

Daemon labels

docker daemon \
  --dns 8.8.8.8 \
  --dns 8.8.4.4 \
  -H unix:///var/run/docker.sock \
  --label com.example.environment="production" \
  --label com.example.storage="ssd"

These labels appear as part of the docker info output for the daemon:

$ docker -D info

Containers: 12
 Running: 5
 Paused: 2
 Stopped: 5
Images: 672
Server Version: 1.9.0
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 697
 Dirperm1 Supported: true
Execution Driver: native-0.2
Logging Driver: json-file
Kernel Version: 3.19.0-22-generic
Operating System: Ubuntu 15.04
CPUs: 24
Total Memory: 62.86 GiB
Name: docker
ID: I54V:OLXT:HVMM:TPKO:JPHQ:CQCD:JNLC:O3BZ:4ZVJ:43XJ:PFHZ:6N2S
Debug mode (server): true
 File Descriptors: 59
 Goroutines: 159
 System Time: 2015-09-23T14:04:20.699842089+08:00
 EventsListeners: 0
 Init SHA1:
 Init Path: /usr/bin/docker
 Docker Root Dir: /var/lib/docker
 Http Proxy: http://test:test@localhost:8080
 Https Proxy: https://test:test@localhost:8080
WARNING: No swap limit support
Username: svendowideit
Registry: [https://index.docker.io/v1/]
Labels:
 com.example.environment=production
 com.example.storage=ssd