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Compose file reference

The compose file is a YAML file where all the top level keys are the name of a service, and the values are the service definition. The default path for a compose file is ./docker-compose.yml.

Each service defined in docker-compose.yml must specify exactly one of image or build. Other keys are optional, and are analogous to their docker run command-line counterparts.

As with docker run, options specified in the Dockerfile (e.g., CMD, EXPOSE, VOLUME, ENV) are respected by default - you don't need to specify them again in docker-compose.yml.

Service configuration reference

This section contains a list of all configuration options supported by a service definition.

build

Path to a directory containing a Dockerfile. When the value supplied is a relative path, it is interpreted as relative to the location of the yml file itself. This directory is also the build context that is sent to the Docker daemon.

Compose will build and tag it with a generated name, and use that image thereafter.

build: /path/to/build/dir

Using build together with image is not allowed. Attempting to do so results in an error.

cap_add, cap_drop

Add or drop container capabilities. See man 7 capabilities for a full list.

cap_add:
  - ALL

cap_drop:
  - NET_ADMIN
  - SYS_ADMIN

command

Override the default command.

command: bundle exec thin -p 3000

cgroup_parent

Specify an optional parent cgroup for the container.

cgroup_parent: m-executor-abcd

container_name

Specify a custom container name, rather than a generated default name.

container_name: my-web-container

Because Docker container names must be unique, you cannot scale a service beyond 1 container if you have specified a custom name. Attempting to do so results in an error.

devices

List of device mappings. Uses the same format as the --device docker client create option.

devices:
  - "/dev/ttyUSB0:/dev/ttyUSB0"

dns

Custom DNS servers. Can be a single value or a list.

dns: 8.8.8.8
dns:
  - 8.8.8.8
  - 9.9.9.9

dns_search

Custom DNS search domains. Can be a single value or a list.

dns_search: example.com
dns_search:
  - dc1.example.com
  - dc2.example.com

dockerfile

Alternate Dockerfile.

Compose will use an alternate file to build with. A build path must also be specified using the build key.

build: /path/to/build/dir
dockerfile: Dockerfile-alternate

Using dockerfile together with image is not allowed. Attempting to do so results in an error.

env_file

Add environment variables from a file. Can be a single value or a list.

If you have specified a Compose file with docker-compose -f FILE, paths in env_file are relative to the directory that file is in.

Environment variables specified in environment override these values.

env_file: .env

env_file:
  - ./common.env
  - ./apps/web.env
  - /opt/secrets.env

Compose expects each line in an env file to be in VAR=VAL format. Lines beginning with # (i.e. comments) are ignored, as are blank lines.

# Set Rails/Rack environment
RACK_ENV=development

environment

Add environment variables. You can use either an array or a dictionary. Any boolean values; true, false, yes no, need to be enclosed in quotes to ensure they are not converted to True or False by the YML parser.

Environment variables with only a key are resolved to their values on the machine Compose is running on, which can be helpful for secret or host-specific values.

environment:
  RACK_ENV: development
  SHOW: 'true'
  SESSION_SECRET:

environment:
  - RACK_ENV=development
  - SHOW=true
  - SESSION_SECRET

expose

Expose ports without publishing them to the host machine - they'll only be accessible to linked services. Only the internal port can be specified.

expose:
 - "3000"
 - "8000"

extends

Extend another service, in the current file or another, optionally overriding configuration.

You can use extends on any service together with other configuration keys. The extends value must be a dictionary defined with a required service and an optional file key.

extends:
  file: common.yml
  service: webapp

The service the name of the service being extended, for example web or database. The file is the location of a Compose configuration file defining that service.

If you omit the file Compose looks for the service configuration in the current file. The file value can be an absolute or relative path. If you specify a relative path, Compose treats it as relative to the location of the current file.

You can extend a service that itself extends another. You can extend indefinitely. Compose does not support circular references and docker-compose returns an error if it encounters one.

For more on extends, see the the extends documentation.

external_links

Link to containers started outside this docker-compose.yml or even outside of Compose, especially for containers that provide shared or common services. external_links follow semantics similar to links when specifying both the container name and the link alias (CONTAINER:ALIAS).

external_links:
 - redis_1
 - project_db_1:mysql
 - project_db_1:postgresql

extra_hosts

Add hostname mappings. Use the same values as the docker client --add-host parameter.

extra_hosts:
 - "somehost:162.242.195.82"
 - "otherhost:50.31.209.229"

An entry with the ip address and hostname will be created in /etc/hosts inside containers for this service, e.g:

162.242.195.82  somehost
50.31.209.229   otherhost

image

Tag or partial image ID. Can be local or remote - Compose will attempt to pull if it doesn't exist locally.

image: ubuntu
image: orchardup/postgresql
image: a4bc65fd

labels

Add metadata to containers using Docker labels. You can use either an array or a dictionary.

It's recommended that you use reverse-DNS notation to prevent your labels from conflicting with those used by other software.

labels:
  com.example.description: "Accounting webapp"
  com.example.department: "Finance"
  com.example.label-with-empty-value: ""

labels:
  - "com.example.description=Accounting webapp"
  - "com.example.department=Finance"
  - "com.example.label-with-empty-value"

links

Link to containers in another service. Either specify both the service name and the link alias (SERVICE:ALIAS), or just the service name (which will also be used for the alias).

links:
 - db
 - db:database
 - redis

An entry with the alias' name will be created in /etc/hosts inside containers for this service, e.g:

172.17.2.186  db
172.17.2.186  database
172.17.2.187  redis

Environment variables will also be created - see the environment variable reference for details.

log_driver

Specify a logging driver for the service's containers, as with the --log-driver option for docker run (documented here).

The default value is json-file.

log_driver: "json-file"
log_driver: "syslog"
log_driver: "none"

Note: Only the json-file driver makes the logs available directly from docker-compose up and docker-compose logs. Using any other driver will not print any logs.

log_opt

Specify logging options with log_opt for the logging driver, as with the --log-opt option for docker run.

Logging options are key value pairs. An example of syslog options:

log_driver: "syslog"
log_opt:
  syslog-address: "tcp://192.168.0.42:123"

net

Networking mode. Use the same values as the docker client --net parameter.

net: "bridge"
net: "none"
net: "container:[name or id]"
net: "host"

pid

pid: "host"

Sets the PID mode to the host PID mode. This turns on sharing between container and the host operating system the PID address space. Containers launched with this flag will be able to access and manipulate other containers in the bare-metal machine's namespace and vise-versa.

ports

Expose ports. Either specify both ports (HOST:CONTAINER), or just the container port (a random host port will be chosen).

Note: When mapping ports in the HOST:CONTAINER format, you may experience erroneous results when using a container port lower than 60, because YAML will parse numbers in the format xx:yy as sexagesimal (base 60). For this reason, we recommend always explicitly specifying your port mappings as strings.

ports:
 - "3000"
 - "3000-3005"
 - "8000:8000"
 - "9090-9091:8080-8081"
 - "49100:22"
 - "127.0.0.1:8001:8001"
 - "127.0.0.1:5000-5010:5000-5010"

security_opt

Override the default labeling scheme for each container.

  security_opt:
    - label:user:USER
    - label:role:ROLE

ulimits

Override the default ulimits for a container. You can either specify a single limit as an integer or soft/hard limits as a mapping.

  ulimits:
    nproc: 65535
    nofile:
      soft: 20000
      hard: 40000

volumes, volume_driver

Mount paths as volumes, optionally specifying a path on the host machine (HOST:CONTAINER), or an access mode (HOST:CONTAINER:ro).

volumes:
 - /var/lib/mysql
 - ./cache:/tmp/cache
 - ~/configs:/etc/configs/:ro

You can mount a relative path on the host, which will expand relative to the directory of the Compose configuration file being used. Relative paths should always begin with . or ...

If you use a volume name (instead of a volume path), you may also specify a volume_driver.

volume_driver: mydriver

Note: No path expansion will be done if you have also specified a volume_driver.

See Docker Volumes and Volume Plugins for more information.

volumes_from

Mount all of the volumes from another service or container, optionally specifying read-only access(ro) or read-write(rw).

volumes_from:
 - service_name
 - container_name
 - service_name:rw

cpu_shares, cpuset, domainname, entrypoint, hostname, ipc, mac_address, mem_limit, memswap_limit, privileged, read_only, restart, stdin_open, tty, user, working_dir

Each of these is a single value, analogous to its docker run counterpart.

cpu_shares: 73
cpuset: 0,1

entrypoint: /code/entrypoint.sh
user: postgresql
working_dir: /code

domainname: foo.com
hostname: foo
ipc: host
mac_address: 02:42:ac:11:65:43

mem_limit: 1000000000
memswap_limit: 2000000000
privileged: true

restart: always

read_only: true
stdin_open: true
tty: true

Variable substitution

Your configuration options can contain environment variables. Compose uses the variable values from the shell environment in which docker-compose is run. For example, suppose the shell contains POSTGRES_VERSION=9.3 and you supply this configuration:

db:
  image: "postgres:${POSTGRES_VERSION}"

When you run docker-compose up with this configuration, Compose looks for the POSTGRES_VERSION environment variable in the shell and substitutes its value in. For this example, Compose resolves the image to postgres:9.3 before running the configuration.

If an environment variable is not set, Compose substitutes with an empty string. In the example above, if POSTGRES_VERSION is not set, the value for the image option is postgres:.

Both $VARIABLE and ${VARIABLE} syntax are supported. Extended shell-style features, such as ${VARIABLE-default} and ${VARIABLE/foo/bar}, are not supported.

You can use a $$ (double-dollar sign) when your configuration needs a literal dollar sign. This also prevents Compose from interpolating a value, so a $$ allows you to refer to environment variables that you don't want processed by Compose.

web:
  build: .
  command: "$$VAR_NOT_INTERPOLATED_BY_COMPOSE"

If you forget and use a single dollar sign ($), Compose interprets the value as an environment variable and will warn you:

The VAR_NOT_INTERPOLATED_BY_COMPOSE is not set. Substituting an empty string.

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