path: root/docs/compose-file.md

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title = "Compose File Reference"
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# Compose file reference

The Compose file is a [YAML](http://yaml.org/) file defining
[services](#service-configuration-reference),
[networks](#network-configuration-reference) and
[volumes](#volume-configuration-reference).
The default path for a Compose file is `./docker-compose.yml`.

A service definition contains configuration which will be applied to each
container started for that service, much like passing command-line parameters to
`docker run`. Likewise, network and volume definitions are analogous to
`docker network create` and `docker volume create`.

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`.

You can use environment variables in configuration values with a Bash-like
`${VARIABLE}` syntax - see [variable substitution](#variable-substitution) for
full details.


## Service configuration reference

> **Note:** There are two versions of the Compose file format – version 1 (the
> legacy format, which does not support volumes or networks) and version 2 (the
> most up-to-date). For more information, see the [Versioning](#versioning)
> section.

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

### build

Configuration options that are applied at build time.

`build` can be specified either as a string containing a path to the build
context, or an object with the path specified under [context](#context) and
optionally [dockerfile](#dockerfile) and [args](#args).

    build: ./dir

    build:
      context: ./dir
      dockerfile: Dockerfile-alternate
      args:
        buildno: 1

If you specify `image` as well as `build`, then Compose names the built image
with the `webapp` and optional `tag` specified in `image`:

    build: ./dir
    image: webapp:tag

This will result in an image named `webapp` and tagged `tag`, built from `./dir`.

> **Note**: In the [version 1 file format](#version-1), `build` is different in
> two ways:
>
> -   Only the string form (`build: .`) is allowed - not the object form.
> -   Using `build` together with `image` is not allowed. Attempting to do so
>     results in an error.

#### context

> [Version 2 file format](#version-2) only. In version 1, just use
> [build](#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:
      context: ./dir

#### dockerfile

Alternate Dockerfile.

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

    build:
      context: .
      dockerfile: Dockerfile-alternate

> **Note**: In the [version 1 file format](#version-1), `dockerfile` is
> different in two ways:

  * It appears alongside `build`, not as a sub-option:

        build: .
        dockerfile: Dockerfile-alternate

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

#### args

> [Version 2 file format](#version-2) only.

Add build arguments, which are environment variables accessible only during the
build process.

First, specify the arguments in your Dockerfile:

    ARG buildno
    ARG password

    RUN echo "Build number: $buildno"
    RUN script-requiring-password.sh "$password"

Then specify the arguments under the `build` key. You can pass either a mapping
or a list:

    build:
      context: .
      args:
        buildno: 1
        password: secret

    build:
      context: .
      args:
        - buildno=1
        - password=secret

You can omit the value when specifying a build argument, in which case its value
at build time is the value in the environment where Compose is running.

    args:
      - buildno
      - password

> **Note**: YAML boolean values (`true`, `false`, `yes`, `no`, `on`, `off`) must
> be enclosed in quotes, so that the parser interprets them as strings.

### 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

The command can also be a list, in a manner similar to [dockerfile](https://docs.docker.com/engine/reference/builder/#cmd):

    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"

### depends_on

Express dependency between services, which has two effects:

- `docker-compose up` will start services in dependency order. In the following
  example, `db` and `redis` will be started before `web`.

- `docker-compose up SERVICE` will automatically include `SERVICE`'s
  dependencies. In the following example, `docker-compose up web` will also
  create and start `db` and `redis`.

Simple example:

    version: '2'
    services:
      web:
        build: .
        depends_on:
          - db
          - redis
      redis:
        image: redis
      db:
        image: postgres

> **Note:** `depends_on` will not wait for `db` and `redis` to be "ready" before
> starting `web` - only until they have been started. If you need to wait
> for a service to be ready, see [Controlling startup order](startup-order.md)
> for more on this problem and strategies for solving it.

### 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

### tmpfs

> [Version 2 file format](#version-2) only.

Mount a temporary file system inside the container. Can be a single value or a list.

    tmpfs: /run
    tmpfs:
      - /run
      - /tmp

### entrypoint

Override the default entrypoint.

    entrypoint: /code/entrypoint.sh

The entrypoint can also be a list, in a manner similar to [dockerfile](https://docs.docker.com/engine/reference/builder/#entrypoint):

    entrypoint:
        - php
        - -d
        - zend_extension=/usr/local/lib/php/extensions/no-debug-non-zts-20100525/xdebug.so
        - -d
        - memory_limit=-1
        - vendor/bin/phpunit


### 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

> **Note:** If your service specifies a [build](#build) option, variables
> defined in environment files will _not_ be automatically visible during the
> build. Use the [args](#args) sub-option of `build` to define build-time
> environment variables.

### 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

> **Note:** If your service specifies a [build](#build) option, variables
> defined in `environment` will _not_ be automatically visible during the
> build. Use the [args](#args) sub-option of `build` to define build-time
> environment variables.

### 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

> **Note:** If you're using the [version 2 file format](#version-2), the
> externally-created containers must be connected to at least one of the same
> networks as the service which is linking to them.

### 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

Specify the image to start the container from. Can either be a repository/tag or
a partial image ID.

    image: redis
    image: ubuntu:14.04
    image: tutum/influxdb
    image: example-registry.com:4000/postgresql
    image: a4bc65fd

If the image does not exist, Compose attempts to pull it, unless you have also
specified [build](#build), in which case it builds it using the specified
options and tags it with the specified tag.

> **Note**: In the [version 1 file format](#version-1), using `build` together
> with `image` is not allowed. Attempting to do so results in an error.

### 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
a link alias (`SERVICE:ALIAS`), or just the service name.

    web:
      links:
       - db
       - db:database
       - redis

Containers for the linked service will be reachable at a hostname identical to
the alias, or the service name if no alias was specified.

Links also express dependency between services in the same way as
[depends_on](#depends-on), so they determine the order of service startup.

> **Note:** If you define both links and [networks](#networks), services with
> links between them must share at least one network in common in order to
> communicate.

### logging

> [Version 2 file format](#version-2) only. In version 1, use
> [log_driver](#log_driver) and [log_opt](#log_opt).

Logging configuration for the service.

    logging:
      driver: syslog
      options:
        syslog-address: "tcp://192.168.0.42:123"

The `driver`  name specifies 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.

    driver: "json-file"
    driver: "syslog"
    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.

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

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

    driver: "syslog"
    options:
      syslog-address: "tcp://192.168.0.42:123"

### log_driver

> [Version 1 file format](#version-1) only. In version 2, use
> [logging](#logging).

Specify a log driver. The default is `json-file`.

    log_driver: syslog

### log_opt

> [Version 1 file format](#version-1) only. In version 2, use
> [logging](#logging).

Specify logging options as key-value pairs. An example of `syslog` options:

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

### net

> [Version 1 file format](#version-1) only. In version 2, use
> [network_mode](#network_mode).

Network mode. Use the same values as the docker client `--net` parameter.
The `container:...` form can take a service name instead of a container name or
id.

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

### network_mode

> [Version 2 file format](#version-2) only. In version 1, use [net](#net).

Network mode. Use the same values as the docker client `--net` parameter, plus
the special form `service:[service name]`.

    network_mode: "bridge"
    network_mode: "host"
    network_mode: "none"
    network_mode: "service:[service name]"
    network_mode: "container:[container name/id]"

### networks

> [Version 2 file format](#version-2) only. In version 1, use [net](#net).

Networks to join, referencing entries under the
[top-level `networks` key](#network-configuration-reference).

    services:
      some-service:
        networks:
         - some-network
         - other-network

#### aliases

Aliases (alternative hostnames) for this service on the network. Other containers on the same network can use either the service name or this alias to connect to one of the service's containers.

Since `aliases` is network-scoped, the same service can have different aliases on different networks.

> **Note**: A network-wide alias can be shared by multiple containers, and even by multiple services. If it is, then exactly which container the name will resolve to is not guaranteed.

The general format is shown here.

    services:
      some-service:
        networks:
          some-network:
            aliases:
             - alias1
             - alias3
          other-network:
            aliases:
             - alias2

In the example below, three services are provided (`web`, `worker`, and `db`), along with two networks (`new` and `legacy`). The `db` service is reachable at the hostname `db` or `database` on the `new` network, and at `db` or `mysql` on the `legacy` network.

    version: '2'

    services:
      web:
        build: ./web
        networks:
          - new

      worker:
        build: ./worker
        networks:
        - legacy

      db:
        image: mysql
        networks:
          new:
            aliases:
              - database
          legacy:
            aliases:
              - mysql

    networks:
      new:
      legacy:

#### ipv4_address, ipv6_address

Specify a static IP address for containers for this service when joining the network.

The corresponding network configuration in the [top-level networks section](#network-configuration-reference) must have an `ipam` block with subnet and gateway configurations covering each static address. If IPv6 addressing is desired, the `com.docker.network.enable_ipv6` driver option must be set to `true`.

An example:

    version: '2'

    services:
      app:
        image: busybox
        command: ifconfig
        networks:
          app_net:
            ipv4_address: 172.16.238.10
            ipv6_address: 2001:3984:3989::10

    networks:
      app_net:
        driver: bridge
        driver_opts:
          com.docker.network.enable_ipv6: "true"
        ipam:
          driver: default
          config:
          - subnet: 172.16.238.0/24
            gateway: 172.16.238.1
          - subnet: 2001:3984:3989::/64
            gateway: 2001:3984:3989::1

### 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

### stop_signal

Sets an alternative signal to stop the container. By default `stop` uses
SIGTERM. Setting an alternative signal using `stop_signal` will cause
`stop` to send that signal instead.

    stop_signal: SIGUSR1

### 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 or named volumes, optionally specifying a path on the host machine
(`HOST:CONTAINER`), or an access mode (`HOST:CONTAINER:ro`).
For [version 2 files](#version-2), named volumes need to be specified with the
[top-level `volumes` key](#volume-configuration-reference).
When using [version 1](#version-1), the Docker Engine will create the named
volume automatically if it doesn't exist.

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 `..`.

    volumes:
      # Just specify a path and let the Engine create a volume
      - /var/lib/mysql

      # Specify an absolute path mapping
      - /opt/data:/var/lib/mysql

      # Path on the host, relative to the Compose file
      - ./cache:/tmp/cache

      # User-relative path
      - ~/configs:/etc/configs/:ro

      # Named volume
      - datavolume:/var/lib/mysql

If you do not use a host path, you may specify a `volume_driver`.

    volume_driver: mydriver

Note that for [version 2 files](#version-2), this driver
will not apply to named volumes (you should use the `driver` option when
[declaring the volume](#volume-configuration-reference) instead).
For [version 1](#version-1), both named volumes and container volumes will
use the specified driver.

> 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``). If no access level is specified,
then read-write will be used.

    volumes_from:
     - service_name
     - service_name:ro
     - container:container_name
     - container:container_name:rw

> **Note:** The `container:...` formats are only supported in the
> [version 2 file format](#version-2). In [version 1](#version-1), you can use
> container names without marking them as such:
>
>     - service_name
>     - service_name:ro
>     - container_name
>     - container_name:rw

### cpu\_shares, cpu\_quota, cpuset, domainname, hostname, ipc, mac\_address, mem\_limit, memswap\_limit, mem\_swappiness, oom\_score\_adj, privileged, read\_only, restart, shm\_size, 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

    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
    mem_swappiness: 10
    privileged: true

    restart: always

    read_only: true
    shm_size: 64M
    stdin_open: true
    tty: true


## Volume configuration reference

While it is possible to declare volumes on the fly as part of the service
declaration, this section allows you to create named volumes that can be
reused across multiple services (without relying on `volumes_from`), and are
easily retrieved and inspected using the docker command line or API.
See the [docker volume](https://docs.docker.com/engine/reference/commandline/volume_create/)
subcommand documentation for more information.

### driver

Specify which volume driver should be used for this volume. Defaults to
`local`. The Docker Engine will return an error if the driver is not available.

     driver: foobar

### driver_opts

Specify a list of options as key-value pairs to pass to the driver for this
volume. Those options are driver-dependent - consult the driver's
documentation for more information. Optional.

     driver_opts:
       foo: "bar"
       baz: 1

### external

If set to `true`, specifies that this volume has been created outside of
Compose. `docker-compose up` will not attempt to create it, and will raise
an error if it doesn't exist.

`external` cannot be used in conjunction with other volume configuration keys
(`driver`, `driver_opts`).

In the example below, instead of attemping to create a volume called
`[projectname]_data`, Compose will look for an existing volume simply
called `data` and mount it into the `db` service's containers.

    version: '2'

    services:
      db:
        image: postgres
        volumes:
          - data:/var/lib/postgresql/data

    volumes:
      data:
        external: true

You can also specify the name of the volume separately from the name used to
refer to it within the Compose file:

    volumes:
      data:
        external:
          name: actual-name-of-volume


## Network configuration reference

The top-level `networks` key lets you specify networks to be created. For a full
explanation of Compose's use of Docker networking features, see the
[Networking guide](networking.md).

### driver

Specify which driver should be used for this network.

The default driver depends on how the Docker Engine you're using is configured,
but in most instances it will be `bridge` on a single host and `overlay` on a
Swarm.

The Docker Engine will return an error if the driver is not available.

    driver: overlay

### driver_opts

Specify a list of options as key-value pairs to pass to the driver for this
network. Those options are driver-dependent - consult the driver's
documentation for more information. Optional.

      driver_opts:
        foo: "bar"
        baz: 1

### ipam

Specify custom IPAM config. This is an object with several properties, each of
which is optional:

-   `driver`: Custom IPAM driver, instead of the default.
-   `config`: A list with zero or more config blocks, each containing any of
    the following keys:
    - `subnet`: Subnet in CIDR format that represents a network segment
    - `ip_range`: Range of IPs from which to allocate container IPs
    - `gateway`: IPv4 or IPv6 gateway for the master subnet
    - `aux_addresses`: Auxiliary IPv4 or IPv6 addresses used by Network driver,
      as a mapping from hostname to IP

A full example:

    ipam:
      driver: default
      config:
        - subnet: 172.28.0.0/16
          ip_range: 172.28.5.0/24
          gateway: 172.28.5.254
          aux_addresses:
            host1: 172.28.1.5
            host2: 172.28.1.6
            host3: 172.28.1.7

### internal

By default, Docker also connects a bridge network to it to provide external connectivity. If you want to create an externally isolated overlay network, you can set this option to `true`.

### external

If set to `true`, specifies that this network has been created outside of
Compose. `docker-compose up` will not attempt to create it, and will raise
an error if it doesn't exist.

`external` cannot be used in conjunction with other network configuration keys
(`driver`, `driver_opts`, `ipam`, `internal`).

In the example below, `proxy` is the gateway to the outside world. Instead of
attemping to create a network called `[projectname]_outside`, Compose will
look for an existing network simply called `outside` and connect the `proxy`
service's containers to it.

    version: '2'

    services:
      proxy:
        build: ./proxy
        networks:
          - outside
          - default
      app:
        build: ./app
        networks:
          - default

    networks:
      outside:
        external: true

You can also specify the name of the network separately from the name used to
refer to it within the Compose file:

    networks:
      outside:
        external:
          name: actual-name-of-network


## Versioning

There are two versions of the Compose file format:

- Version 1, the legacy format. This is specified by omitting a `version` key at
  the root of the YAML.
- Version 2, the recommended format. This is specified with a `version: '2'` entry
  at the root of the YAML.

To move your project from version 1 to 2, see the [Upgrading](#upgrading)
section.

> **Note:** If you're using
> [multiple Compose files](extends.md#different-environments) or
> [extending services](extends.md#extending-services), each file must be of the
> same version - you cannot mix version 1 and 2 in a single project.

Several things differ depending on which version you use:

- The structure and permitted configuration keys
- The minimum Docker Engine version you must be running
- Compose's behaviour with regards to networking

These differences are explained below.


### Version 1

Compose files that do not declare a version are considered "version 1". In
those files, all the [services](#service-configuration-reference) are declared
at the root of the document.

Version 1 is supported by **Compose up to 1.6.x**. It will be deprecated in a
future Compose release.

Version 1 files cannot declare named
[volumes](#volume-configuration-reference), [networks](networking.md) or
[build arguments](#args).

Example:

    web:
      build: .
      ports:
       - "5000:5000"
      volumes:
       - .:/code
      links:
       - redis
    redis:
      image: redis


### Version 2

Compose files using the version 2 syntax must indicate the version number at
the root of the document. All [services](#service-configuration-reference)
must be declared under the `services` key.

Version 2 files are supported by **Compose 1.6.0+** and require a Docker Engine
of version **1.10.0+**.

Named [volumes](#volume-configuration-reference) can be declared under the
`volumes` key, and [networks](#network-configuration-reference) can be declared
under the `networks` key.

Simple example:

    version: '2'
    services:
      web:
        build: .
        ports:
         - "5000:5000"
        volumes:
         - .:/code
      redis:
        image: redis

A more extended example, defining volumes and networks:

    version: '2'
    services:
      web:
        build: .
        ports:
         - "5000:5000"
        volumes:
         - .:/code
        networks:
          - front-tier
          - back-tier
      redis:
        image: redis
        volumes:
          - redis-data:/var/lib/redis
        networks:
          - back-tier
    volumes:
      redis-data:
        driver: local
    networks:
      front-tier:
        driver: bridge
      back-tier:
        driver: bridge


### Upgrading

In the majority of cases, moving from version 1 to 2 is a very simple process:

1. Indent the whole file by one level and put a `services:` key at the top.
2. Add a `version: '2'` line at the top of the file.

It's more complicated if you're using particular configuration features:

-   `dockerfile`: This now lives under the `build` key:

        build:
          context: .
          dockerfile: Dockerfile-alternate

-   `log_driver`, `log_opt`: These now live under the `logging` key:

        logging:
          driver: syslog
          options:
            syslog-address: "tcp://192.168.0.42:123"

-   `links` with environment variables: As documented in the
    [environment variables reference](link-env-deprecated.md), environment variables
    created by
    links have been deprecated for some time. In the new Docker network system,
    they have been removed. You should either connect directly to the
    appropriate hostname or set the relevant environment variable yourself,
    using the link hostname:

        web:
          links:
            - db
          environment:
            - DB_PORT=tcp://db:5432

-   `external_links`: Compose uses Docker networks when running version 2
    projects, so links behave slightly differently. In particular, two
    containers must be connected to at least one network in common in order to
    communicate, even if explicitly linked together.

    Either connect the external container to your app's
    [default network](networking.md), or connect both the external container and
    your service's containers to an
    [external network](networking.md#using-a-pre-existing-network).

-   `net`: This is now replaced by [network_mode](#network_mode):

        net: host    ->  network_mode: host
        net: bridge  ->  network_mode: bridge
        net: none    ->  network_mode: none

    If you're using `net: "container:[service name]"`, you must now use
    `network_mode: "service:[service name]"` instead.

        net: "container:web"  ->  network_mode: "service:web"

    If you're using `net: "container:[container name/id]"`, the value does not
    need to change.

        net: "container:cont-name"  ->  network_mode: "container:cont-name"
        net: "container:abc12345"   ->  network_mode: "container:abc12345"

-   `volumes` with named volumes: these must now be explicitly declared in a
    top-level `volumes` section of your Compose file. If a service mounts a
    named volume called `data`, you must declare a `data` volume in your
    top-level `volumes` section. The whole file might look like this:

        version: '2'
        services:
          db:
            image: postgres
            volumes:
              - data:/var/lib/postgresql/data
        volumes:
          data: {}

    By default, Compose creates a volume whose name is prefixed with your
    project name. If you want it to just be called `data`, declare it as
    external:

        volumes:
          data:
            external: 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 `EXTERNAL_PORT=8000` and you supply
this configuration:

    web:
      build: .
      ports:
        - "${EXTERNAL_PORT}:5000"

When you run `docker-compose up` with this configuration, Compose looks for
the `EXTERNAL_PORT` environment variable in the shell and substitutes its
value in. In this example, Compose resolves the port mapping to `"8000:5000"`
before creating the `web` container.

If an environment variable is not set, Compose substitutes with an empty
string. In the example above, if `EXTERNAL_PORT` is not set, the value for the
port mapping is `:5000` (which is of course an invalid port mapping, and will
result in an error when attempting to create the container).

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)