# Compose file reference The compose file is a [YAML](http://yaml.org/) 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 Either a path to a directory containing a Dockerfile, or a url to a git repository. When the value supplied is a relative path, it is interpreted as relative to the location of the Compose file. 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](extends.md#extending-services). ### 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](https://docs.docker.com/engine/userguide/labels-custom-metadata/). 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](env.md) for details. ### log_driver Specify a logging driver for the service's containers, as with the ``--log-driver`` option for docker run ([documented here](https://docs.docker.com/engine/reference/logging/overview/)). 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](https://docs.docker.com/engine/userguide/dockervolumes/) and [Volume Plugins](https://docs.docker.com/engine/extend/plugins_volume/) 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, cpu\_quota, 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](https://docs.docker.com/engine/reference/run/) counterpart. cpu_shares: 73 cpu_quota: 50000 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. ## Compose documentation - [User guide](index.md) - [Installing Compose](install.md) - [Get started with Django](django.md) - [Get started with Rails](rails.md) - [Get started with WordPress](wordpress.md) - [Command line reference](./reference/index.md)