---
description: Compose file reference
keywords: fig, composition, compose, docker
redirect_from:
- /compose/yml
- /compose/compose-file-v3/
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

Here is a sample Compose file from the voting app sample used in the
[Docker for Beginners lab](https://github.com/docker/labs/tree/master/beginner/)
topic on [Deploying an app to a Swarm](https://github.com/docker/labs/blob/master/beginner/chapters/votingapp.md):

<div class="panel panel-default">
    <div class="panel-heading collapsed" data-toggle="collapse" data-target="#collapseSample1" style="cursor: pointer">
    Example Compose file version 3
    <i class="chevron fa fa-fw"></i></div>
    <div class="collapse block" id="collapseSample1">
<pre><code>
version: "{{ site.compose_file_v3 }}"
services:

  redis:
    image: redis:alpine
    ports:
      - "6379"
    networks:
      - frontend
    deploy:
      replicas: 2
      update_config:
        parallelism: 2
        delay: 10s
      restart_policy:
        condition: on-failure

  db:
    image: postgres:9.4
    volumes:
      - db-data:/var/lib/postgresql/data
    networks:
      - backend
    deploy:
      placement:
        max_replicas_per_node: 1
        constraints:
          - "node.role==manager"

  vote:
    image: dockersamples/examplevotingapp_vote:before
    ports:
      - "5000:80"
    networks:
      - frontend
    depends_on:
      - redis
    deploy:
      replicas: 2
      update_config:
        parallelism: 2
      restart_policy:
        condition: on-failure

  result:
    image: dockersamples/examplevotingapp_result:before
    ports:
      - "5001:80"
    networks:
      - backend
    depends_on:
      - db
    deploy:
      replicas: 1
      update_config:
        parallelism: 2
        delay: 10s
      restart_policy:
        condition: on-failure

  worker:
    image: dockersamples/examplevotingapp_worker
    networks:
      - frontend
      - backend
    deploy:
      mode: replicated
      replicas: 1
      labels: [APP=VOTING]
      restart_policy:
        condition: on-failure
        delay: 10s
        max_attempts: 3
        window: 120s
      placement:
        constraints:
          - "node.role==manager"

  visualizer:
    image: dockersamples/visualizer:stable
    ports:
      - "8080:8080"
    stop_grace_period: 1m30s
    volumes:
      - "/var/run/docker.sock:/var/run/docker.sock"
    deploy:
      placement:
        constraints:
          - "node.role==manager"

networks:
  frontend:
  backend:

volumes:
  db-data:
</code></pre>
    </div>
</div>

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.

## Service configuration reference

The Compose file is a [YAML](https://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 that is 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, such as `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:

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  webapp:
    build: ./dir
```

Or, as an object with the path specified under [context](#context) and
optionally [Dockerfile](#dockerfile) and [args](#args):

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  webapp:
    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`:

```yaml
build: ./dir
image: webapp:tag
```

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

> Note when using docker stack deploy
>
> The `build` option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md)
> The `docker stack` command does not build images before deploying.
{: .important }

#### 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 builds and tags it with a generated name, and uses that image
thereafter.

```yaml
build:
  context: ./dir
```

#### dockerfile

Alternate Dockerfile.

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

```yaml
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:

```dockerfile
ARG buildno
ARG gitcommithash

RUN echo "Build number: $buildno"
RUN echo "Based on commit: $gitcommithash"
```

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

```yaml
build:
  context: .
  args:
    buildno: 1
    gitcommithash: cdc3b19
```

```yaml
build:
  context: .
  args:
    - buildno=1
    - gitcommithash=cdc3b19
```

> Scope of build-args
>
> In your Dockerfile, if you specify `ARG` before the `FROM` instruction,
> `ARG` is not available in the build instructions under `FROM`.
> If you need an argument to be available in both places, also specify it under
> the `FROM` instruction. Refer to the [understand how ARGS and FROM interact](../../engine/reference/builder.md#understand-how-arg-and-from-interact)
> section in the documentation for usage details.

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.

```yaml
args:
  - buildno
  - gitcommithash
```

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

#### cache_from

> Added in [version 3.2](compose-versioning.md#version-32) file format

A list of images that the engine uses for cache resolution.

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

#### labels

> Added in [version 3.3](compose-versioning.md#version-33) file format

Add metadata to the resulting image using [Docker labels](../../config/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.

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

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

#### network

> Added in [version 3.4](compose-versioning.md#version-34) file format

Set the network containers connect to for the `RUN` instructions during
build.

```yaml
build:
  context: .
  network: host
```

```yaml
build:
  context: .
  network: custom_network_1
```

Use `none` to disable networking during build:

```yaml
build:
  context: .
  network: none
```

#### shm_size

> Added in [version 3.5](compose-versioning.md#version-35) file format

Set the size of the `/dev/shm` partition for this build's containers. Specify
as an integer value representing the number of bytes or as a string expressing
a [byte value](#specifying-byte-values).

```yaml
build:
  context: .
  shm_size: '2gb'
```

```yaml
build:
  context: .
  shm_size: 10000000
```

#### target

> Added in [version 3.4](compose-versioning.md#version-34) file format

Build the specified stage as defined inside the `Dockerfile`. See the
[multi-stage build docs](../../develop/develop-images/multistage-build.md) for
details.

```yaml
build:
  context: .
  target: prod
```

### cap_add, cap_drop

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

```yaml
cap_add:
  - ALL

cap_drop:
  - NET_ADMIN
  - SYS_ADMIN
```

> Note when using docker stack deploy
>
> The `cap_add` and `cap_drop` options are ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md)
{: .important }

### cgroup_parent

Specify an optional parent cgroup for the container.

```yaml
cgroup_parent: m-executor-abcd
```

> Note when using docker stack deploy
>
> The `cgroup_parent` option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md)
{: .important }

### command

Override the default command.

```yaml
command: bundle exec thin -p 3000
```

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

```yaml
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 fails.

For more information on configs, see [configs](../../engine/swarm/configs.md).

#### 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.

> Added in [version 3.3](compose-versioning.md#version-33) file format.
> 
> `config` definitions are only supported in version 3.3 and higher  of the
> compose file format.

```yaml
version: "{{ site.compose_file_v3 }}"
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 to be mounted in the service's
  task containers. Defaults to `/<source>` if not specified.
- `uid` and `gid`: The numeric UID or GID that owns 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 is 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" rel="noopener" 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.

```yaml
version: "{{ site.compose_file_v3 }}"
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.

### container_name

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

```yaml
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 when using docker stack deploy
>
> The `container_name` option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md)
{: .important }

### credential_spec

> Added in [version 3.3](compose-versioning.md#version-33) file format.
> 
> The `credential_spec` option was added in v3.3. Using group Managed Service
> Account (gMSA) configurations with compose files is supported in file format
> version 3.8 or up.

Configure the credential spec for managed service account. This option is only
used for services using Windows containers. The `credential_spec` must be in the
format `file://<filename>` or `registry://<value-name>`.

When using `file:`, the referenced file must be present in the `CredentialSpecs`
subdirectory in the Docker data directory, which defaults to `C:\ProgramData\Docker\`
on Windows. The following example loads the credential spec from a file named

```
C:\ProgramData\Docker\CredentialSpecs\my-credential-spec.json
```

```yaml
credential_spec:
  file: my-credential-spec.json
```

When using `registry:`, the credential spec is read from the Windows registry on
the daemon's host. A registry value with the given name must be located in:

    HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Virtualization\Containers\CredentialSpecs

The following example load the credential spec from a value named `my-credential-spec`
in the registry:

```yaml
credential_spec:
  registry: my-credential-spec
```

#### Example gMSA configuration
When configuring a gMSA credential spec for a service, you only need
to specify a credential spec with `config`, as shown in the following example:

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  myservice:
    image: myimage:latest
    credential_spec:
      config: my_credential_spec

configs:
  my_credentials_spec:
    file: ./my-credential-spec.json|
```

### depends_on

Express dependency between services. Service dependencies cause the following
behaviors:

- `docker-compose up` starts services in dependency order. In the following
  example, `db` and `redis` are started before `web`.
- `docker-compose up SERVICE` automatically includes `SERVICE`'s
  dependencies. In the example below, `docker-compose up web` also
  creates and starts `db` and `redis`.
- `docker-compose stop` stops services in dependency order. In the following
  example, `web` is stopped before `db` and `redis`.

Simple example:

```yaml
version: "{{ site.compose_file_v3 }}"
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` does 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.
> - 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.

### deploy

> Added in [version 3](compose-versioning.md#version-3) file format.

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`.

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  redis:
    image: redis:alpine
    deploy:
      replicas: 6
      placement:
        max_replicas_per_node: 1
      update_config:
        parallelism: 2
        delay: 10s
      restart_policy:
        condition: on-failure
```

Several sub-options are available:

#### endpoint_mode

> Added in [version 3.2](compose-versioning.md#version-32) file format.

Specify a service discovery method for external clients connecting to a swarm.

* `endpoint_mode: vip` - Docker assigns the service a virtual IP (VIP)
that acts as the front end for clients to reach the service on a
network. Docker routes requests between the client and available worker
nodes for the service, without client knowledge of how many nodes
are participating in the service or their IP addresses or ports.
(This is the default.)

* `endpoint_mode: dnsrr` -  DNS round-robin (DNSRR) service discovery does
not use a single virtual IP. Docker sets up DNS entries for the service
such that a DNS query for the service name returns a list of IP addresses,
and the client connects directly to one of these. DNS round-robin is useful
in cases where you want to use your own load balancer, or for Hybrid
Windows and Linux applications.

```yaml
version: "{{ site.compose_file_v3 }}"

services:
  wordpress:
    image: wordpress
    ports:
      - "8080:80"
    networks:
      - overlay
    deploy:
      mode: replicated
      replicas: 2
      endpoint_mode: vip

  mysql:
    image: mysql
    volumes:
       - db-data:/var/lib/mysql/data
    networks:
       - overlay
    deploy:
      mode: replicated
      replicas: 2
      endpoint_mode: dnsrr

volumes:
  db-data:

networks:
  overlay:
```

The options for `endpoint_mode` also work as flags on the swarm mode CLI command
[docker service create](../../engine/reference/commandline/service_create.md). For a
quick list of all swarm related `docker` commands, see
[Swarm mode CLI commands](../../engine/swarm/index.md#swarm-mode-key-concepts-and-tutorial).

To learn more about service discovery and networking in swarm mode, see
[Configure service discovery](../../engine/swarm/networking.md#configure-service-discovery)
in the swarm mode topics.


#### labels

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

```yaml
version: "{{ site.compose_file_v3 }}"
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`:

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  web:
    image: web
    labels:
      com.example.description: "This label will appear on all containers for the web service"
```

#### 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.md#replicated-and-global-services)
in the [swarm](../../engine/swarm/index.md) topics.)


```yaml
version: "{{ site.compose_file_v3 }}"
services:
  worker:
    image: dockersamples/examplevotingapp_worker
    deploy:
      mode: global
```

#### placement

Specify placement of constraints and preferences. See the docker service create
documentation for a full description of the syntax and available types of
[constraints](../../engine/reference/commandline/service_create.md#specify-service-constraints---constraint),
[preferences](../../engine/reference/commandline/service_create.md#specify-service-placement-preferences---placement-pref),
and [specifying the maximum replicas per node](../../engine/reference/commandline/service_create.md#specify-maximum-replicas-per-node---replicas-max-per-node)

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  db:
    image: postgres
    deploy:
      placement:
        constraints:
          - "node.role==manager"
          - "engine.labels.operatingsystem==ubuntu 18.04"
        preferences:
          - spread: node.labels.zone
```

#### max_replicas_per_node

> Added in [version 3.8](compose-versioning.md#version-38) file format.

If the service is `replicated` (which is the default), [limit the number of replicas](../../engine/reference/commandline/service_create.md#specify-maximum-replicas-per-node---replicas-max-per-node)
that can run on a node at any time.

When there are more tasks requested than running nodes, an error
`no suitable node (max replicas per node limit exceed)` is raised.

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  worker:
    image: dockersamples/examplevotingapp_worker
    networks:
      - frontend
      - backend
    deploy:
      mode: replicated
      replicas: 6
      placement:
        max_replicas_per_node: 1
```

#### replicas

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

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  worker:
    image: dockersamples/examplevotingapp_worker
    networks:
      - frontend
      - backend
    deploy:
      mode: replicated
      replicas: 6
```

#### resources

Configures resource constraints.

> Changed in compose-file version 3
>
> The `resources` section replaces  the [older resource constraint options](compose-file-v2.md#cpu-and-other-resources)
> in Compose files prior to version 3 (`cpu_shares`, `cpu_quota`, `cpuset`,
> `mem_limit`, `memswap_limit`, `mem_swappiness`).
> Refer to [Upgrading version 2.x to 3.x](compose-versioning.md#upgrading)
> to learn about differences between version 2 and 3 of the compose-file format.

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

In this general example, the `redis` service is constrained to use no more than
50M of memory and `0.50` (50% of a single core) of available processing time (CPU),
and has `20M` of memory and `0.25` CPU time reserved (as always available to it).

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  redis:
    image: redis:alpine
    deploy:
      resources:
        limits:
          cpus: '0.50'
          memory: 50M
        reservations:
          cpus: '0.25'
          memory: 20M
```

The topics below describe available options to set resource constraints on
services or containers in a swarm.

> Looking for options to set resources on non swarm mode containers?
>
> The options described here are specific to the
`deploy` key and swarm mode. If you want to set resource constraints
on non swarm deployments, use
[Compose file format version 2 CPU, memory, and other resource options](compose-file-v2.md#cpu-and-other-resources).
If you have further questions, refer to the discussion on the GitHub
issue [docker/compose/4513](https://github.com/docker/compose/issues/4513){: target="_blank" rel="noopener" class="_"}.
{: .important}

##### 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](../../config/containers/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: 5s).
- `max_attempts`: How many times to attempt to restart a container before giving
  up (default: never give up). If the restart does not succeed within the configured
  `window`, this attempt doesn't count toward the configured `max_attempts` value.
  For example, if `max_attempts` is set to '2', and the restart fails on the first
  attempt, more than two restarts may be attempted.
- `window`: How long to wait before deciding if a restart has succeeded,
  specified as a [duration](#specifying-durations) (default:
  decide immediately).

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  redis:
    image: redis:alpine
    deploy:
      restart_policy:
        condition: on-failure
        delay: 5s
        max_attempts: 3
        window: 120s
```

#### rollback_config

> Added in [version 3.7](compose-versioning.md#version-37) file format.

Configures how the service should be rollbacked in case of a failing
update.

- `parallelism`: The number of containers to rollback at a time. If set to 0, all containers rollback simultaneously.
- `delay`: The time to wait between each container group's rollback (default 0s).
- `failure_action`: What to do if a rollback 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 5s) **Note**: Setting to 0 will use the default 5s.
- `max_failure_ratio`: Failure rate to tolerate during a rollback (default 0).
- `order`: Order of operations during rollbacks. One of `stop-first` (old task is stopped before starting new one), or `start-first` (new task is started first, and the running tasks briefly overlap) (default `stop-first`).

#### 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`, `rollback`, or `pause`
  (default: `pause`).
- `monitor`: Duration after each task update to monitor for failure `(ns|us|ms|s|m|h)` (default 5s) **Note**: Setting to 0 will use the default 5s.
- `max_failure_ratio`: Failure rate to tolerate during an update.
- `order`: Order of operations during updates. One of `stop-first` (old task is stopped before starting new one), or `start-first` (new task is started first, and the running tasks briefly overlap) (default `stop-first`) **Note**: Only supported for v3.4 and higher.

> Added in [version 3.4](compose-versioning.md#version-34) file format.
>
> The `order` option is only supported by v3.4 and higher of the compose
> file format.

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  vote:
    image: dockersamples/examplevotingapp_vote:before
    depends_on:
      - redis
    deploy:
      replicas: 2
      update_config:
        parallelism: 2
        delay: 10s
        order: stop-first
```

#### 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)
- [tmpfs](#tmpfs)
- [external_links](#external_links)
- [links](#links)
- [network_mode](#network_mode)
- [restart](#restart)
- [security_opt](#security_opt)
- [userns_mode](#userns_mode)

> Tip
>
> See 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 to work with swarms and services, they must be configured 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.

```yaml
devices:
  - "/dev/ttyUSB0:/dev/ttyUSB0"
```

> Note when using docker stack deploy
>
> The `devices` option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md)
{: .important }

### dns

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

```yaml
dns: 8.8.8.8
```

```yaml
dns:
  - 8.8.8.8
  - 9.9.9.9
```

### dns_search

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

```yaml
dns_search: example.com
```

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

### entrypoint

Override the default entrypoint.

```yaml
entrypoint: /code/entrypoint.sh
```

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

```yaml
entrypoint: ["php", "-d", "memory_limit=-1", "vendor/bin/phpunit"]
```

> **Note**
>
> Setting `entrypoint` both overrides any default entrypoint set on the service's
> image with the `ENTRYPOINT` Dockerfile instruction, *and* clears out any default
> command on the image - meaning that if there's a `CMD` instruction in the
> Dockerfile, it is 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 declared in the [environment](#environment) section
_override_ these values &ndash; this holds true even if those values are
empty or undefined.

```yaml
env_file: .env
```

```yaml
env_file:
  - ./common.env
  - ./apps/web.env
  - /opt/runtime_opts.env
```

Compose expects each line in an env file to be in `VAR=VAL` format. Lines
beginning with `#` are treated as comments and are ignored. Blank lines are
also ignored.

```console
# Set Rails/Rack environment
RACK_ENV=development
```

> **Note**
>
> If your service specifies a [build](#build) option, variables defined in
> environment files are _not_ 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 are 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:

```console
# a.env
VAR=1
```

and

```console
# b.env
VAR=hello
```

`$VAR` is `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.

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

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

> **Note**
>
> If your service specifies a [build](#build) option, variables defined in
> `environment` are _not_ 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.

```yaml
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`).

```yaml
external_links:
  - redis_1
  - project_db_1:mysql
  - project_db_1:postgresql
```

> **Note**
>
> The externally-created  containers must be connected to at least one of the same
> networks as the service that is linking to them. [Links](compose-file-v2#links)
> are a legacy option. We recommend using [networks](#networks) instead.

> Note when using docker stack deploy
>
> The `external_links` option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md)
{: .important }

### extra_hosts

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

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

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

```console
162.242.195.82  somehost
50.31.209.229   otherhost
```

### healthcheck

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.

```yaml
healthcheck:
  test: ["CMD", "curl", "-f", "http://localhost"]
  interval: 1m30s
  timeout: 10s
  retries: 3
  start_period: 40s
```

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

> Added in [version 3.4](compose-versioning.md#version-34) file format.
>
> The `start_period` option was added in file format 3.4.

`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.

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

As above, but wrapped in `/bin/sh`. Both forms below are equivalent.

```yaml
test: ["CMD-SHELL", "curl -f http://localhost || exit 1"]
```

```yaml
test: curl -f https://localhost || exit 1
```

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

```yaml
healthcheck:
  disable: true
```

### image

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

```yaml
image: redis
```
```yaml
image: ubuntu:18.04
```
```yaml
image: tutum/influxdb
```
```yaml
image: example-registry.com:4000/postgresql
```
```yaml
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.

### init

> Added in [version 3.7](compose-versioning.md#version-37) file format.

Run an init inside the container that forwards signals and reaps processes.
Set this option to `true` to enable this feature for the service.

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  web:
    image: alpine:latest
    init: true
```

> The default init binary that is used is [Tini](https://github.com/krallin/tini),
> and is installed in `/usr/libexec/docker-init` on the daemon host. You can
> configure the daemon to use a custom init binary through the
> [`init-path` configuration option](/engine/reference/commandline/dockerd/#daemon-configuration-file).

### 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](../../config/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.

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

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

### links

>**Warning**
>
> The `--link` flag is a legacy feature of Docker. It may eventually be removed.
> Unless you absolutely need to continue using it, we recommend that you use
> [user-defined networks](../networking.md)
> to facilitate communication between two containers instead of using `--link`.
>
> One feature that user-defined networks do not support that you can do with
> `--link` is sharing environmental variables between containers. However, you
> can use other mechanisms such as volumes to share environment variables between
> containers in a more controlled way.
{:.warning}

Link to containers in another service. Either specify both the service name and
a link alias (`"SERVICE:ALIAS"`), or just the service name.

```yaml
web:
  links:
    - "db"
    - "db:database"
    - "redis"
```

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

Links are not required to enable services to communicate - by default,
any service can reach any other service at that service’s name. (See also, the
[Links topic in Networking in Compose](../networking.md#links).)

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 to
> communicate.

> Note when using docker stack deploy
>
> The `links` option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md)
{: .important }

### logging

Logging configuration for the service.

```yaml
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](../../config/containers/logging/configure.md)).

The default value is json-file.

```yaml
driver: "json-file"
```
```yaml
driver: "syslog"
```
```yaml
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
> does 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:

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

The default driver [json-file](../../config/containers/logging/json-file.md), 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:

```yaml
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:

```yaml
version: "{{ site.compose_file_v3 }}"
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](../../config/containers/logging/json-file.md).
> These particular options are not available on other logging drivers.
> For a full list of supported logging drivers and their options, refer to the
> [logging drivers](../../config/containers/logging/configure.md) documentation.

### network_mode

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

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

> **Note**
>
> * This option is ignored when
>   [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md).
> * `network_mode: "host"` cannot be mixed with [links](#links).
{: .important }

### networks

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

```yaml
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 resolves to is not
> guaranteed.

The general format is shown here.

```yaml
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.

```yaml
version: "{{ site.compose_file_v3 }}"

services:
  web:
    image: "nginx:alpine"
    networks:
      - new

  worker:
    image: "my-worker-image:latest"
    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`](compose-file-v2.md#enable_ipv6)
> option must be set, and you must use a [version 2.x Compose file](compose-file-v2.md#ipv4_address-ipv6_address).
> _IPv6 options do not currently work in swarm mode_.

An example:

```yaml
version: "{{ site.compose_file_v3 }}"

services:
  app:
    image: nginx:alpine
    networks:
      app_net:
        ipv4_address: 172.16.238.10
        ipv6_address: 2001:3984:3989::10

networks:
  app_net:
    ipam:
      driver: default
      config:
        - subnet: "172.16.238.0/24"
        - subnet: "2001:3984:3989::/64"
```

### pid

```yaml
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 can access and manipulate other
containers in the bare-metal machine's namespace and vice versa.

### ports

Expose ports.

> **Note**
>
> Port mapping is incompatible with `network_mode: host`

#### Short syntax

Either specify both ports (`HOST:CONTAINER`), or just the container
port (an ephemeral host port is 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
> parses numbers in the format `xx:yy` as a base-60 value. For this reason,
> we recommend always explicitly specifying your port mappings as strings.

```yaml
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"
  - "12400-12500:1240"
```

#### 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 to be load balanced.

```yaml
ports:
  - target: 80
    published: 8080
    protocol: tcp
    mode: host
```

> Added in [version 3.2](compose-versioning.md#version-32) file format.
>
> The long syntax is new in the v3.2 file format.

### restart

`no` is the default [restart policy](../../config/containers/start-containers-automatically.md#use-a-restart-policy), and it does 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. `unless-stopped` always restarts a container, except when the
container is stopped (manually or otherwise).

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

> Note when using docker stack deploy
>
> The `restart` option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md).
{: .important }

### secrets

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

> Note when using docker stack deploy
>
> The secret must already exist or be
> [defined in the top-level `secrets` configuration](#secrets-configuration-reference)
> of the compose file, or stack deployment fails.
{: .important }

For more information on secrets, see [secrets](../../engine/swarm/secrets.md).

#### 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.

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.

```yaml
version: "{{ site.compose_file_v3 }}"
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 to 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 that owns the file within
  `/run/secrets/` in the service's task containers. Both default to `0` if not
  specified.
- `mode`: The permissions for the file to 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 is
  be `0444` in newer versions. 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" rel="noopener" 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.

```yaml
version: "{{ site.compose_file_v3 }}"
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.

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

> Note when using docker stack deploy
>
> The `security_opt` option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md).
{: .important }

### 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`](#stop_signal)), before sending SIGKILL. Specified
as a [duration](#specifying-durations).

```yaml
stop_grace_period: 1s
```

```yaml
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` causes
`stop` to send that signal instead.

```yaml
stop_signal: SIGUSR1
```

### sysctls

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

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

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

You can only use sysctls that are namespaced in the kernel. Docker does not
support changing sysctls inside a container that also modify the host system.
For an overview of supported sysctls, refer to
[configure namespaced kernel parameters (sysctls) at runtime](../../engine/reference/commandline/run.md#configure-namespaced-kernel-parameters-sysctls-at-runtime).

> Note when using docker stack deploy
>
> This option requires Docker Engine 19.03 or up when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md).

### tmpfs

> Added in [version 3.6](compose-versioning.md#version-36) file format.

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

```yaml
tmpfs: /run
```

```yaml
tmpfs:
  - /run
  - /tmp
```

> Note when using docker stack deploy
>
> This option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md)
> with a (version 3-3.5) Compose file.

Mount a temporary file system inside the container. Size parameter specifies the size
of the tmpfs mount in bytes. Unlimited by default.

```yaml
- type: tmpfs
  target: /app
  tmpfs:
    size: 1000
```

### 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.

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

### userns_mode

```yaml
userns_mode: "host"
```

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

> Note when using docker stack deploy
>
> The `userns_mode` option is ignored when
> [deploying a stack in swarm mode](../../engine/reference/commandline/stack_deploy.md).
{: .important }

### volumes

Mount host paths or named volumes, specified as sub-options to a service.

You can mount a host path as part of a definition for a single service, and
there is no need to define it in the top level `volumes` key.

But, if you want to reuse a volume across multiple services, then define a named
volume 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).

> Changed in [version 3](compose-versioning.md#version-3) file format.
>
> The top-level [volumes](#volume-configuration-reference) key 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.

This example shows a named volume (`mydata`) being used by the `web` service,
and a bind mount defined for a single service (first path under `db` service
`volumes`). The `db` service also uses a named volume called `dbdata` (second
path under `db` service `volumes`), but defines it using the old string format
for mounting a named volume. Named volumes must be listed under the top-level
`volumes` key, as shown.

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  web:
    image: nginx:alpine
    volumes:
      - type: volume
        source: mydata
        target: /data
        volume:
          nocopy: true
      - type: bind
        source: ./static
        target: /opt/app/static

  db:
    image: postgres:latest
    volumes:
      - "/var/run/postgres/postgres.sock:/var/run/postgres/postgres.sock"
      - "dbdata:/var/lib/postgresql/data"

volumes:
  mydata:
  dbdata:
```

> **Note**
> 
> For general information on volumes, refer to the [use volumes](../../storage/volumes.md)
> and [volume plugins](/engine/extend/plugins_volume/) sections in the documentation.


#### Short syntax

The short syntax uses the generic `[SOURCE:]TARGET[:MODE]` format, where
`SOURCE` can be either a host path or volume name. `TARGET` is the container
path where the volume is mounted. Standard modes are `ro` for read-only
and `rw` for read-write (default).

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

```yaml
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

> Added in [version 3.2](compose-versioning.md#version-32) file format.

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

- `type`: the mount type `volume`, `bind`, `tmpfs` or `npipe`
- `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). Not applicable for a tmpfs mount.
- `target`: the path in the container where the volume is 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
- `tmpfs`: configure additional tmpfs options
  - `size`: the size for the tmpfs mount in bytes

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  web:
    image: nginx:alpine
    ports:
      - "80:80"
    volumes:
      - type: volume
        source: mydata
        target: /data
        volume:
          nocopy: true
      - type: bind
        source: ./static
        target: /opt/app/static

networks:
  webnet:

volumes:
  mydata:
```

> **Note**
>
> When creating bind mounts, using the long syntax requires the
> referenced folder to be created beforehand. Using the short syntax
> creates the folder on the fly if it doesn't exist.
> See the [bind mounts documentation](../../storage/bind-mounts.md#differences-between--v-and---mount-behavior)
> for more information.

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

> Note when using docker stack deploy
>
> 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, and this 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 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:

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  db:
    image: postgres:9.4
    volumes:
      - db-data:/var/lib/postgresql/data
    networks:
      - backend
    deploy:
      placement:
        constraints: [node.role == manager]
```

### 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. Note that `mac_address` is a legacy option.

```yaml
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
[`check`](#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`.

## Specifying byte values

Some configuration options, such as the `shm_size` sub-option for
[`build`](#build), accept a byte value as a string in a format
that looks like this:

    2b
    1024kb
    2048k
    300m
    1gb

The supported units are `b`, `k`, `m` and `g`, and their alternative notation `kb`,
`mb` and `gb`. Decimal values are not supported at this time.

## Volume configuration reference

While it is possible to declare [volumes](#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](../../engine/reference/commandline/volume_create.md)
subcommand documentation for more information.

See [use volumes](../../storage/volumes.md) and [volume
plugins](/engine/extend/plugins_volume/) 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:

```yaml
version: "{{ site.compose_file_v3 }}"

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
uses 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 returns an error when
`docker-compose up` tries to create the volume.

```yaml
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.

```yaml
volumes:
  example:
    driver_opts:
      type: "nfs"
      o: "addr=10.40.0.199,nolock,soft,rw"
      device: ":/docker/example"
```

### external

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

For version 3.3 and below of the format, `external` cannot be used in
conjunction with other volume configuration keys (`driver`, `driver_opts`,
`labels`). This limitation no longer exists for
[version 3.4](compose-versioning.md#version-34) and above.

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

```yaml
version: "{{ site.compose_file_v3 }}"

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

volumes:
  data:
    external: true
```

> Deprecated in [version 3.4](compose-versioning.md#version-34) file format.
>
> external.name was deprecated in version 3.4 file format use `name` instead.
{: .important }

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

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

> Note when using docker stack deploy
>
> External volumes that do not exist _are created_ if you use [docker stack deploy](#deploy) 
> to launch the app in [swarm mode](../../engine/swarm/index.md) (instead of
> [docker compose up](../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).
{: .important }

### labels

Add metadata to containers using
[Docker labels](../../config/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.

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

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

### name

> Added in [version 3.4](compose-versioning.md#version-34) file format.

Set a custom name for this volume. The name field can be used to reference
volumes that contain special characters. The name is used as is
and will **not** be scoped with the stack name.

```yaml
version: "{{ site.compose_file_v3 }}"
volumes:
  data:
    name: my-app-data
```

It can also be used in conjunction with the `external` property:

```yaml
version: "{{ site.compose_file_v3 }}"
volumes:
  data:
    external: true
    name: my-app-data
```

## 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 is `bridge` on a single host and `overlay` on a
Swarm.

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

```yaml
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/index.md).

* 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).

#### host or none

Use the host's networking stack, or no networking. Equivalent to
`docker run --net=host` or `docker run --net=none`. Only used if you use
`docker stack` commands. If you use the `docker-compose` command,
use [network_mode](#network_mode) instead.

If you want to use a particular network on a common build, use [network] as
mentioned in the second yaml file example.

The syntax for using built-in networks such as `host` and `none` is a little
different. Define an external network with the name `host` or `none` (that
Docker has already created automatically) and an alias that Compose can use
(`hostnet` or `nonet` in the following examples), then grant the service access to that
network using the alias.

```yaml
version: "{{ site.compose_file_v3 }}"
services:
  web:
    networks:
      hostnet: {}

networks:
  hostnet:
    external: true
    name: host
```

```yaml
services:
  web:
    ...
    build:
      ...
      network: host
      context: .
      ...
```

```yaml
services:
  web:
    ...
    networks:
      nonet: {}

networks:
  nonet:
    external: true
    name: none
```

### 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.

```yaml
driver_opts:
  foo: "bar"
  baz: 1
```

### attachable

> Added in [version 3.2](compose-versioning.md#version-32) file format.

Only used when the `driver` is set to `overlay`. If set to `true`, then
standalone containers can attach to this network, in addition to services. If a
standalone container attaches to an overlay network, it can communicate with
services and standalone containers that are also attached to the overlay
network from other Docker daemons.

```yaml
networks:
  mynet1:
    driver: overlay
    attachable: true
```

### enable_ipv6

Enable IPv6 networking on this network.

> Not supported in Compose File version 3
>
> `enable_ipv6` requires you to use a version 2 Compose file, as this directive
> is not yet supported in Swarm mode.
{: .warning }

### 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:

```yaml
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](../../config/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.

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

```yaml
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` does not attempt to create it, and raises
an error if it doesn't exist.

For version 3.3 and below of the format, `external` cannot be used in
conjunction with other network configuration keys (`driver`, `driver_opts`,
`ipam`, `internal`). This limitation no longer exists for
[version 3.4](compose-versioning.md#version-34) and above.

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

```yaml
version: "{{ site.compose_file_v3 }}"

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

networks:
  outside:
    external: true
```

> Deprecated in [version 3.5](compose-versioning.md#version-35) file format.
>
> external.name was deprecated in version 3.5 file format use `name` instead.
{: .important }

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

```yaml
version: "{{ site.compose_file_v3 }}"
networks:
  outside:
    external:
      name: actual-name-of-network
```

### name

> Added in [version 3.5](compose-versioning.md#version-35) file format.

Set a custom name for this network. The name field can be used to reference
networks which contain special characters. The name is used as is
and will **not** be scoped with the stack name.

```yaml
version: "{{ site.compose_file_v3 }}"
networks:
  network1:
    name: my-app-net
```

It can also be used in conjunction with the `external` property:

```yaml
version: "{{ site.compose_file_v3 }}"
networks:
  network1:
    external: true
    name: my-app-net
```

## configs configuration reference

The top-level `configs` declaration defines or references
[configs](../../engine/swarm/configs.md) that 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 does not attempt to create it, and if it does not exist, a
  `config not found` error occurs.
- `name`: The name of the config object in Docker. This field can be used to
  reference configs that contain special characters. The name is used as is
  and will **not** be scoped with the stack name. Introduced in version 3.5
  file format.
- `driver` and `driver_opts`: The name of a custom secret driver, and driver-specific
  options passed as key/value pairs. Introduced in version 3.8 file format, and
  only supported when using `docker stack`.
- `template_driver`: The name of the templating driver to use, which controls
  whether and how to evaluate the secret payload as a template. If no driver
  is set, no templating is used. The only driver currently supported is `golang`,
  which uses a `golang`. Introduced in version 3.8 file format, and only supported
  when using `docker stack`. Refer to [use a templated config](../../engine/swarm/configs.md#example-use-a-templated-config)
  for a examples of templated configs.

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

```yaml
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 exists within the service. The following
example modifies the previous one to use the external config called
`redis_config`.

```yaml
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) that 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 does not attempt to create it, and if it does not exist, a
  `secret not found` error occurs.
- `name`: The name of the secret object in Docker. This field can be used to
  reference secrets that contain special characters. The name is used as is
  and will **not** be scoped with the stack name. Introduced in version 3.5
  file format.
- `template_driver`: The name of the templating driver to use, which controls
  whether and how to evaluate the secret payload as a template. If no driver
  is set, no templating is used. The only driver currently supported is `golang`,
  which uses a `golang`. Introduced in version 3.8 file format, and only
  supported when using `docker stack`.

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

```yaml
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 exists within the service. The following
example modifies the previous one to use the external secret called
`redis_secret`.

### Compose File v3.5 and above

```yaml
secrets:
  my_first_secret:
    file: ./secret_data
  my_second_secret:
    external: true
    name: redis_secret
```

### Compose File v3.4 and under

```yaml
  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 %}

## Extension fields

> Added in [version 3.4](compose-versioning.md#version-34) file format.

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

## Compose documentation

- [User guide](../index.md)
- [Installing Compose](../install.md)
- [Compose file versions and upgrading](compose-versioning.md)
- [Sample apps with Compose](../samples-for-compose.md)
- [Command line reference](../reference/index.md)
