---
description: Compose file reference
keywords: fig, composition, compose, docker
redirect_from:
- /compose/yml
- /compose/compose-file-v3.md
title: Compose file version 3 reference
toc_max: 4
toc_min: 1
---

## Reference and guidelines

These topics describe version 3 of the Compose file format. This is the newest
version.

## Compose and Docker compatibility matrix

There are several versions of the Compose file format – 1, 2, 2.x, and 3.x. The
table below is a quick look. For full details on what each version includes and
how to upgrade, see **[About versions and upgrading](compose-versioning.md)**.

{% include content/compose-matrix.md %}

## Compose file structure and examples

The topics on this reference page are organized alphabetically by top-level key
to reflect the structure of the Compose file itself. Top-level keys that define
a section in the configuration file such as `build`, `deploy`, `depends_on`,
`networks`, and so on, are listed with the options that support them as
sub-topics. This maps to the `<key>: <option>: <value>` indent structure of the
Compose file.

The best way to grok the layout and syntax of a Compose file is to
read [Get started with Docker Compose](/compose/gettingstarted/) and look
at files included in [sample applications](https://docs.docker.com/samples/).

A good place to start is the [Getting Started](/get-started/index.md) tutorial
which uses version 3 Compose stack files to implement multi-container apps,
service definitions, and swarm mode. Here are some Compose files used in the
tutorial.

- [Your first docker-compose.yml File](/get-started/part3.md#your-first-docker-composeyml-file)

- [Adding a new service and redeploying](/get-started/part5.md#adding-a-new-service-and-redeploying)

Another example of a Compose file is included in the Docker Labs topic, [Deploying an app to a Swarm](https://github.com/docker/labs/blob/master/beginner/chapters/votingapp.md).


## Service configuration 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`.

>**Tip**: You can use either a `.yml` or `.yaml` extension for this file.
They both work.

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.

This section contains a list of all configuration options supported by a service
definition in version 3.

### 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**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file. The `docker stack` command accepts only pre-built images.

#### context

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

#### args

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.

#### cache_from

> **Note:** This option is new in v3.2

A list of images that the engine will use for cache resolution.

    build:
      context: .
      cache_from:
        - alpine:latest
        - corp/web_app:3.14

#### labels

> **Note:** This option is new in v3.3

Add metadata to the resulting image using [Docker labels](/engine/userguide/labels-custom-metadata.md).
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.

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


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

### 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

> **Note**: These options are ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### command

Override the default command.

    command: bundle exec thin -p 3000

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

    command: ["bundle", "exec", "thin", "-p", "3000"]

### configs

Grant access to configs on a per-service basis using the per-service `configs`
configuration. Two different syntax variants are supported.

> **Note**: The config must already exist or be
> [defined in the top-level `configs` configuration](#configs-configuration-reference)
> of this stack file, or stack deployment will fail.

#### Short syntax

The short syntax variant only specifies the config name. This grants the
container access to the config and mounts it at `/<config_name>`
within the container. The source name and destination mountpoint are both set
to the config name.

The following example uses the short syntax to grant the `redis` service
access to the `my_config` and `my_other_config` configs. The value of
`my_config` is set to the contents of the file `./my_config.txt`, and
`my_other_config` is defined as an external resource, which means that it has
already been defined in Docker, either by running the `docker config create`
command or by another stack deployment. If the external config does not exist,
the stack deployment fails with a `config not found` error.

> **Note**: `config` definitions are only supported in version 3.3 and higher
>  of the compose file format.

```none
version: "3.3"
services:
  redis:
    image: redis:latest
    deploy:
      replicas: 1
    configs:
      - my_config
      - my_other_config
configs:
  my_config:
    file: ./my_config.txt
  my_other_config:
    external: true
```

#### Long syntax

The long syntax provides more granularity in how the config is created within
the service's task containers.

- `source`: The name of the config as it exists in Docker.
- `target`: The path and name of the file that will be mounted in the service's
  task containers. service's task containers. Defaults to `/<source>` if not
  specified.
- `uid` and `gid`: The numeric UID or GID which will own the mounted config file
  within in the service's task containers. Both default to `0` on Linux if not
  specified. Not supported on Windows.
- `mode`: The permissions for the file that will be mounted within the service's
  task containers, in octal notation. For instance, `0444`
  represents world-readable. The default is `0444`. Configs cannot be writable
  because they are mounted in a temporary filesystem, so if you set the writable
  bit, it is ignored. The executable bit can be set. If you aren't familiar with
  UNIX file permission modes, you may find this
  [permissions calculator](http://permissions-calculator.org/){: target="_blank" class="_" }
  useful.

The following example sets the name of `my_config` to `redis_config` within the
container, sets the mode to `0440` (group-readable) and sets the user and group
to `103`. The `redis` service does not have access to the `my_other_config`
config.

```none
version: "3.3"
services:
  redis:
    image: redis:latest
    deploy:
      replicas: 1
    configs:
      - source: my_config
        target: /redis_config
        uid: '103'
        gid: '103'
        mode: 0440
configs:
  my_config:
    file: ./my_config.txt
  my_other_config:
    external: true
```

You can grant a service access to multiple configs and you can mix long and
short syntax. Defining a config does not imply granting a service access to it.

### cgroup_parent

Specify an optional parent cgroup for the container.

    cgroup_parent: m-executor-abcd

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### 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.

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### credential_spec

> **Note:** this option was added in v3.3

Configure the credential spec for managed service account (Windows only).

    credential_spec:
      file: c:/WINDOWS/my-credential-spec.txt

    credential_spec:
      registry: HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Virtualization\Containers\CredentialSpecs

### deploy

> **[Version 3](compose-versioning.md#version-3) only.**

Specify configuration related to the deployment and running of services. This
only takes effect when deploying to a [swarm](/engine/swarm/index.md) with
[docker stack deploy](/engine/reference/commandline/stack_deploy.md), and is
ignored by `docker-compose up` and `docker-compose run`.

    version: '3'
    services:
      redis:
        image: redis:alpine
        deploy:
          replicas: 6
          update_config:
            parallelism: 2
            delay: 10s
          restart_policy:
            condition: on-failure


Several sub-options are available:

#### mode

Either `global` (exactly one container per swarm node) or `replicated` (a
specified number of containers). The default is `replicated`. (To learn more,
see [Replicated and global
services](/engine/swarm/how-swarm-mode-works/services/#replicated-and-global-services)
in the [swarm](/engine/swarm/) topics.)


    version: '3'
    services:
      worker:
        image: dockersamples/examplevotingapp_worker
        deploy:
          mode: global

#### replicas

If the service is `replicated` (which is the default), specify the number of
containers that should be running at any given time.

    version: '3'
    services:
      worker:
        image: dockersamples/examplevotingapp_worker
        networks:
          - frontend
          - backend
        deploy:
          mode: replicated
          replicas: 6

#### placement

Specify placement constraints. For a full description of the syntax and
available types of constraints, see the
[docker service create](/engine/reference/commandline/service_create.md#specify-service-constraints-constraint)
documentation.

    version: '3'
    services:
      db:
        image: postgres
        deploy:
          placement:
            constraints:
              - node.role == manager
              - engine.labels.operatingsystem == ubuntu 14.04

#### update_config

Configures how the service should be updated. Useful for configuring rolling
updates.

- `parallelism`: The number of containers to update at a time.
- `delay`: The time to wait between updating a group of containers.
- `failure_action`: What to do if an update fails. One of `continue` or `pause`
  (default: `pause`).
- `monitor`: Duration after each task update to monitor for failure `(ns|us|ms|s|m|h)` (default 0s).
- `max_failure_ratio`: Failure rate to tolerate during an update.

```none
version: '3'
services:
  vote:
    image: dockersamples/examplevotingapp_vote:before
    depends_on:
      - redis
    deploy:
      replicas: 2
      update_config:
        parallelism: 2
        delay: 10s
```

#### resources

Configures resource constraints. This replaces the older resource constraint
options in Compose files prior to version 3 (`cpu_shares`, `cpu_quota`,
`cpuset`, `mem_limit`, `memswap_limit`, `mem_swappiness`).

Each of these is a single value, analogous to its
[docker service create](/engine/reference/commandline/service_create.md) counterpart.

```none
version: '3'
services:
  redis:
    image: redis:alpine
    deploy:
      resources:
        limits:
          cpus: '0.001'
          memory: 50M
        reservations:
          cpus: '0.0001'
          memory: 20M
```

##### Out Of Memory Exceptions (OOME)

If your services or containers attempt to use more memory than the system has
available, you may experience an Out Of Memory Exception (OOME) and a container,
or the Docker daemon, might be killed by the kernel OOM killer. To prevent this
from happening, ensure that your application runs on hosts with adequate memory
and see [Understand the risks of running out of
memory](/engine/admin/resource_constraints.md#understand-the-risks-of-running-out-of-memory).


#### restart_policy

Configures if and how to restart containers when they exit. Replaces
[`restart`](compose-file-v2.md#orig-resources).

- `condition`: One of `none`, `on-failure` or `any` (default: `any`).
- `delay`: How long to wait between restart attempts, specified as a
  [duration](#specifying-durations) (default: 0).
- `max_attempts`: How many times to attempt to restart a container before giving
  up (default: never give up).
- `window`: How long to wait before deciding if a restart has succeeded,
  specified as a [duration](#specifying-durations) (default:
  decide immediately).

```none
version: "3"
services:
  redis:
    image: redis:alpine
    deploy:
      restart_policy:
        condition: on-failure
        delay: 5s
        max_attempts: 3
        window: 120s
```

#### labels

Specify labels for the service. These labels will *only* be set on the service,
and *not* on any containers for the service.

    version: "3"
    services:
      web:
        image: web
        deploy:
          labels:
            com.example.description: "This label will appear on the web service"

To set labels on containers instead, use the `labels` key outside of `deploy`:

    version: "3"
    services:
      web:
        image: web
        labels:
          com.example.description: "This label will appear on all containers for the web service"

#### Not supported for `docker stack deploy`

The following sub-options (supported for `docker compose up` and `docker compose run`) are _not supported_ for `docker stack deploy` or the `deploy` key.

- [build](#build)
- [cgroup_parent](#cgroup_parent)
- [container_name](#container_name)
- [devices](#devices)
- [dns](#devices)
- [dns_search](#dns_search)
- [tmpfs](#tmpfs)
- [external_links](#external_links)
- [links](#links)
- [network_mode](#network_mode)
- [security_opt](#security_opt)
- [stop_signal](#stop_signal)
- [sysctls](#sysctls)
- [userns_mode](#userns_mode)

>**Tip:** See also, the section on [how to configure volumes
for services, swarms, and docker-stack.yml
files](#volumes-for-services-swarms-and-stack-files).  Volumes _are_ supported
but in order to work with swarms and services, they must be configured properly,
as named volumes or associated with services that are constrained to nodes with
access to the requisite volumes.

### devices

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

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

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### 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: '3'
    services:
      web:
        build: .
        depends_on:
          - db
          - redis
      redis:
        image: redis
      db:
        image: postgres

> There are several things to be aware of when using `depends_on`:
>
> - `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](/compose/startup-order.md)
>   for more on this problem and strategies for solving it.
>
> - Version 3 no longer supports the `condition` form of `depends_on`.
>
> - The `depends_on` option is ignored when
>   [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
>   with a version 3 Compose file.
{: .note-vanilla}


### 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

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### 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

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### tmpfs

> [Version 2 file format](compose-versioning.md#version-2) and up.

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

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

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### entrypoint

Override the default entrypoint.

    entrypoint: /code/entrypoint.sh

The entrypoint can also be a list, in a manner similar to
[dockerfile](/engine/reference/builder.md#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

> **Note**: Setting `entrypoint` will both override any default entrypoint set
> on the service's image with the `ENTRYPOINT` Dockerfile instruction, *and*
> clear out any default command on the image - meaning that if there's a `CMD`
> instruction in the Dockerfile, it will be ignored.

### 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](#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.

The value of `VAL` is used as is and not modified at all. For example if the
value is surrounded by quotes (as is often the case of shell variables), the
quotes will be included in the value passed to Compose.

Keep in mind that _the order of files in the list is significant in determining
the value assigned to a variable that shows up more than once_. The files in the
list are processed from the top down. For the same variable specified in file
`a.env` and assigned a different value in file `b.env`, if `b.env` is
listed below (after), then the value from `b.env` stands. For example, given the
following declaration in `docker_compose.yml`:

```yaml
services:
  some-service:
    env_file:
      - a.env
      - b.env
```

And the following files:

```none
# a.env
VAR=1
```

and

```none
# b.env
VAR=hello
```

$VAR will be `hello`.

### 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"

### 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 the legacy option `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

> **Notes:**
>
> If you're using the [version 2 or above file format](compose-versioning.md#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. Starting with Version 2, [links](compose-file-v2#links) are a
legacy option. We recommend using [networks](#networks) instead.
>
> This option is ignored when [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
with a (version 3) Compose file.
{: .note-vanilla}

### 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

### healthcheck

> [Version 2.1 file format](compose-versioning.md#version-21) and up.

Configure a check that's run to determine whether or not containers for this
service are "healthy". See the docs for the
[HEALTHCHECK Dockerfile instruction](/engine/reference/builder.md#healthcheck)
for details on how healthchecks work.

    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost"]
      interval: 1m30s
      timeout: 10s
      retries: 3

`interval` and `timeout` are specified as
[durations](#specifying-durations).

`test` must be either a string or a list. If it's a list, the first item must be
either `NONE`, `CMD` or `CMD-SHELL`. If it's a string, it's equivalent to
specifying `CMD-SHELL` followed by that string.

    # Hit the local web app
    test: ["CMD", "curl", "-f", "http://localhost"]

    # As above, but wrapped in /bin/sh. Both forms below are equivalent.
    test: ["CMD-SHELL", "curl -f http://localhost && echo 'cool, it works'"]
    test: curl -f https://localhost && echo 'cool, it works'

To disable any default healthcheck set by the image, you can use `disable:
true`. This is equivalent to specifying `test: ["NONE"]`.

    healthcheck:
      disable: true

### 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.

### isolation

Specify a container’s isolation technology. On Linux, the only supported value
is `default`. On Windows, acceptable values are `default`, `process` and
`hyperv`. Refer to the
[Docker Engine docs](/engine/reference/commandline/run.md#specify-isolation-technology-for-container---isolation)
for details.

### labels

Add metadata to containers using [Docker labels](/engine/userguide/labels-custom-metadata.md). 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](#dependson), so they determine the order of service startup.

> **Notes**
>
> * 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.
>
> *  This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.
{: .note-vanilla}

### logging

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](/engine/admin/logging/overview.md)).

The default value is json-file.

    driver: "json-file"
    driver: "syslog"
    driver: "none"

> **Note**: Only the `json-file` and `journald` drivers make 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"

The default driver [json-file](/engine/admin/logging/overview.md#json-file), has options to limit the amount of logs stored. To do this, use a key-value pair for maximum storage size and maximum number of files:

    options:
      max-size: "200k"
      max-file: "10"

The example shown above would store log files until they reach a `max-size` of
200kB, and then rotate them. The amount of individual log files stored is
specified by the `max-file` value. As logs grow beyond the max limits, older log
files are removed to allow storage of new logs.

Here is an example `docker-compose.yml` file that limits logging storage:

    services:
      some-service:
        image: some-service
        logging:
          driver: "json-file"
          options:
            max-size: "200k"
            max-file: "10"

> Logging options available depend on which logging driver you use
>
> The above example for controlling log files and sizes uses options
specific to the [json-file driver](/engine/admin/logging/overview.md#json-file).
These particular options are not available on other logging drivers.
For a full list of supported logging drivers and their options, see
[logging drivers](/engine/admin/logging/overview.md).
{: .note-vanilla}

### network_mode

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]"

> **Notes**
>
>* This option is ignored when
[deploying a stack in swarm
 mode](/engine/reference/commandline/stack_deploy.md) with a (version 3) Compose
 file.
>
>* `network_mode: "host"` cannot be mixed with [links](#links).
{: .note-vanilla}

### networks

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 configurations covering each static address. If IPv6 addressing is desired, the [`enable_ipv6`](#enableipv6) option must be set.

An example:

    version: '2.1'

    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
        enable_ipv6: true
        ipam:
          driver: default
          config:
          -
            subnet: 172.16.238.0/24
          -
            subnet: 2001:3984:3989::/64

### 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.

#### Short syntax

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"
     - "6060:6060/udp"

#### Long syntax

The long form syntax allows the configuration of additional fields that can't be
expressed in the short form.

- `target`: the port inside the container
- `published`: the publicly exposed port
- `protocol`: the port protocol (`tcp` or `udp`)
- `mode`: `host` for publishing a host port on each node, or `ingress` for a swarm
   mode port which will be load balanced.

```none
ports:
  - target: 80
    published: 8080
    protocol: tcp
    mode: host

```

> **Note:** The long syntax is new in v3.2

### secrets

Grant access to secrets on a per-service basis using the per-service `secrets`
configuration. Two different syntax variants are supported.

> **Note**: The secret must already exist or be
> [defined in the top-level `secrets` configuration](#secrets-configuration-reference)
> of this stack file, or stack deployment will fail.

#### Short syntax

The short syntax variant only specifies the secret name. This grants the
container access to the secret and mounts it at `/run/secrets/<secret_name>`
within the container. The source name and destination mountpoint are both set
to the secret name.

> Limitations of short syntax in Docker 1.13.1
>
> Due to a bug in Docker 1.13.1, using the short syntax currently
> mounts the secret with permissions `000`, which means secrets defined using
> the short syntax are unreadable within the container if the command does not
> run as the `root` user. The workaround is to use the long syntax instead if
> you use Docker 1.13.1 and the secret must be read by a non-`root` user.
{: .warning-vanilla}

The following example uses the short syntax to grant the `redis` service
access to the `my_secret` and `my_other_secret` secrets. The value of
`my_secret` is set to the contents of the file `./my_secret.txt`, and
`my_other_secret` is defined as an external resource, which means that it has
already been defined in Docker, either by running the `docker secret create`
command or by another stack deployment. If the external secret does not exist,
the stack deployment fails with a `secret not found` error.

```none
version: "3.1"
services:
  redis:
    image: redis:latest
    deploy:
      replicas: 1
    secrets:
      - my_secret
      - my_other_secret
secrets:
  my_secret:
    file: ./my_secret.txt
  my_other_secret:
    external: true
```

#### Long syntax

The long syntax provides more granularity in how the secret is created within
the service's task containers.

- `source`: The name of the secret as it exists in Docker.
- `target`: The name of the file that will be mounted in `/run/secrets/` in the
  service's task containers. Defaults to `source` if not specified.
- `uid` and `gid`: The numeric UID or GID which will own the file within
  `/run/secrets/` in the service's task containers. Both default to `0` if not
  specified.
- `mode`: The permissions for the file that will be mounted in `/run/secrets/`
  in the service's task containers, in octal notation. For instance, `0444`
  represents world-readable. The default in Docker 1.13.1 is `0000`, but will
  be `0444` in the future. Secrets cannot be writable because they are mounted
  in a temporary filesystem, so if you set the writable bit, it is ignored. The
  executable bit can be set. If you aren't familiar with UNIX file permission
  modes, you may find this
  [permissions calculator](http://permissions-calculator.org/){: target="_blank" class="_" }
  useful.

The following example sets name of the `my_secret` to `redis_secret` within the
container, sets the mode to `0440` (group-readable) and sets the user and group
to `103`. The `redis` service does not have access to the `my_other_secret`
secret.

```none
version: "3.1"
services:
  redis:
    image: redis:latest
    deploy:
      replicas: 1
    secrets:
      - source: my_secret
        target: redis_secret
        uid: '103'
        gid: '103'
        mode: 0440
secrets:
  my_secret:
    file: ./my_secret.txt
  my_other_secret:
    external: true
```

You can grant a service access to multiple secrets and you can mix long and
short syntax. Defining a secret does not imply granting a service access to it.

### security_opt

Override the default labeling scheme for each container.

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

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### stop_grace_period

Specify how long to wait when attempting to stop a container if it doesn't
handle SIGTERM (or whatever stop signal has been specified with
[`stop_signal`](#stopsignal)), before sending SIGKILL. Specified
as a [duration](#specifying-durations).

    stop_grace_period: 1s
    stop_grace_period: 1m30s

By default, `stop` waits 10 seconds for the container to exit before sending
SIGKILL.

### 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

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### sysctls

Kernel parameters to set in the container. You can use either an array or a
dictionary.

    sysctls:
      net.core.somaxconn: 1024
      net.ipv4.tcp_syncookies: 0

    sysctls:
      - net.core.somaxconn=1024
      - net.ipv4.tcp_syncookies=0

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### 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

### userns_mode

    userns_mode: "host"

Disables the user namespace for this service, if Docker daemon is configured with user namespaces.
See [dockerd](/engine/reference/commandline/dockerd.md#disable-user-namespace-for-a-container) for
more information.

> **Note**: This option is ignored when
> [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)
> with a (version 3) Compose file.

### volumes

> **Note**: The top-level
> [volumes](#volume-configuration-reference) option defines
> a named volume and references it from each service's `volumes` list. This replaces `volumes_from` in earlier versions of the Compose file format. (See [Docker Volumes](/engine/userguide/dockervolumes.md) and
[Volume Plugins](/engine/extend/plugins_volume.md) for general information on volumes.)

Mount host paths or named volumes. Named volumes must be defined in the
[top-level `volumes` key](#volume-configuration-reference). Use named volumes with [services, swarms, and stack files](#volumes-for-services-swarms-and-stack-files).

#### Short syntax

Optionally specify a path on the host machine
(`HOST:CONTAINER`), or an access mode (`HOST:CONTAINER: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 `..`.

    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


#### Long syntax

The long form syntax allows the configuration of additional fields that can't be
expressed in the short form.

- `type`: the mount type `volume` or `bind`
- `source`: the source of the mount, a path on the host for a bind mount, or the
  name of a volume defined in the
  [top-level `volumes` key](#volume-configuration-reference)
- `target`: the path in the container where the volume will be mounted
- `read_only`: flag to set the volume as read-only
- `bind`: configure additional bind options
  - `propagation`: the propagation mode used for the bind
- `volume`: configure additional volume options
  - `nocopy`: flag to disable copying of data from a container when a volume is
    created


```none
volumes:
  - type: volume
    source: mydata
    target: /data
    volume:
      nocopy: true
  - type: bind
    source: ./static
    target: /opt/app/static
```

> **Note:** The long syntax is new in v3.2


#### Volumes for services, swarms, and stack files

When working with services, swarms, and `docker-stack.yml` files, keep in mind
that the tasks (containers) backing a service can be deployed on any node in a
swarm, which may be a different node each time the service is updated.

In the absence of having named volumes with specified sources, Docker creates an
anonymous volume for each task backing a service. Anonymous volumes do not
persist after the associated containers are removed.

If you want your data to persist, use a named volume and a volume driver that
is multi-host aware, so that the data is accessible from any node. Or, set
constraints on the service so that its tasks are deployed on a node that has the
volume present.

As an example, the `docker-stack.yml` file for the
[votingapp sample in Docker
Labs](https://github.com/docker/labs/blob/master/beginner/chapters/votingapp.md) defines a service called `db` that runs a `postgres` database. It is
configured as a named volume in order to persist the data on the swarm,
_and_ is constrained to run only on `manager` nodes. Here is the relevant snip-it from that file:

```
version: "3"
services:
  db:
    image: postgres:9.4
    volumes:
      - db-data:/var/lib/postgresql/data
    networks:
      - backend
    deploy:
      placement:
        constraints: [node.role == manager]
```

#### Caching options for volume mounts (Docker for Mac)

On Docker 17.04 CE Edge and up, including 17.06 CE Edge and Stable, you can
configure container-and-host consistency requirements for bind-mounted
directories in Compose files to allow for better performance on read/write of
volume mounts. These options address issues specific to `osxfs` file sharing,
and therefore are only applicable on Docker for Mac.

The flags are:

* `consistent`: Full consistency. The container runtime and the
host maintain an identical view of the mount at all times.  This is the default.

* `cached`: The host's view of the mount is authoritative. There may be
delays before updates made on the host are visible within a container.

* `delegated`: The container runtime's view of the mount is
authoritative. There may be delays before updates made in a container
are visible on the host.

Here is an example of configuring a volume as `cached`:

```
version: '3'
services:
  php:
    image: php:7.1-fpm
    ports:
      - 9000
    volumes:
      - .:/var/www/project:cached
```

Full detail on these flags, the problems they solve, and their
`docker run` counterparts is in the Docker for Mac topic [Performance tuning for
volume mounts (shared filesystems)](/docker-for-mac/osxfs-caching.md).


### restart

`no` is the default restart policy, and it will not restart a container under
any circumstance. When `always` is specified, the container always restarts. The
`on-failure` policy restarts a container if the exit code indicates an
on-failure error.

    restart: "no"
    restart: always
    restart: on-failure
    restart: unless-stopped

### domainname, hostname, ipc, mac\_address, privileged, read\_only, shm\_size, stdin\_open, tty, user, working\_dir

Each of these is a single value, analogous to its
[docker run](/engine/reference/run.md) counterpart.

    user: postgresql
    working_dir: /code

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

    privileged: true


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


## Specifying durations

Some configuration options, such as the `interval` and `timeout` sub-options for
[`healthcheck`](#healthcheck), accept a duration as a string in a
format that looks like this:

    2.5s
    10s
    1m30s
    2h32m
    5h34m56s

The supported units are `us`, `ms`, `s`, `m` and `h`.


## Volume configuration reference

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

See [Docker Volumes](/engine/userguide/dockervolumes.md) and [Volume
Plugins](/engine/extend/plugins_volume.md) for general information on volumes.

Here's an example of a two-service setup where a database's data directory is
shared with another service as a volume so that it can be periodically backed
up:

    version: "3"

    services:
      db:
        image: db
        volumes:
          - data-volume:/var/lib/db
      backup:
        image: backup-service
        volumes:
          - data-volume:/var/lib/backup/data

    volumes:
      data-volume:

An entry under the top-level `volumes` key can be empty, in which case it will
use the default driver configured by the Engine (in most cases, this is the
`local` driver). Optionally, you can configure it with the following keys:

### driver

Specify which volume driver should be used for this volume. Defaults to whatever
driver the Docker Engine has been configured to use, which in most cases is
`local`. If the driver is not available, the Engine will return an error when
`docker-compose up` tries to create the volume.

     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 attempting 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

> External volumes are always created with docker stack deploy
>
External volumes that do not exist _will be created_ if you use [docker stack
deploy](#deploy) to launch the app in [swarm mode](/engine/swarm/index.md)
(instead of [docker compose up](/compose/reference/up.md)). In swarm mode, a
volume is automatically created when it is defined by a service. As service
tasks are scheduled on new nodes,
[swarmkit](https://github.com/docker/swarmkit/blob/master/README.md) creates the
volume on the local node. To learn more, see
[moby/moby#29976](https://github.com/moby/moby/issues/29976).
{: .note-vanilla}

### labels

Add metadata to containers using
[Docker labels](/engine/userguide/labels-custom-metadata.md). 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: "Database volume"
      com.example.department: "IT/Ops"
      com.example.label-with-empty-value: ""

    labels:
      - "com.example.description=Database volume"
      - "com.example.department=IT/Ops"
      - "com.example.label-with-empty-value"


## 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 and all
network driver options, see the [Networking guide](../networking.md).

* For [Docker Labs](https://github.com/docker/labs/blob/master/README.md)
tutorials on networking, start with [Designing Scalable, Portable Docker
Container
Networks](https://github.com/docker/labs/blob/master/networking/README.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

#### bridge

Docker defaults to using a `bridge` network on a single host. For examples of
how to work with bridge networks, see the Docker Labs tutorial on [Bridge
networking](https://github.com/docker/labs/blob/master/networking/A2-bridge-networking.md).

#### overlay

The `overlay` driver creates a named network across multiple nodes in a
[swarm](/engine/swarm/).

* For a working example of how to build and use an
`overlay` network with a service in swarm mode, see the Docker Labs tutorial on
[Overlay networking and service
discovery](https://github.com/docker/labs/blob/master/networking/A3-overlay-networking.md).

* For an in-depth look at how it works under the hood, see the
networking concepts lab on the [Overlay Driver Network
Architecture](https://github.com/docker/labs/blob/master/networking/concepts/06-overlay-networks.md).

### 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

### enable_ipv6

Enable IPv6 networking on this network.

### 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

A full example:

    ipam:
      driver: default
      config:
        - subnet: 172.28.0.0/16

> **Note**: Additional IPAM configurations, such as `gateway`, are only honored for version 2 at the moment.

### 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`.

### labels

Add metadata to containers using
[Docker labels](/engine/userguide/labels-custom-metadata.md). 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: "Financial transaction network"
      com.example.department: "Finance"
      com.example.label-with-empty-value: ""

    labels:
      - "com.example.description=Financial transaction network"
      - "com.example.department=Finance"
      - "com.example.label-with-empty-value"

### 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
attempting 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

## configs configuration reference

The top-level `configs` declaration defines or references
[configs](/engine/swarm/configs.md) which can be granted to the services in this
stack. The source of the config is either `file` or `external`.

- `file`: The config is created with the contents of the file at the specified
  path.
- `external`: If set to true, specifies that this config has already been
  created. Docker will not attempt to create it, and if it does not exist, a
  `config not found` error occurs.

In this example, `my_first_config` will be created (as
`<stack_name>_my_first_config)`when the stack is deployed,
and `my_second_config` already exists in Docker.

```none
configs:
  my_first_config:
    file: ./config_data
  my_second_config:
    external: true
```

Another variant for external configs is when the name of the config in Docker
is different from the name that will exist within the service. The following
example modifies the previous one to use the external config called
`redis_config`.

```none
configs:
  my_first_config:
    file: ./config_data
  my_second_config:
    external:
      name: redis_config
```

You still need to [grant access to the config](#configs) to each service in the
stack.



## secrets configuration reference

The top-level `secrets` declaration defines or references
[secrets](/engine/swarm/secrets.md) which can be granted to the services in this
stack. The source of the secret is either `file` or `external`.

- `file`: The secret is created with the contents of the file at the specified
  path.
- `external`: If set to true, specifies that this secret has already been
  created. Docker will not attempt to create it, and if it does not exist, a
  `secret not found` error occurs.

In this example, `my_first_secret` will be created (as
`<stack_name>_my_first_secret)`when the stack is deployed,
and `my_second_secret` already exists in Docker.

```none
secrets:
  my_first_secret:
    file: ./secret_data
  my_second_secret:
    external: true
```

Another variant for external secrets is when the name of the secret in Docker
is different from the name that will exist within the service. The following
example modifies the previous one to use the external secret called
`redis_secret`.

```none
secrets:
  my_first_secret:
    file: ./secret_data
  my_second_secret:
    external:
      name: redis_secret
```

You still need to [grant access to the secrets](#secrets) to each service in the
stack.

## Variable substitution

{% include content/compose-var-sub.md %}

## Compose documentation

- [User guide](/compose/index.md)
- [Installing Compose](/compose/install/)
- [Compose file versions and upgrading](compose-versioning.md)
- [Get started with Docker](/get-started/)
- [Samples](/samples/)
- [Command line reference](/compose/reference/)